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IC 
 
 8955 
 
 Bureau of Mines Information Circular/1984 
 
 Underground Mine Communications, 
 Control and Monitoring 
 
 By Staff, Pittsburgh Research Center 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
fflb^^ ^^^^^fe^^-M/t^^ cr^'HA:^^ 
 
 Information Circular]8955 
 
 4/ 
 
 Underground Mine Communications, 
 Control and Monitoring 
 
 By Staff, Pittsburgh Research Center 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 James G. Watt, Secretary 
 
 BUREAU OF MINES 
 Robert C. Norton, Director 
 
\>H 
 
 (\i" 
 
 1°^ 
 
 
 
 Library of Congress Cataloging in Publication Data: 
 
 Underground 
 
 mine communications, control 
 
 and monitoring. 
 
 
 (Information circular / United States D 
 reau of Mines ; 8955) 
 
 epartment of the 
 
 Interior, Bu- 
 
 Includes 
 
 bibliographical references. 
 
 
 
 Supt. of Docs, no.: I 28.27:8955. 
 
 
 
 1. Mine 
 II. Series: 
 8955. 
 
 communication systems. I. 
 Information circular (United 
 
 Pittsburgh Research Center. 
 States. Bureau of Mines) ; 
 
 TN295.U4 
 
 [TN344] 622s [622'. 2] 
 
 83-600288 
 
 
<v^ PREFACE 
 
 OO 
 
 "^^ Since 1969, the Bureau of Mines, U.S. Department of Interior, has 
 ^ sponsored numerous programs aimed at improving methods of underground 
 communication. As a result of these research and development programs , 
 a wealth of information has been made available to the mining industry. 
 Unfortunately, some of this material is highly analytical, and most is 
 written in terms best understood by communication specialists. Because 
 of the volume of data (over 100 studies have been performed) and its 
 highly technical nature, most of the information is not readily avail- 
 able for practical application by mine operators. This manual brings 
 together relevant data from all previous reports, studies, and other 
 sources, and presents these data in such a way that they may be applied 
 by the mining industry to improve communications in underground mines. 
 
 This report is intended as a guideline and not as a comprehensive 
 documentary of mine equipment. Installation of equipment in a mine 
 should be done only by people thoroughly qualified to do such work. 
 Installations should follow procedures recommended by the equipment man- 
 ufacturer and should comply with good safety practices. All installa- 
 tions should also comply with applicable codes and regulations. 
 
 The views and conclusions contained in this document are those of the contractor, 
 Collins Avionic Division, and should not be interpreted as necessarily representing 
 the official policies or recommendations of the Interior Department's Bureau of Mines 
 or of the U.S. Government. 
 
 
 cD 
 
iii 
 
 CONTENTS 
 
 Page 
 
 Preface 1 
 
 Chapter 1. — Introduction 1 
 
 1.1 History of Underground Comnunl cat ions 1 
 
 1.2 Productivity and Safety 2 
 
 1 . 3 Constraints on Equipment 4 
 
 1.4 Communication Requirements 5 
 
 1.4.1 The Working Section Crew 5 
 
 1.4.2 The Maintenance Crew 6 
 
 1.4.3 Motormen 6 
 
 1.4.4 Inspectors and Management Personnel 6 
 
 1.4.5 The Dispatcher 6 
 
 1.4.6 Hoist Communications 7 
 
 1.5 Present Communication Systems 7 
 
 1.6 Summary 8 
 
 Bibliography 9 
 
 Chapter 2. — Communication Systems 10 
 
 2. 1 Introduction 10 
 
 2.2 Wired Phone Systems 10 
 
 2.2.1 General Telephone Theory 11 
 
 a. Magneto-Type Telephones 11 
 
 b. Sound-Powered Telephones 11 
 
 c. Paging Telephones 11 
 
 d. Dlal-and-Page Telephones 13 
 
 2.2.2 Distribution Systems 13 
 
 a. Single Pair 14 
 
 b. Figure-8 Multipair Cable 14 
 
 c. Coaxial Cable 15 
 
 d. Multiplex Systems..... 15 
 
 I. Frequency Division Multiplexing 15 
 
 11. Time Division Multiplexing 16 
 
 2.2.3 Telephone Exchanges 17 
 
 a. Manual Switchboard 17 
 
 b. Private Automatic Branch Exchange 17 
 
 c. Computer-Controlled Switches 18 
 
 2.3 Radio Systems 18 
 
 2.3.1 General Radio System Theory 18 
 
 a. Amplitude Modulation 18 
 
 b. Frequency Modulation 18 
 
 2.3.2 Distribution Systems 19 
 
 a. Antenna Theory 20 
 
 1. Half-Wave Dlpole Antenna 20 
 
 II. Quarter-Wave Antenna 21 
 
 ill. Long-Wire Antenna 21 
 
 Iv. Loop Antenna 21 
 
 b. Leaky Feeder Systems 22 
 
 c. Waveguide Propagation 22 
 
 d. Repeaters 23 
 
 1. Fl-Fl Repeater 23 
 
 11. F1-F2 Repeater 24 
 
 2.3.3 Through-the-Earth Radio 24 
 
 2.3.4 Radio Pagers 25 
 
CONTENTS — Continued 
 
 Page 
 
 2.4 Carrier Current Systems 25 
 
 2.4.1 Trolley Carrier Phone 26 
 
 2.4.2 Hoist Rope Radio 26 
 
 2.4.3 Medium-Frequency Radio , 27 
 
 2.5 Hybrid Systems ,. 31 
 
 2.5.1 Improvements in System Versatility 31 
 
 2.5.2 Dial Phone-Pager Phone Systems 32 
 
 2.6 Other Systems , 33 
 
 2.6.1 Seismic Systems 33 
 
 2.6.2 Stench System , 33 
 
 2.6.3 Hoist Bell Signaling... 33 
 
 2.6.4 Visual Pagers , 34 
 
 2.7 Summary , 34 
 
 Bibliography 35 
 
 Chapter 3. — Solutions to the Communication Requirements 37 
 
 3. 1 Introduction. 37 
 
 3.2 The Mine Entrance 38 
 
 3.2.1 Bell Signaling Systems 39 
 
 3.2.2 Trailing Cable Systems 40 
 
 3.2.3 Radio Systems 40 
 
 3.2.4 Hoist Rope Carrier Current System ., 41 
 
 3.2.5 Hoist Signaling Summary 41 
 
 3.3 Permanent and Semipermanent In-Mine Locations 42 
 
 3.3.1 Single-Pair Pager Phones 42 
 
 3.3.2 Multipair Systems 43 
 
 3.3.3 Multiplexed Systems 44 
 
 3.4 Mining Area ■, 45 
 
 3.4.1 Radio Systems .,,. 45 
 
 3.4.2 Longwall Mining 49 
 
 3 . 5 Haulageways 52 
 
 3.5.1 Trolley Haulage 52 
 
 a. Dedicated Wire 54 
 
 b . Summary 56 
 
 3.5.2 Nontrolley Haulage 56 
 
 a. Leaky-Coax Systems 56 
 
 b. UHF Reflective Techniques in Underground Mines 59 
 
 c. Dedicated-Wire Radio Systems 60 
 
 d. Wireless Radio System 61 
 
 i . Interference 61 
 
 ii. Signal Attenuation in the Haulageway 63 
 
 iii. Signal Attenuation Around Corners 63 
 
 3.5.3 Belt Haulage 64 
 
 3.6 Special Requirements 64 
 
 3.6.1 The Roving or Isolated Miner 64 
 
 a. One-Way-Voice (Pocket) Pagers 65 
 
 3.6.2 Mo to rman- to- Snapper 67 
 
 a. Telephone and Trolley-Carrier Phone System 67 
 
 b. Walkie-Talkie System 68 
 
 3 . 7 Emergency Communications 69 
 
 3.7.1 Detecting and Locating the Trapped Miner 69 
 
CONTENTS—Continued 
 
 Page 
 
 3.7.2 Refuge Shelter 72 
 
 3.7.3 Rescue Team Comminicatlons 73 
 
 3.7.4 Medium-Frequency Rescue Systems 73 
 
 a. Specific Application of MF Communications to Rescue Teams... 74 
 
 b. System Concepts 75 
 
 c. Location and Communications Systems for the Rescue of 
 
 Trapped Miners 76 
 
 d . Performance Data 79 
 
 3.7.5 Emergency Warning Systems 79 
 
 Bibliography. 82 
 
 Chapter 4. — Computerized Mine Monitoring 85 
 
 4. 1 Introduction. 85 
 
 4.2 Uses of a Mine Monitoring System 85 
 
 4.3 Petitions for Modification 87 
 
 4.4 Technical Factors 88 
 
 4.4.1 Sensors 88 
 
 4.4.2 Telemetry 89 
 
 4.4.3 Reliability 89 
 
 4.5 Commercially Available Mine Monitoring Equipment 90 
 
 4.5.1 Introduction 90 
 
 4.5.2 Telemetry-Analysis and Display Systems 91 
 
 a. Systems Suppliers 93 
 
 b . Summary 98 
 
 4.5.3 Sensors , 99 
 
 a. Air Velocity Sensors 100 
 
 b. Methane Sensors 101 
 
 c. Carbon Monoxide Sensors 103 
 
 d. Dust sensors 104 
 
 4.6 Existing Mine Monitoring Systems 104 
 
 4.6.1 U.S. Underground Coal Mines 105 
 
 4.6.2 U.S. Underground Metal-Nonmetal Mines 115 
 
 4.6.3 Foreign Mines 115 
 
 References. 119 
 
 Chapter 5. — Communication System Design and Improvement 121 
 
 5.1 Introduction 121 
 
 5.2 New Phone System Design 121 
 
 5.2.1 Wired Phone Systems 122 
 
 a. Single-Pair Systems 122 
 
 b. Multlpair Systems 124 
 
 c. Multiplexed Phone Systems 126 
 
 5.2.2 Cable Selection 127 
 
 5.2.3 Summary 130 
 
 5.3 Improving Existing (In-Place) Phone Systems 131 
 
 5.3.1 Trolley Carrier Phone Systems 131 
 
 a. Isolating Loads at the Carrier Frequency 132 
 
 i. Rectifiers 133 
 
 ii. Heaters 136 
 
 iii. Vehicle Lights 137 
 
 iv. Other Loads 137 
 
 b. Using a Dedicated Wire 138 
 
vi 
 
 CONTENTS—Cont inued 
 
 Page 
 
 c. Using a Remote Transceiver 139 
 
 d. Summary 140 
 
 5.3.2 Improving Telephone Systems 140 
 
 a. Loopback Methods 141 
 
 b. Sectionalizing the Underground Network 142 
 
 5.3.3 Summary 142 
 
 Bibliography 143 
 
 Chapter 6. — Installation Techniques 144 
 
 6.1 The Basic Philosophy 144 
 
 6.2 Pager Phone Installation 144 
 
 6.2.1 Mounting 144 
 
 6.2.2 Connections 145 
 
 6.2.3 Batteries 145 
 
 6.2.4 Fuses 146 
 
 6.2.5 Amplifier Loudness 146 
 
 6.3 Phone Lines and Transmission Cables 146 
 
 6.3.1 Phone Lines 146 
 
 6.3.2 Leaky Feeder Cable 147 
 
 6.4 Carrier Phone Installation 147 
 
 6.4.1 The Dispatcher Location 147 
 
 6.4.2 Vehicle Installations 150 
 
 6.5 Carrier Current Hoist Phone 154 
 
 6.5.1 Cage 154 
 
 6.5.2 Holstroom and Headframe 155 
 
 6.6 Summary 155 
 
 Bibliography 158 
 
 Chapter 7. — ^Maintenance. 159 
 
 7 . 1 General 1 59 
 
 7.2 Preventive Maintenance and Inspections 159 
 
 7.2.1 Cables 159 
 
 7.2.2 Pager Phones 159 
 
 a. Listen Circuit 159 
 
 b. Page Circuit and Talk Circuit 159 
 
 c. General Comments 160 
 
 d. Battery Condition 160 
 
 e. Battery Testing 160 
 
 7.2.3 Carrier Phones 161 
 
 a. Microphone 161 
 
 b. Batteries 162 
 
 c. Wet Cell Maintenance 163 
 
 d. Gelled Electrolyte Battery 163 
 
 e. Troubleshooting on the Vehicle 163 
 
 f. Mapping Signal Levels 166 
 
 7 . 3 Summary 168 
 
 Bibliography 167 
 
 Appendix A. — Communication System Examples 170 
 
 Appendix B. — Federal Regulations 197 
 
 Appendix C. — Equipment Suppliers 202 
 
 Appendix D. — Glossary of Terms 208 
 
CHAPTER 1.— INTRODUCTION 
 
 1.1 History of Underground 
 Communications 
 
 Although the technology involved in 
 removing material from below the earth's 
 surface has a long history, communication 
 systems in underground workings are rela- 
 tively new to the industry. Communica- 
 tion equipment did not begin appearing in 
 underground mines until the early 1900' s. 
 Figure 1-1 shows a miner using a Western 
 Electric standard telephone set for 
 underground mines in 1913. These early 
 phones were essentially the same as those 
 used aboveground, except that they were 
 enclosed in cast-iron boxes as protection 
 against moisture, acid fumes, and gases. 
 
 In the 1950' s, the Chesapeake and 
 Potomac Telephone Co. of West Virginia 
 introduced a telephone set for use in 
 explosive atmospheres that was designed 
 around the philosophy of explosion 
 containment. To contain an explosion 
 within the telephone set required a 
 39-pound casing, which greatly limited 
 portability. 
 
 FIGURE !•!. - Coal miner talking on an under- 
 ground telephone in 1913. 
 
 Both of these telephone sets 
 required "pipe" or conduit, not a very 
 practical item for a 100-square-mile coal 
 mine. A conmion thread prevalent in the 
 first 40 years of design and development 
 is that the primary effort was placed on 
 the telephone set. 
 
 In the early 1970' s, as work began 
 
 on the design of modern conmiunication 
 
 systems for use in underground mine 
 
 environments, the following requirements 
 were established: 
 
 1. Must meet intrinsic safety 
 standards. — This applies not only to 
 coal mines, but to other explosive 
 environments. 
 
 2. Must be compatible with the 
 environment. — The system must with- 
 stand dust, moisture, and corrosive 
 conditions. 
 
 3. Must be rugged in structure. — 
 The system must withstand maintenance by 
 10-inch pliers and 4-pound hammers, as 
 well as impact from a falling piece of 
 roof. 
 
 4. Must be size-flexible. — Whatever 
 is built must be sized for the small 
 operator as well as the large companies. 
 
 5. Must be a total system in 
 design. — The system must be intrinsically 
 safe, including cable, power supply, and 
 station set. 
 
 6. Must work with present tele- 
 phone system. — The system should be com- 
 patible with aboveground communications 
 already in place; it should not be the 
 single cause of change in aboveground 
 communications . 
 
 7. Value-added pricing. — The system 
 should be reasonably priced so that sav- 
 ings created by its introduction will 
 more than offset the installation cost. 
 
8. Full-service full-support con- 
 cept. — The service offering the system 
 must provide for maintenance, training 
 (initial and continuing), route design, 
 and transmission engineering. 
 
 In recent history, the most common 
 method of underground mine communications 
 consisted of louds peaking- or paging- 
 type telephones, or alternatively, mag- 
 neto ringing telephones. In most cases 
 these phones were connected on a common 
 party line with one telephone for each 
 working section and additional phones at 
 other key locations, such as maintenance 
 shops, both underground and on the sur- 
 face. As mining operations became more 
 mechanized, underground rail haulage sys- 
 tems were developed. Eventually, these 
 were driven by electrically powered loco- 
 motives, and trolley carrier current com- 
 munication systems were developed. While 
 a few mines have begun to use dial-type 
 telephones underground, their use to date 
 has been very limited. It is expected, 
 however, that more and more mines will 
 install dial-and-page telephones and 
 radio-type communication systems in the 
 future. Today's rail transportation sys- 
 tems are typically equipped with carrier 
 current phones to provide communications 
 between vehicles and between vehicles and 
 a central dispatcher. 
 
 Early shaft communications consisted 
 of bells or whistle signaling systems. 
 Today these systems have been replaced 
 with radio and carrier current systems 
 that allow two-way voice communication to 
 and from personnel in the cage. 
 
 It is recognized tnat presently 
 available mine communication systems need 
 improvement. Research and development 
 programs are continually being conducted 
 by the Bureau of Mines , the mining indus- 
 try, and equipment manufacturers to 
 improve communication technology and 
 techniques. The primary objective of all 
 of these programs is to increase the pro- 
 ductivity of the mining industry and 
 increase the safety of miners. 
 
 1.2 Productivity and Safety 
 
 Underground mining operations, like 
 other industrial enterprises, are tied 
 together by communications. Adequate 
 communication within a mine and between 
 the surface and the underground work sta- 
 tions is a vital part of the proper 
 operation of any underground facility. 
 This communication capability is not only 
 an important factor in the concept of 
 safety, but also is an aid to the day- 
 to-day operations and the task of 
 extracting and moving the product to the 
 surface. If a rapid, accurate flow of 
 data is automatically and continuously 
 presented to management, then decisions 
 can be made sooner and more accurately. 
 Some people consider the operation of a 
 mine to be a relatively static operation 
 planned many months, even years, in 
 advance. However, the productive time 
 per shift and productivity trends indi- 
 cate that a large amount of waste time 
 accumulates due to unpredictable daily 
 events. If management were aware of 
 breakdowns within minutes, attention 
 could be more quickly focused on solving 
 the problems, thereby increasing long- 
 term production. 
 
 Safety can certainly be enhanced by 
 accurate fast communication channels. 
 Quicker medical assistance, faster evalu- 
 ation of the situation underground, and 
 accurate location of problems will be 
 direct benefits. In addition to these 
 obvious advantages, remote monitoring and 
 control of equipment and conditions 
 underground will allow management per- 
 sonnel to prevent accidents and other 
 causes of production losses. 
 
 A study of 9,300 injury-causing 
 accidents that occurred in underground 
 bituminous coal mines during 1974 found 
 that the financial cost totaled almost 
 $57 million, or an average of $6,100 per 
 accident. This figure did not include 
 the intangible cost of reduced efficiency 
 in fellow workers resulting from the 
 accident. The study detected a marked 
 
tendency of mine crews in or near the 
 area where the accident occurred to slow 
 down their pace of operations for a 
 period of time, especially after a seri- 
 ous accident; consequently, production 
 would drop. Such a drop was in addition 
 to the accident itself, and to the time 
 required to clean up after the accident. 
 About 41%, or an estimated $23.6 million, 
 of the total (current and future) cost of 
 underground coal mine accidents in 1974 
 was borne by mining companies in the form 
 of compensation payments to accident vic- 
 tims and survivors, lost coal production, 
 and the expense of investigating the 
 accidents. Any devices or techniques, 
 including good communication systems, 
 that will reduce accidents will quickly 
 pay for themselves. 
 
 The evolution of mining technology, 
 including underground communications, is 
 inevitable as coal assumes a more sig- 
 nificant role in the national energy 
 plan. Another factor that will aid this 
 development is the application of modern 
 remote control and monitoring methods to 
 increase production. Advances in remote 
 monitoring and control of equipment and 
 conditions underground will involve 
 increased use of computers. Mine moni- 
 toring systems using a coii5)uter have 
 already been installed in some mines. 
 Basically, these systems consist of 
 sensors placed at strategic locations, 
 data relay stations, and a digital com- 
 puter in the mine office. The computer 
 is programed to process the data, deter- 
 mine when and where abnormal conditions 
 exist, and alert mine personnel. 
 
 Six types of sensors in common use 
 detect methane, carbon monoxide, tempera- 
 ture, relative humidity, airflow, or dif- 
 ferential air pressure. Systems can 
 include an electronic display of the mine 
 layout, electric typewriters, and video 
 display screens connected to the com- 
 puter. The computer receives data from 
 the remote stations in the form of elec- 
 trical signals, which are translated into 
 numerical measurements and checked 
 against a set of standards to see if all 
 factors are within normal limits. If any 
 factors at a remote station exceed normal 
 
 limits, an alarm may be initiated. At 
 the same time, the electric typewriter 
 connected to the computer types out 
 either a warning or a danger message to 
 specify what is wrong and at which sensor 
 the trouble is being detected. 
 
 In the United Kingdom, a commercial 
 mine monitoring and control system, MINOS 
 (Mine Operating System), is being devel- 
 oped. The heart of the system is a digi- 
 tal computer (fig. 1-2) located within 
 the system control center. This computer 
 is programed to obtain various data from 
 remote sensors and monitor the satis- 
 factory operation of the mine, giving 
 audible and visual alarms as well as 
 initiating automatic shutdown of equip- 
 ment when necessary. Control of the con- 
 veyor system including startup, shutdown, 
 and feed rate is also possible. Print- 
 outs and daily summaries of specified 
 information are outputted automatically 
 or are available on demand by key- 
 board commands entered at the system 
 center. Remote control and monitoring 
 of surface facilities and preparation 
 plants as well as underground equipment 
 such as pumps and fans can also be 
 acconqjlished with the computer-controlled 
 systems. 
 
 Any remote monitor and control sys- 
 tem is relatively sophisticated, and an 
 
 VISUAL DISPLAY SCREENS 
 
 INDICATOR LAMPS 
 
 CONTROL PANEL 
 
 FIGURE 1-2. - MINOS system center. 
 
economic analysis of the tradeoffs 
 involved in the mechanization is 
 required. However, minewide remote moni- 
 toring and control, coupled with an 
 effective communication system, is one of 
 the tools that can be used to increase 
 safety and productivity in underground 
 
 1.3 Constraints on Equipment 
 
 The environmental constraints on 
 equipment expected to operate in any 
 underground mine are severe. Equipment 
 designed for surface operation, even if 
 it would work underground, could not be 
 expected to last for any length of time 
 in the harsh mine environment. 
 
 Equipment must be protected from, or 
 imnune to, high-moisture atmospheres (0 
 to 100% humidity levels) and remain oper- 
 able over wide temperature ranges. Dust 
 can be expected to clog airflow passages, 
 plug relay contacts, and cause switches 
 to stick. Dust accumulation on elec- 
 tronic components can also cause heat 
 buildup and even "short circuits." Some 
 mine atmospheres are highly corrosive, 
 and equipment in these mines must be con- 
 structed from materials that are resist- 
 ant to the corrosion, or else protected 
 from it. The very dry atmosphere in some 
 mines causes gaskets and seals to quickly 
 dry out and start leaking. Static elec- 
 tricity in these mines can also pose a 
 problem for certain types of solid state 
 electrical circuits. 
 
 In addition to the environmental 
 constraints, physical considerations must 
 be taken into account. Space is often at 
 a premium in underground mines, espe- 
 cially in low coal seams. Communica- 
 tions, control, and monitoring equipment 
 must be small in size and should be light 
 in weight. Because it must exist in 
 close proximity with heavy mining machin- 
 ery, communication equipment must also be 
 ruggedly designed and shock protected. 
 Reliability and ease of maintenance are 
 other reasons why most equipment designed 
 for surface operation cannot be utilized 
 in underground mines. 
 
 Much of the communication equipment 
 in coal mines today is located in venti- 
 lated areas where there is less likeli- 
 hood of an explosion. However, if a ven- 
 tilation system is required to control 
 the gas or dust hazard and basic commini- 
 cations must be maintained in the event 
 of a ventilation failure, then this com- 
 munication equipment must not be capable 
 of generating an ignition spark. Thus, 
 much of today's mine communication equip- 
 ment carries a "Permissibility" label 
 granted by the Mine Safety and Health 
 Administration. To achieve the permissi- 
 bility rating without explosion-proof 
 packaging, all sources of sparks must be 
 controlled by limiting voltages, cur- 
 rents, and the amounts of stored energy — 
 such as in batteries, capacitors, and 
 inductors — to safe levels. Such a device 
 is defined as "intrinsically safe." 
 
 The amount of spark current in a 
 resistive circuit required to ignite 
 a methane-air mixture varies with the 
 open-circuit voltage. Guidelines to 
 safe operating limits, such as shown 
 in figure 1-3, indicate a hazard at 
 
 MINIMUM IGNITION CURRENT lAMPERESI 
 
 FIGURE 1-3. - Typical ignition hazard curve. 
 
2 amperes in a 20-volt resistive circuit. 
 However, no greater hazard exists with 
 10 amperes in a 12-volt resistive cir- 
 cuit. When reactive circuits (circuits 
 with inductors or capacitors) or energy 
 storage elements (batteries) are con- 
 sidered, limits on safe circuit design 
 are even tighter. These safety consider- 
 ations have directed the design of all 
 the "permissible" communication devices 
 on the market today. One of the most 
 recent pager-type phones does not contain 
 any inductive components other than the 
 speaker and handset and uses only a 
 single 12-volt battery. 
 
 In gassy mines, only permissible or 
 intrinsically safe communication equip- 
 ment should be acceptable for use in all 
 locations under all conditions. 
 
 The permissible rating requirement 
 rules out use of most systems designed 
 for surface applications. For instance, 
 a permissible rating cannot be given to 
 a standard telephone connected to a 
 telephone switchboard because of high 
 voltages (48 volts dc while on hook and 
 as much as 120 volts ac while ringing) 
 and use of many highly inductive devices 
 in the circuit. While placing the tele- 
 phone in an explosion-proof housing could 
 be considered as a means of making 
 it "permissible," the cost would be 
 unreasonable. 
 
 Other restrictions on communication 
 equipment used underground are specified 
 in the U.S. Code of Federal Regulations 
 (See appendix B of this manual.) 
 
 1.4 Communication Requirements 
 
 In an ideal communication system an 
 individual should be able to initiate and 
 receive calls regardless of his or her 
 position in the mine. To accomplish 
 this, more than one communication system 
 may be required. Communication require- 
 ments are more readily defined by sep- 
 arating the underground personnel in 
 present-day mine operations into four 
 functional groups: 
 
 Working section crew 
 
 Maintenance crew 
 
 Motormen 
 
 Inspectors and management personnel 
 
 The position of dispatcher is con- 
 sidered separately because he or she gen- 
 erally coordinates the communications as 
 well as the haulage traffic. 
 
 In small mines and belt-haulage-type 
 mines the conmiunication center may be the 
 responsibility of the hoist engineer, the 
 supply man, or the maintenance foreman. 
 Because hoist communication requirements 
 are unique, they also can be treated 
 separately. 
 
 1.4.1 The Working Section Crew 
 
 Under normal operating conditions 
 the section forman communicates by fixed 
 phone to the shift foreman to request 
 supplies and maintenance services and to 
 file periodic productivity reports. 
 Under emergency conditions he must be 
 able to request medical aid for personnel 
 and report hazardous conditions in his 
 area. 
 
 The high acoustic noise level cre- 
 ated by the mining machinery greatly 
 reduces the effective communications 
 between the foreman and his crew. This 
 noise also interferes with the foreman 
 receiving calls. Often a motorman must 
 deliver a call-in message to the foreman 
 when he is transferring hauling cars in 
 his section. Some belt haulage mines 
 must even resort to turning off the con- 
 veyor system, thereby causing all the 
 section foremen to call in. Communica- 
 tion requirements of the section crew can 
 be satisfied by loudspeaking pager phones 
 that can easily be moved to keep them 
 within hearing range, or by some form of 
 radio link (two-way radio, pocket pager, 
 or "beeper"). 
 
1.4.2 The Maintenance Crew 
 
 Unlike a working section crew, the 
 maintenance crew is spread throughout the 
 mine. The maintenance foreman must be 
 able to receive repair requests and dis- 
 patch his crews for both emergency and 
 scheduled repair work. He should also be 
 able to maintain communications with the 
 individual crew members while they are in 
 transit or after they have arrived at the 
 repair site. The dispatcher may provide 
 assistance by routing messages for equip- 
 ment repair and parts to the foreman from 
 the maintenance crew. 
 
 Wireless mobile communication equip- 
 ment, linking the maintenance foreman and 
 his crew together, would be ideal for the 
 above tasks. However, any portable radio 
 equipment used must be small and light- 
 weight because crew members already have 
 much to carry. 
 
 (Newer phones have backup batteries 
 installed in each phone so this may not 
 be a problem. ) 
 
 In spite of these drawbacks, carrier 
 phone systems usually meet the require- 
 ments of the motormen, except for an 
 emergency that severs the trolley wire or 
 otherwise removes power from the wire. 
 
 In mines without tracked-trolley 
 haulage systems, a radio link must be 
 established to allow motormen to remain 
 in contact with one another. 
 
 1.4.4 Inspectors and Management 
 Personnel 
 
 These people are underground pri- 
 marily to observe mine conditions and 
 personnel. They should be able to stay 
 in continuous contact with the communica- 
 tion center for the following reasons: 
 
 1.4.3 Motormen 
 
 The motormen are responsible for 
 coal or ore haulage and the delivery of 
 men and supplies to the working sections. 
 Rights-of-way and the disposition of 
 haulage cars must be known to all motor- 
 men to avoid accidents. In mines using 
 tracked-trolley haulage, activities can 
 be coordinated by a trolley wire (car- 
 rier) phone system. These are known as 
 "carrier" systems because the communica- 
 tion is "carried" on a wire not intended 
 for communication, in this case the 
 trolley line; the term "carrier," how- 
 ever, refers to the technique, not the 
 wire itself. This single-channel network 
 keeps the dispatcher and all motormen in 
 continuous contact with one another. 
 This phone system also allows the dis- 
 patcher to notify all motormen of any 
 mine emergency. The two drawbacks to 
 this system are 
 
 1. Dead zones, which are sections 
 of track where the phone is inoperative 
 owing to excess electrical noise or 
 excess attenuation of signal strength. 
 
 2. Trolley wire power failures, 
 which cause the phones to go dead. 
 
 To be informed of any emergencies 
 that might arise. 
 
 To keep the center informed of their 
 location. 
 
 To receive calls from other parts of 
 the mine. 
 
 These requirements could be satis- 
 fied by an effective, extensive wireless 
 mobile communication system. A vehicle- 
 mounted system may be sufficient in some 
 cases, such as the trolley carrier phones 
 in track haulage mines. 
 
 1.4.5 The Dispatcher 
 
 The dispatcher's location in some 
 mines has developed into a communication 
 center for all underground operations. 
 He is in direct contact with all motormen 
 via the trolley wire phone system, and 
 directs all vehicle traffic in the mine. 
 In some mines, he also controls the fixed 
 phone circuits via a small switchboard. 
 He locates personnel by the paging phones 
 or by relaying messages through the 
 motormen to the sections. He serves as 
 the human coupler between the different 
 phone systems, and he is in the best 
 
position to quickly notify all under- 
 ground personnel of any emergency 
 condition. 
 
 If this evolution continues, the 
 dispatcher's job will expand to the point 
 where he may become overworked. For 
 safety and productivity reasons the voice 
 traffic control and the monitoring func- 
 tion of the dispatcher's job cannot 
 interfere with his prime responsibility 
 of vehicle traffic control. Therefore, 
 it may be desirable to transfer these 
 responsibilities to other personnel or to 
 automatic dialing and alarm equipment. 
 For this reason some mines have estab- 
 lished a separate communication system 
 "operator" position. This person has 
 responsibility for voice traffic control, 
 and also for monitoring environmental 
 conditions in the mine. The operating 
 conditions of the haulage and mining 
 equipment could also be monitored from 
 the communication center. 
 
 1.4.6 Hoist Communications 
 
 A hoist-shaft communication system 
 should satisfy the requirements for com- 
 munication throughout the full travel 
 of the cage, providing two-way voice com- 
 munication between the cage and the 
 hoistman. The system should also 
 allow for shaft-inspection communication 
 between the inspector and the "hoistman. " 
 For the modern, automated shaft, signals 
 are also required for a slack-rope indi- 
 cation, to permit selection of level, 
 enable interface with interlocks, and jog 
 for exact position at any level or shaft 
 station. Cage equipment must be small 
 and capable of being located so that it 
 cannot be damaged by any of the various 
 uses of the cage such as transporting 
 supplies and machinery into the mine. 
 
 Additional microphone-speaker sta- 
 tions should be located on each cage 
 level when multilevel "man trip" cages 
 are used. Until recently, bell signaling 
 was the only form of hoist-shaft communi- 
 cation. A disadvantage of the bell sig- 
 naling system is that communication 
 with the cage when it is between levels 
 
 is impossible. This deficiency is espe- 
 cially crucial during shaft inspection or 
 repair, where movement of the cage must 
 be controlled precisely. Today, however, 
 equipment is available that allows two- 
 way voice communication between persons 
 in the hoist cage and the hoistman or 
 other locations at the shaft top or 
 bottom. 
 
 Reliable hoist-shaft communication 
 should be considered as a vital part of 
 the overall communication system, espe- 
 cially during or following an accident or 
 disaster situation. Experience has 
 taught that the hoist often becomes a 
 bottleneck during rescue or evacuation 
 operations, and good communication to and 
 from the cage is essential. 
 
 1.5 Present Communication Systems 
 
 Communication systems currently used 
 in many mines generally consist of two 
 systems, the trolley wire carrier phones 
 and the fixed pager phones. The trolley 
 wire channel must be a party line to keep 
 all motormen informed of one another's 
 location. The pager phone system is 
 often divided into multiparty circuits 
 which are controlled by the dispatcher. 
 For example, one mine studied used eight 
 party line phone circuits terminated at a 
 simple switchboard in the dispatcher's 
 office. A logical partitioning of the 
 pager phone network into a multichannel 
 private line system would be to give each 
 working section a separate line, or at 
 the most two sections per line, and have 
 one common line for all haulageway 
 phones. Individual section phone lines 
 would eliminate peak traffic demand dur- 
 ing production reporting time and provide 
 the section foreman with a private line 
 during an emergency situation. Other 
 phone lines could be used for monitoring 
 the environment and equipment. Based on 
 data and survey results to date, it 
 appears that about two to eight phone 
 lines, depending on activity of the mine, 
 would be sufficient to provide efficient 
 service for the working sections and also 
 provide a common private line for the 
 haulageways. 
 
1.6 Summary 
 
 The need for effective communication 
 between locations underground and between 
 underground and surface locations has 
 been recognized for some time. Unfor- 
 tunately, the equipment to totally sat- 
 isfy these requirements in underground 
 mines has only recently become available. 
 Part of the reason for lack of equip- 
 ment can be blamed on the uniquely 
 harsh environment present in underground 
 mines. 
 
 Despite past deficiencies, equipment 
 is now available to meet most of the 
 requirements for effective underground 
 comnunication systems. The operational 
 requirement of the ultimate system may be 
 simply stated as follows: Each indi- 
 vidual should be able to initiate and 
 receive communications regardless of his 
 location within the mine. In practice, 
 this ultimate requirement will normally 
 be modified. The size and age of a mine, 
 operating conditions, and economic con- 
 siderations will affect the degree to 
 which a system fully meets this ultimate 
 requirement. 
 
 Vehicles operating on rails within a 
 mine should have a communication unit 
 mounted in every powered vehicle 
 (required in some States). Operational 
 safety is increased when there is 
 an intervehicular communication system. 
 Operators can report to each other or to 
 a central dispatcher, thereby reducing 
 the chances of a collision. Carried to 
 the ultimate, the central dispatcher can 
 control the movement of all vehicles at 
 all times. 
 
 Often there is a need to call a man 
 who is not near any vehicle or phone. As 
 a minimum, some form of paging capability 
 should be included in the overall mine 
 phone system that can be used to tell a 
 called party to go to the nearest phone 
 and return the call. The ability to com- 
 municate with men underground, no matter 
 what their location, is essential to the 
 efficiency of any mine. Safety and pro- 
 ductivity are directly related, and both 
 depend upon good communications. In 
 
 addition to the obvious advantages of 
 reliable and effective communications, 
 there are intangible benefits that may 
 not be recognized: 
 
 1. The general attitude of the 
 underground workforce will be improved If 
 they know that they are not "cut off" 
 communicationwise from the surface. 
 
 2. Mines with good communication 
 systems should be able to more effec- 
 tively compete in the labor market. High 
 turnover rates, which are costly owing to 
 training requirements, will be reduced. 
 
 3. Mines with effective comminica- 
 tion systems and good safety records are 
 usually subjected to inspections less 
 frequently, and since studies have shown 
 that production rates decrease when it is 
 known that inspection personnel are on 
 the property, the effect on production 
 should be good. 
 
 Adequate communications within a 
 mine and to the surface is a vital part 
 of the proper operation of an under- 
 ground facility. This communication 
 capability is not only an important 
 factor in the concept of safety precau- 
 tions, but also an aid to the day-to-day 
 operations and the task of moving the 
 mined product to the surface. The mining 
 industry exists to bring the product out 
 and to bring it out safely and econom- 
 ically. Adequate communication is one of 
 the tools available to assist in this 
 task. 
 
 In a like manner, a judicious choice 
 of remote monitoring and control of 
 parameters in the underground environment 
 and on selected machinery will yield a 
 cost savings in production and augment 
 safety. Many man-hours and dollars can 
 be saved by knowing conditions before 
 they become a problem. Situations that 
 could be disastrous can be predicted and 
 proper solutions implemented before the 
 disaster occurs. Proper environmental 
 and machine monitoring is another key 
 to safer, more productive underground 
 mining. 
 
BIBLIOGRAPHY 
 
 1. Betsh, K. W. , G. Trace, and P. B. 
 Day. A Permissive Dial/Page Telephone 
 for Coal Mine Communications. Proc. 2d 
 WVU Conf. on Coal Mine Electrotechnology , 
 Morgantown, W. Va. , June 12-14, 1974, 
 pp. 8-1—8-13. 
 
 2. Collins Radio Co. (Cedar Rapids, 
 Iowa). Research and Development Contract 
 for Coal Mine Communication System, Vol- 
 ume I, Summary and Results of System 
 Study. BuMines OFR 69(l)-75, Novem- 
 ber 1974, 46 pp.; available from NTIS 
 PB 244 169. 
 
 3. Drake, J. L. and J. M. Sticklen. 
 An Industrial Communications System, 
 
 Paper in Underground Mine Communications. 
 1. Mine Telephone Systems. BuMines 
 IC 8742, 1977, pp. 27-41. 
 
 4. Lagace, R. L. , W. G. Bender, J. D. 
 Foulkes, and P. F. O'Brien. Techni- 
 cal Services for Mine Communications 
 Research. Applicability of Available 
 Multiplex Carrier Equipment for Mine Tel- 
 ephone Systems. BuMines OFR 20(1 )-76, 
 July 1975, 95 pp.; NTIS PB 249 829. 
 
 5. Parkinson, H. E. Mine Communica- 
 tions. Proc. 2d WVU Conf. on Coal Mine 
 Electrotechnology, Morgantown, W. Va. 
 June 12-14, 1974, pp. 5-1—5-5. 
 
10 
 
 CHAPTER 2. — COMMUNICATION SYSTEMS 
 
 2. 1 Introduction 
 
 Any coniniinication system requires at 
 least three elements in order to func- 
 tion: a transmitting device, a receiving 
 device, and a transmission line or propa- 
 gation medium. Even the device children 
 use, tin cans connected with string, con- 
 sists or these three elements. One 
 speaks into one can (transmitter), which 
 vibrates at the same frequencies as the 
 voice. The string (transmission path) 
 picks up the vibrations of the can and 
 carries them along its entire length. 
 The other can (receiver) detects the 
 vibration and reproduces the original 
 sounds to a lesser extent depending on 
 distance, tightness of string, type of 
 string, etc. All communication systems 
 depend on these three elements: trans- 
 mitter, transmission path, and receiver. 
 
 Communication systems can be divided 
 into three fundamental categories: wired 
 phone systems, radio systems, and carrier 
 current systems. Sections 2.2, 2.3, and 
 2.4, respectively, describe these systems 
 and explain the basic principles of how 
 each works. 
 
 Hybrid systems are those systems 
 that use various combinations of 
 the three basic comminication meth- 
 ods. Hybrid systems are described in 
 section 2.5. 
 
 There are some other methods of sig- 
 naling (stench warning, bell signaling, 
 etc. ) that can be used in underground 
 mines to transmit or convey information. 
 These systems, although they cannot be 
 considered true communication systems 
 since they do not provide voice or even 
 two-way communication, are briefly 
 described in section 2.6. 
 
 2. 2 Wired Phone Systems 
 
 Wired phone systems are all those 
 that depend on a wire connection between 
 phones with the wire carrying the voice 
 signals. Figure 2-1 is a diagram of two 
 typical wired phone systems. The top 
 
 panel shows a simple single pair party 
 line system. In this system each phone 
 is connected to a common pair of wires, 
 and a person speaking into one phone will 
 be heard at all the other phones on the 
 line. The bottom panel shows a multipair 
 private line phone system. In this type 
 of system, each phone is connected by its 
 own individual pair of wires to a central 
 switch or telephone exchange. To estab- 
 lish a call between two phones in this 
 system, the lines between the two phones 
 must be connected (switched together) 
 within the telephone exchange. 
 
 In early exchanges, the connections 
 were made manually by an operator. These 
 exchanges are called Private Branch 
 Exchanges (PBX's). Today, equipment 
 within the exchange can automatically 
 connect each phone to any other phone in 
 the system. These exchanges are called 
 Private Automatic Branch Exchanges 
 (PABX's). There are many different types 
 of automatic exchanges. Some utilize 
 switches to physically make each connec- 
 tion according to the number dialed. 
 Other, more advanced exchanges are com- 
 pletely solid state and may even be com- 
 puter controlled. 
 
 DISTRIBUTION 
 SYSTEM 
 
 al. SINGLE PAIR (PARTY LINE) SYSTEM 
 
 DISTRIBUTION 
 SYSTEM 
 
 bl. MULTIPAIR (PRIVATE LINEI SYSTEM 
 
 FIGURE 2-1. - Wired phone systems. 
 
11 
 
 Telephone exchanges are described in 
 section 2.2.3. The various types of 
 phones in use today and a description of 
 how they operate is given in sec- 
 tion 2.2.1. Distribution systems are 
 described in section 2.2.2. 
 
 2.2.1 General Telephone Theory 
 
 There are many different types of 
 phones in use today, including 
 
 Magneto type 
 
 Sound-powered 
 
 Paging 
 
 Dial 
 
 Dial -and-p age 
 
 These phones and the basic princi- 
 ples of how they operate are described 
 in the following paragraphs. 
 
 2.2.1a Magneto-Type Telephones 
 
 One of the earliest types of under- 
 ground communication instruments was the 
 magneto phone, also known as the crank 
 ringer phone. These phones consisted of 
 a transmitter, receiver, hookswitch, 
 ringer, battery, and hand generator (mag- 
 neto). As the spindle handle was turned, 
 80 to 100 volts at 15 to 18 cycles per 
 second was produced by the magneto. This 
 current caused the other phones to ring. 
 Once the called phone was answered, 
 talking power was supplied by battery 
 voltage. 
 
 Magneto phones were connected in 
 party line fashion with a code of short 
 and long rings to identify the called 
 station. Some mines still use this type 
 of system. However, as mines expanded in 
 size, the system proved to lack adequate 
 signal strength to power a large number 
 of phones. 
 
 2.2.1b Sound-Powered Telephones 
 
 A sound-powered set is one that pro- 
 vides a means of voice communications 
 with the use of no energy except that 
 
 furnished by the speaker's voice. These 
 phones have highly efficient transmitters 
 and receivers for converting voice into 
 electrical signals. 
 
 The sound-powered handset is compar- 
 able in size and appearance to the famil- 
 iar battery-powered handset. Two of 
 these phones were usually connected 
 by a single line to constitute an 
 intercom circuit. Such a circuit will 
 reproduce speech with reasonable good 
 quality for short distances in quiet 
 surroundings. 
 
 For special communication applica- 
 tions requiring exceptionally rugged and 
 durable sets for private comminication 
 purposes, and particularly when the line 
 loops are short, sound-powered sets are 
 well adapted. 
 
 their prin- 
 as indepen- 
 simplicity 
 ng without 
 ability and 
 d telephone 
 ndings are 
 
 Sound-powered sets find 
 cipal application in mines, 
 dent intercom systems. The 
 and convenience of operati 
 batteries and the service reli 
 ruggedness of the sound-powere 
 when used in adverse surrou 
 points in their favor. 
 
 2.2.1c Paging Telephones 
 
 The most common type of communica- 
 tion system used in underground mines is 
 a paging telephone system. These phones 
 are sometimes referred to as squawk 
 phones or squawk boxes because of the 
 harsh sound of the speaker. Each phone 
 in the system is usually connected to a 
 twisted pair cable in party line fashion. 
 Each pager phone has internal batteries 
 that power audio amplifiers to boost sig- 
 nal level for normal communication. A 
 paging amplifier allows each phone to 
 broadcast a page call on a loudspeaker 
 that is also housed within each phone 
 (fig. 2-2). 
 
 The paging telephone has gained 
 widespread use for two reasons. First, 
 it permits persons and station areas to 
 be paged by name and thus does not 
 require the miners to learn ringing codes 
 or telephone numbers. Second, the use of 
 page amplifiers in each phone makes the 
 
12 
 
 T mSIEU PAm PHOHl Lwe 
 
 I f t t I 
 
 m 
 
 TMMswrrcR- 
 
 I l*^SS TO 
 r TiLK 
 SWITCH 
 
 UTTtirr 
 
 "1 
 
 t 
 
 FIGURE 2-2. • Generalized two-wire pager phone. 
 
 system less affected by poor line splices 
 and induced noise. The loudspeaker also 
 yields a higher sound level at the 
 receiver, which is important in the 
 vicinity of noisy machinery. The primary 
 disadvantage of paging telephone systems 
 is that the telephone line must be used 
 in a party line arrangement. This pre- 
 vents simultaneous conversations in the 
 system and reduces its usefulness for 
 discussing maintenance problems or other 
 uses which can tie up the system for long 
 periods of time. 
 
 In a large mine there may be 30 to 
 40 phones on a single twisted pair cable. 
 However, as the mine develops and the 
 miles of twisted wire pair increase, a 
 limit is reached. The limiting factor is 
 the power .available to signal the paging 
 amplifier in other phones to turn on. 
 The application of an electronic switch, 
 in place of the low-voltage "page relay," 
 has extended the operating range of newer 
 paging phones. 
 
 A schematic of a generalized two- 
 wire pager phone is shown in figure 2-2. 
 When the ("Page") switch on the page 
 phone is pressed, a dc voltage from the 
 battery is placed on the line. All tele- 
 phones connected to the line are ener- 
 gized through their "page relay," and the 
 
 paging amplifier at each station is 
 turned on. The person making the call 
 then squeezes the press-to-talk button on 
 his handset and makes an announcement. 
 The handset signal is amplified and then 
 transmitted over the two-wire network to 
 all other phones. After completing the 
 page, the caller releases the "Page" 
 switch. The individual paged can respond 
 by squeezing the press-to-talk button and 
 talking into the handset. Because the 
 two-wire line is common to all phones, 
 any conversation on the party line may be 
 heard at all stations. 
 
 Most of the pager phones that are 
 available are directly interchangeable 
 within a system or can easily be modified 
 to be interchangeable. Two major areas 
 of difference among available equipment 
 are the battery voltage used for the 
 page-call function, and the character- 
 istics of the internal relay that 
 responds to the dc page signal. The two 
 choices of battery voltage are presently 
 12 volts and 24 volts. The 24-volt sys- 
 tem was the standard for several years. 
 Most installations also used electro- 
 mechanical relays to "switch in" the pag- 
 ing amplifiers. 
 
 In recent years, new designs using 
 solid state switching circuits that oper- 
 ate at 12 volts have become popular. 
 Besides being more reliable, solid state 
 systems have a high impedance and thus 
 present a minimum load to the paging 
 circuit. In a large complex multiphone 
 system, with several miles of intercon- 
 necting cable, the minimum loading caused 
 by these phones means that more phones 
 can be installed. 
 
 Options included by the manufac- 
 turers of some paging phones include a 
 battery-test button, an improve-hearing 
 button, and a flashing light to help 
 attract attention in noisy areas. The 
 battery-test button, when pressed, lights 
 a lamp on the front panel to indicate 
 that the battery is in good condition. 
 When the improve-hearing button is 
 pressed, the gain of the receiver ampli- 
 fier in the handset is increased. To 
 
13 
 
 assist the loudspeaker in attracting a 
 miner's attention, the flashing light is 
 turned on when the page is initiated. 
 
 2. 2. Id Dial-and-Page Telephones 
 
 The use of normal surface-type tele- 
 phones in underground mines has two 
 disadvantages: The potential hazard, in 
 a methane atmosphere, from the 120-volt 
 20-hertz bell-ringing voltage; and the 
 inability to locate a person who is not 
 in his inmiediate work area. Surface-type 
 telephones also have two advantages, how- 
 ever: The selective call feature and the 
 multiple private lines. 
 
 A unique system combines the dial 
 telephone with the page phone. A surface 
 interface is provided to isolate the 
 potentially hazardous voltages from the 
 underground line, and a converter changes 
 ring voltage into the low-voltage direct 
 current required to turn on the paging 
 speaker in the dial-selected pager phone. 
 The handset switch elin?inates the need 
 for a hook switch. Pressing the handset 
 switch accomplishes all functions nor- 
 mally accomplished by lifting the handset 
 of a conventional phone from the cradle. 
 When a call is dialed, the interface mod- 
 ifies signaling to interface another 
 underground phone or a conventional tele- 
 phone on the surface. 
 
 Figure 2-3 is a diagram of a dial- 
 and-page phone. The telephone consists 
 of a handset, containing a transmitter in 
 the mouthpiece and a receiver in the ear- 
 piece, and a main housing. The main 
 housing contains a speech dialer network, 
 which isolates the outgoing and incoming 
 signals, a pulsing switch that is actu- 
 ated by the rotary dial to signal a num- 
 ber to the surface interface, and the 
 paging speaker. The speech network fil- 
 ters noise and processes the talker's 
 voice. A common-page button permits 
 paging all phones, as is required when 
 searching for a roving miner or for mak- 
 ing a general announcement. 
 
 Automatic dial systems are used at 
 several mines that operate their own Pri- 
 vate Automatic Branch Exchange (PABX) for 
 
 OUTPUT 
 TERMINALS 
 TO SURFACE 
 INTERFACE 
 
 FIGURE 2-3. - Mine dial phone. 
 
 both inside and outside telephone ser- 
 vice. Dial systems provide for many 
 simultaneous conversations, but do need 
 to use some form of signal multiplexing 
 or multiconductor cables. These problems 
 can be minimized by using multiconductor 
 cables only in the main haulageway where 
 few roof falls or line breaks are likely 
 to occur and by taking conventional two- 
 wire cable up to each section. However, 
 this means all telephones in one area 
 must be on a party line. 
 
 2.2.2 Distribution Systems 
 
 The distribution system for wired 
 phones is the equivalent of the string 
 between the two tin cans described in the 
 introduction. This is the propagation 
 medium that carries the voice signal. 
 For wired phones, the distribution system 
 may be single pair, multipair, or a mul- 
 tiplex system. 
 
 All wired systems used in mines are 
 inherently unreliable. That is, if a 
 telephone line is broken or shorted by a 
 roof fall, for example, all telephones 
 beyond that point are severed from com- 
 munication to the outside. If the line 
 
14 
 
 is shorted, communications in the entire 
 system may be severely affected or lost 
 con5)letely . 
 
 2.2.2a Single Pair 
 
 A single pair of wires is the mini- 
 mum requirement for wired comminications. 
 A single twisted pair is used for magneto 
 phones, paging phones, and intercom 
 phones, connected in party line fash- 
 ion. Single-pair wires are generally 
 14 AWG (0.06408-inch diameter). This 
 relatively heavy gage wire is used in 
 order to minimize signal loss over long 
 distance. 
 
 The signal loss per mile, or attenu- 
 ation as it is called, is dependent upon 
 the frequency of the signal and the size 
 of wire gage as shown in table 2-1. This 
 table shows that as wire size increases 
 and gage decreases, the signal loss 
 (attenuation) decreases. 
 
 Each attenuation of 3 dB means that 
 the signal power has decreased by one- 
 half. For example, if the output of a 
 communication device was 4 mW, the -3 dB 
 attenuated value of this signal is one- 
 half the output, or 2 mW. A 6-dB attenu- 
 ation of a 4-mW signal results in a 1-mW 
 (4 X 1/2 X 1/2) signal level. 
 
 FIGURE 2"4. " Figure-8 multipair cable. 
 2.2.2b Figure-8 Multipair Cable 
 
 Figure-8 aerial distribution wire is 
 shown in figure 2-4. In cross section, 
 the wire looks like a figure-8 with a 
 steel support wire, called the messenger, 
 in the top half and a core of twisted 
 pairs in the lower half. The outer 
 jacket covering the messenger cable and 
 twisted pair bundle is highly resistant 
 to abrasion, moisture, and environmental 
 stress cracking. Each wire is solid, 
 commercially pure, annealed copper. One 
 conductor of each pair is usually color 
 coded by a solid body color with a con- 
 trasting spiraling color strip. The 
 other conductor has the complementary 
 color combination. Figure-8 cable is 
 recommended for mine applications be- 
 cause the messenger cable adds consider- 
 able strength to the cable and the in- 
 stallation is similar to that of trolley 
 wire. 
 
 TABLE 2-1. - Cable attenuations 
 
 
 Wire size 
 
 Frequency 
 
 13 
 
 gage 
 
 16 gage 
 
 19 gage 
 
 (kHz) 
 
 Decibels 
 
 Percent of 
 
 Decibels 
 
 Percent of 
 
 Decibels 
 
 Percent of 
 
 
 per mile 
 
 signal power 
 remaining 
 
 per mile 
 
 signal power 
 remaining 
 
 per mile 
 
 signal power 
 remaining 
 
 10 
 
 0.80 
 
 83 
 
 1.32 
 
 74 
 
 2.43 
 
 57 
 
 20 
 
 1.04 
 
 79 
 
 1.55 
 
 70 
 
 2.77 
 
 53 
 
 30 
 
 1.27 
 
 75 
 
 1.78 
 
 66 
 
 3.02 
 
 50 
 
 50 
 
 1.75 
 
 67 
 
 2.24 
 
 60 
 
 3.53 
 
 44 
 
 100 
 
 2.72 
 
 54 
 
 3.31 
 
 47 
 
 4.80 
 
 33 
 
 150 
 
 3.60 
 
 44 
 
 4.27 
 
 37 
 
 6.00 
 
 25 
 
15 
 
 2.2.2c Coaxial Cable 
 
 Coaxial cable consists of an inner 
 conductor and an outer conductor, as 
 shown in figure 2-5. This type of cable 
 has two main advantages over twisted pair 
 for transmission. First, the coax usu- 
 ally has lower attenuation. Second, the 
 shield over the central conductor keeps 
 the electrostatic and electromagnetic 
 fields contained within the coax, thereby 
 minimizing crosstalk and interference 
 problems. 
 
 2. 2. 2d Multiplex Systems 
 
 The term "multiplex" is applied to 
 any system in which a single wire or wire 
 pair is used for the transmission of more 
 than one simultaneous signal. In this 
 type of system, a means must be provided 
 for inserting the individual signals onto 
 the conmion transmission line and then 
 separating these individual signals at 
 the output of the line. There are two 
 principal methods of multiplexing sig- 
 nals. One is based on frequency transla- 
 tions and is called frequency-division 
 multiplexing, or FDM. The other is based 
 on time-sharing the transmission line and 
 is called time-division multiplexing, or 
 TDM. 
 
 2.2.2d.i Frequency Division Multiplexing 
 
 Frequency division multiplexing is a 
 process by which two or more signals are 
 sent over the same line by transmitting 
 each signal at a different frequency. 
 The FDM concept can be illustrated by 
 considering how the commercial radio 
 broadcasting systems operate. Each radio 
 station transmits at a specific frequency 
 that has been assigned by the Federal 
 Communications Commission (FCC). The 
 signal from each radio station is trans- 
 mitted by a common path (the atmosphere) , 
 with many stations being on the air at 
 the same time. To receive a particular 
 station, a person merely tunes his radio 
 (receiver) to the frequency of the 
 desired radio station. 
 
 This same principle, transmitting 
 more than one voice signal, each at a 
 different frequency, over a common path 
 
 (in this case a single pair of wires) , 
 can be employed in underground communica- 
 tion systems. 
 
 Figure 2-6 illustrates the FDM con- 
 cept. At the transmitting terminal, each 
 of the voice channels (CH 1 through CH n) 
 is applied to a modulator. Each modu- 
 lator shifts that voice signal to an 
 assigned frequency (f^ through fn) and 
 transmits the resulting signal over the 
 common line to the receiving terminal. 
 
 At the receiving terminal, a bank of 
 filters separates the signals according 
 to frequency. Individual demodulators 
 recover the original voice signal. Note 
 that the multiplexing system shown in 
 figure 2-6 operates in only one direc- 
 tion. Most communication systems require 
 two-way voice transmission. This is 
 accomplished by a complete duplication of 
 
 INSULATION 
 
 OUTER JACKET 
 
 CENTER CONDUCTOR 
 
 OUTER CONDUCTOR 
 
 FIGURE 2-5. - Coaxial cable, cross-sectional view. 
 
 TRANSMITTING TERMINAL 
 
 RECEIVING TERMINAL 
 
 — » FILTER — ij 
 
 COMMON LINE 
 FDM SIGNAL 
 If, *U*U* 
 
 FILTER 
 
 J^ 
 
 DEM 
 
 
 
 
 FILTER 
 
 i 
 
 DEM 
 
 
 FIGURE 2-6.- Frequency division multiplex (FDM)t 
 
 SINGLE WIRE PAIR OR COAX 
 
 y jL A 
 
 MODEM *■] 
 
 XX 
 
 e 
 
 xr 
 
 © 
 
 FIGURE 2-7, - Multiplex phone system. 
 
16 
 
 multiplexing facilities, with the compon- 
 ents in reverse order and with the signal 
 waves traveling from right to left. Each 
 terminal has a transmitting modulator and 
 a receiving filter and demodulator com- 
 bined to form a "modem." 
 
 A block diagram of the multiplexing 
 principle is shown in figure 2-7. Each 
 phone is connected to a subscriber termi- 
 nal unit or modem. The transmission line 
 is a twisted pair or coaxial cable whose 
 gage depends on the system size. Repeat- 
 ers can be inserted in the line to com- 
 pensate for attenuation. 
 
 2.2.2d.ii Time Division Multiplexing 
 
 Time division multiplexing is a 
 process by which two or more signals are 
 transmitted over the same line by allo- 
 cating a different time interval for the 
 transmission of each signal. The time 
 available is divided up into small slots, 
 and each of these is occupied by a piece 
 of one of the signals to be sent. The 
 multiplexing equipment scans the input 
 signals in a sequential round-robin 
 fashion so that only one signal occupies 
 the TDM line at any one time. 
 
 The basic concept, showing two sig- 
 nals (A and B) being time division 
 multiplexed together, is illustrated in 
 figure 2-8. During time slot 1, signal B 
 
 12 3 4 5 6 7 
 
 FIGURE 2-8. - Time multiplexing two signals. 
 
 is connected to the TDM line. During 
 time slot 2, signal B is removed and sig- 
 nal A is placed on the TEM line. This 
 process is repeated with each signal 
 alternately occupying a time slot on the 
 TDM line. The bottom waveform in 
 figure 2-8 shows the resulting time divi- 
 sion multiplexed signal. The TDM signal 
 consists of signal A during the even- 
 numbered time slots and signal B during 
 the odd-numbered time slots. To separate 
 the signals, when they are received at 
 the other end of the TDM line, a demulti- 
 plexer must be used. The demultiplexer 
 is similar to a multiplexer except that 
 the input and output are reversed. The 
 demultiplexer reconstructs the original 
 signals (A and B) from the multiplexed 
 signal. 
 
 In order to illustrate the TDM con- 
 cept, the preceding discussion considered 
 multiplexing only two signals together. 
 This concept can be extended to multiplex 
 many signals together onto a single TDM 
 line. For instance, if eight different 
 signals are to be multiplexed, then each 
 signal would be placed onto the TDM line 
 each eighth time slot. Figures 2-9 and 
 2-10 show eight switches at one location 
 and eight lamps at another which must be 
 controlled by those switches. 
 
 The system shown in figure 2-9 is 
 simple and easy to understand; however, 
 eight separate wires must be strung 
 between the switches and the lamps. This 
 can be quite costly and impractical when 
 the switches and lamps are separated by 
 large distances. Figure 2-10 shows how 
 each lamp can still be controlled by its 
 associated switch using a single wire 
 (TDM line) between the switches and 
 lamps. With the wiper on each of the 
 multipole switches synchronized, this 
 system would sample the status of the 
 first input switch (SI) at the trans- 
 mitting end and communicate that informa- 
 tion to the receiving end. At the next 
 interval of time, both multipole switches 
 would step to position 2. Control of the 
 second lamp would be accomplished by sam- 
 pling the status of the S2 input switch 
 at the transmitting end. At the next 
 time interval, both scanner switches 
 would step to position 3 and a similar 
 
17 
 
 INDICATOR 
 LAMPS 
 
 i * o— o— 
 
 SI 
 S2 
 
 S3 
 
 'Si 
 
 S6 
 
 POWER I , 
 
 SUPPLY ^ 
 
 FIGURE 2=9,- Nomultiplexing (eight wiresrequired). 
 
 INDICATOR 
 LAMPS 
 
 FIGURE 2-10, - Time division multiplexing (one wire), 
 
 control action would result for the third 
 lamp. After all eight positions had been 
 scanned, the scanner switch would return 
 to position 1 and start the sequence over 
 again. If the scanning were fast enough 
 the lights would appear to glow steady 
 and not blink. In a like manner, if this 
 were a voice system, it would not sound 
 chopped. 
 
 2.2.3 Telephone Exchanges 
 
 The function of any telephone 
 exchange is to connect a calling phone to 
 a called phone. The earliest method of 
 connection was the manual switchboard 
 
 located within a private branch exchange 
 (PBX). As the number of phones in use 
 increased, the size and complexity of 
 manual switchboards also increased. This 
 led to the development of an automatic 
 switching system called a private auto- 
 matic branch exchange (PABX). Since tel- 
 ephone lines are currently used as data 
 links for computers, teletypwriters , and 
 various other data equipment, faster, 
 more reliable exchanges were needed. To 
 fill this need, the computer is presently 
 used to control large solid state 
 exchanges and perform many other central 
 office (CO) functions. 
 
 _L 2.2.3a Manual Switchboard 
 
 The earliest switchboards allowed 
 the operator to manually patch two cir- 
 cuits together. When the central office 
 received ringing current, the operator 
 inserted the answering plug into the jack 
 of the caller. After verbally receiving 
 the called number, the operator inserted 
 the calling plug of the same circuit into 
 the called party's jack and applied ring- 
 ing voltage on the ringer of the called 
 party. After the conversation has been 
 concluded, both parties would ring off, 
 informing the operator that the plugs 
 could be disconnected. 
 
 2.2.3b Private Automatic Branch Exchange 
 
 The private automatic branch 
 exchange performs the same end function 
 as the operator at the old manual switch- 
 board. It makes a connection between the 
 phone line of a caller to the line of the 
 phone being called. Connections through 
 PABX's can be made using electro- 
 mechanical devices (rotary switches, 
 relays, etc.) or solid state electronic 
 circuits. 
 
 In rotary dial phone system, the 
 phone loop, which includes the two speech 
 wires and the telephone set, is momentar- 
 ily interrupted by the dial switch as the 
 dial runs down. The number of loop 
 interruption electrical pulses thus gen- 
 erated corresponds to the number dialed. 
 In pushbutton-type phone systems, the 
 numerical information (each digit dialed) 
 
18 
 
 is transmitted to the switching equipment 
 in the form of coded frequency or voltage 
 signals. In either case, the PABX 
 switching equipment must receive and 
 decode the "number dialed" signals from 
 the calling phone to determine what phone 
 is being called. 
 
 2.2.3c Computer-Controlled Switches 
 
 Modern solid state exchanges under 
 computer control can efficiently handle 
 thousands of phone lines. In addition to 
 being faster and more reliable, computer- 
 controlled equipment occupies only a 
 fraction of the space required by systems 
 using mechanical relays or rotary 
 switches. 
 
 Alterations to these systems no 
 longer require a lot of time and hardware 
 since the switching is under control of a 
 computer program. Changes such as adding 
 phones, deleting phones, changing a phone 
 number, etc. , can be made by simply 
 changing the program. The computer also 
 can keep track of billing functions and 
 special events. Special features, such 
 as conference calls, automatic call for- 
 warding, and abbreviated dialing, can be 
 incorporated into computer-controlled 
 exchanges by making changes to the com- 
 puter program. 
 
 System maintenance can also be han- 
 dled by computer program. Instead of 
 many labor hours spent in attempting to 
 locate and correct a fault, the computer 
 can cycle through all parts of the system 
 and locate a trouble spot within a matter 
 of minutes. Because computer operations 
 are very fast, they can be performing 
 maintenance functions even while handling 
 the switching function for thousands of 
 calls. 
 
 2.3 Radio Systems 
 
 Radio systems do not depend on a 
 wire connection between transmitter and 
 receiver. There are many types of sys- 
 tems in this category: one-way voice, 
 one-way signal, and two-way voice. In 
 one-way operations, the transmitter sends 
 a code or voice signal to the receiver. 
 
 Two-way voice utilizes a device called a 
 transceiver (combined form of transmitter 
 and receiver). 
 
 2.3.1 General Radio System Theory 
 
 Voice frequency (VF) signals could 
 be transmitted directly from one antenna 
 to another; however, because of the low 
 frequencies (and therefore long wave- 
 lengths) involved, the antennas required 
 would be very large. For this reason, VF 
 signals are combined with higher fre- 
 quency RF (radio frequency) carrier sig- 
 nals which can be effectively transmitted 
 and received by antennas of reasonable 
 size. The two primary methods of com- 
 bining VF signals and RF carriers are 
 amplitude modulation (AM) and frequency 
 modulation (FM). 
 
 2.3.1a Amplitude Modulation 
 
 In amplitude modulation the height, 
 or amplitude, of the RF carrier is made 
 to vary with the VF signal. This prin- 
 ciple is illustrated in figure 2-11. The 
 top waveform shown in figure 2-11 is a 
 typical VF signal. The middle waveform 
 of figure 2-11 represents an RF carrier 
 that will easily propagate between an- 
 tennas of convenient size. The bottom 
 waveform shows the result of "amplitude 
 modulating" the RF carrier with the VF 
 signal. This signal retains the basic 
 shape of the original VF signal but will 
 also easily propagate between antennas 
 because it is being transmitted at the RF 
 carrier frequency. The original voice 
 signal is regenerated by demodulation 
 circuits in the receiver. 
 
 2.3.1b Frequency Modulation 
 
 A voice signal may also be super- 
 imposed on a carrier frequency through 
 the use of frequency modulation tech- 
 niques. In FM, the frequency of the RF 
 carrier is made to vary at the VF signal 
 rate. As the amplitude of the VF signal 
 changes, the frequency of the RF carrier 
 (instead of the amplitude) changes. 
 
 Figure 2-12 shows the principles of 
 frequency modulation. The voice signal 
 
19 
 
 A A A A ^'' 
 
 I I I I I CARRIER 
 
 RF CARRIER 
 AFTER 
 AMPLITUDE 
 MODULATION 
 
 FIGURE 2=11. . Amplitude modulation (AM). 
 
 Is shown by the top waveform and the RF 
 carrier by the middle waveform. The bot- 
 tom waveform shows the resultant 
 frequency -modulated RF carrier. As the 
 voice signal increases to its maximum 
 value, the carrier frequency increases 
 (the waves bunch up). As the voice sig- 
 nal decreases to its minimum value, the 
 carrier frequency decreases (the wave 
 spreads out). The amount of carrier fre- 
 quency change is referred to as frequency 
 deviation or carrier deviation. The 
 unmodulated carrier frequency is referred 
 to as the center frequency. The amount 
 of carrier deviation is proportional to 
 the amplitude of the voice signal, with 
 maximum carrier deviation occurring at 
 the peaks of the voice signal. 
 
 VF 
 SIGNAL 
 
 RF 
 CARRIER 
 
 RF CARRIER 
 AFTER 
 FREQUENCY 
 MODULATION 
 
 A 
 
 [\ 
 
 u u u 
 
 (\ (\ 
 
 \J \J 
 
 (\ A 
 
 u 
 
 II A A A 
 
 _ 
 
 
 Hiu U U 
 
 J \j \j \. 
 
 FIGURE 2-12. = Frequency modulation (Ff> 
 
 FM receivers are less susceptible to 
 noise when receiving a signal of only 
 moderate strength or when the background 
 electromagnetic (EM) noise is almost as 
 "loud" as the signal. This advantage of 
 FM over AM can become an important con- 
 sideration in underground mining opera- 
 tions where electrical equipment is being 
 operated and the amount of EM noise being 
 generated is large. 
 
 2.3.2 Distribution Systems 
 
 For the most part, the distribution 
 system or propagating medium for radio 
 transmission is not hardwired but takes 
 the form of electromagnetic (radio) waves 
 in the air. (Radio waves at certain 
 frequencies will also propagate directly 
 through the earth. ) Electromagnetic 
 energy (radio waves) in empty space 
 travel at the speed of light. Because 
 the speed of any traveling wave is its 
 
20 
 
 wavelength times its frequency, we have a 
 formula of propagation (fA = speed of 
 light) where X is the wavelength and f is 
 the frequency. \ and f are thus 
 inversely proportional (as f increases, X 
 decreases), as shown in figure 2-13. 
 Commonly used units are X in meters and f 
 in hertz (Hz) (cycles per second). If 
 the wave is traveling through anything 
 other than empty space, its speed is 
 reduced depending upon the electrical 
 properties of the medium through which it 
 is passing. Radio waves are slowed down 
 only slightly by the earth's atmosphere. 
 In solid insulating materials the speed 
 is generally much slower; for example, in 
 distilled water (which is a good insu- 
 lator) the waves travel only one-ninth as 
 fast as they do in free space. In good 
 conductors such as metals the speed is so 
 low that opposing fields induced in the 
 conductor by the wave almost cancel the 
 wave itself. This is the reason why thin 
 metal enclosures make good shields for 
 electrical fields at radio frequencies. 
 
 2.3.2a Antenna Theory 
 
 In normal electronic circuits the 
 physical size of a circuit is small com- 
 pared with the wavelength of the fre- 
 quencies being used. When this is the 
 case, most of the electromagnetic energy 
 stays in the circuit itself or is con- 
 verted into heat. However, when the 
 physical dimensions of wiring or compon- 
 ents approach the size of the wavelength 
 being used, some of the energy escapes by 
 radiation in the form of electromagnetic, 
 or radio, waves. Antennas can be con- 
 sidered as special circuits intentionally 
 designed so that a large part of the 
 energy input to the antenna will be 
 radiated as electromagnetic energy. 
 
 Usually an antenna is a straight 
 section of conductor, either a wire or 
 hollow metal tubing, which is suspended 
 in space. When a radio transmitter is 
 connected to the antenna, rapidly varying 
 electrical currents are set up in the 
 antenna. These currents cause electro- 
 magnetic waves to radiate from the 
 antenna and travel through the atmosphere 
 or other surrounding medium. When these 
 waves strike another antenna they induce 
 electrical currents in it similar to the 
 current flowing in the transmitting 
 antenna. These currents, although they 
 may be very small if the antennas are far 
 apart or if they are transmitting through 
 the earth, can be amplified by electronic 
 circuits (receivers) to reproduce the 
 original signal. The range of radio dis- 
 tribution systems can be extended by 
 leaky feeder cable (special coaxial cable 
 designed to allow radio waves to "leak" 
 from the cable to the surrounding atmos- 
 phere and/or radio repeater stations. 
 
 2.3.2a.i Half-Wave Dipole Antenna 
 
 The strength of the electromagnetic 
 field radiated from an antenna is propor- 
 tional to the amount of current flowing 
 in the antenna. It is, therefore, desir- 
 able to make the current as large as pos- 
 sible. This can be accomplished by 
 adjusting the length of the antenna so 
 that it resonates at the operating 
 frequency. 
 
 If a straight wire, or antenna ele- 
 ment, were to be suspended in space, the 
 lowest frequency at which it would 
 resonate has a wavelength of twice the 
 length of the wire. When used to trans- 
 mit or receive RF energy that has a wave- 
 length of twice the length of the wire. 
 
 I02 io3 10* lO' 10® 
 
 FREQUENCY IN HERTZ 
 lOB io9 lO'O 10" 10'2 lo" I 
 
 VOICE FRCO 
 
 (vr) 
 
 RADIO FREO - 
 IRFl 
 
 1 I I I 
 
 INFDA-RED ! I 
 
 iMEAT-WfcVESl 
 
 liV 
 
 I ULTRA I 
 l< VIOUT«J 
 RAYSI . 
 
 SUN^ rays' 
 
 *REACH~^ 
 EARTH I ' 
 
 3»I0* 
 
 3XI02 
 
 3XI0'2 
 
 WAVELENGTH IN METRES 
 
 FIGURE 2-13. ■= Electromagnetic energy spectrum„ 
 
21 
 
 FIGURE 2=14. - Half-wave antenna, voltage and 
 current distribution. 
 
 1/2 X 
 
 FIGURE 2-15. - Dipole antenna. 
 
 the wire is known as a half-wave antenna. 
 The current and voltage distributions 
 along such a wire are shown in figure 2- 
 14. Such an antenna, when connected to a 
 receiver as shown in figure 2-15, is 
 called a dipole. 
 
 2.3.2a.ii Quarter-Wave Antenna 
 
 An antenna may also be a quarter 
 wave in length. This is possible because 
 of its connection to ground, which elec- 
 trically acts as the other quarter- 
 wavelength. Refer to figure 2-16. The 
 ground plane reflects the quarter-wave 
 antenna so it has electrical character- 
 istics similar to those of a half -wave 
 antenna. 
 
 An antenna of this sort may be 
 any odd multiple of a quarter-wavelength: 
 1/4A, 3/4A, 5/4X, 7/4A, etc. These 
 antennas are commonly used for low- and 
 medium-frequency applications. If a 
 quarter-wave whip antenna is installed on 
 a vehicle, the vehicle becomes the ground 
 plane. A modified quarter-wave antenna 
 is commonly used for citizens' band (CB) 
 radios on vehicles. 
 
 FIGURE 2-16. - Quarter-Vi^ave antenna, 
 voltage and current distribution. 
 
 2.3.2a.iii Long-Wire Antenna 
 
 A long-wire antenna is one that is 
 long with respect to the wavelength of 
 the incoming and outgoing signals. The 
 length should be an integral number of 
 half -wavelengths (2A, 2-1/2A., 3A, 3-1/2A, 
 etc.) to radiate effectively. A 1/2X 
 (dipole antenna) is said to operate 
 on the fundamental frequency, A oper- 
 ates on the second harmonic, 1-1/2A 
 operates on the third harmonic, 2A oper- 
 ates on the fourth harmonic, and so 
 on. 
 
 2.3.2a.iv Loop Antenna 
 
 Loop antennas can be utilized for 
 through-the-earth radio transmissions or 
 as receiving antennas in direction- 
 finding systems. These antennas can be 
 composed of one or more turns of wire on 
 a round or square form, or the loop can 
 be established by simply laying the wire 
 in a loop on the ground or floor of a 
 mine tunnel. 
 
22 
 
 2.3.2b Leaky Feeder Systems 
 
 Figure 2-17 shows a cross-section 
 view of a standard coaxial cable and the 
 lateral variation of its associated 
 fields. In such cables, the bulk of the 
 radio frequency electromagnetic energy is 
 transported down the cable between the 
 center conductor and the shield. How- 
 ever, the shields of most coaxial cables 
 do not provide perfect containment of the 
 internal electromagnetic fields or isola- 
 tion from external fields. As shown in 
 figure 2-17, a small fraction of the 
 cable's internal field is leaked to the 
 external space. External fields also 
 leak into the cable in a similar manner. 
 
 The leaky feeder system is based on 
 the use of semiflexible cable with spe- 
 cially designed shielding that has a 
 greater coupling to the external space. 
 Therefore, this cable easily leaks radi- 
 ated signals and saturates the area 
 around the cable with these signals. One 
 type of leaky feeder cable is shown in 
 figure 2-18. The cable has a solid cop- 
 per shield in which holes have been ma- 
 chined to increase the amount of leakage 
 to and from external space. In large 
 mines, repeaters may also be used to 
 amplify and retransmit incoming and out- 
 going signals to roving miners carrying 
 portable radios. The spacing of these 
 repeaters along the cable is governed 
 primarily by the receiver sensitivity, 
 the longitudinal attenuation rate of the 
 cable, the coupling loss from the cable 
 to the portable units, and the trans- 
 mitter power. Since the portable unit's 
 transmitter power is generally lower than 
 that available for fixed repeater or base 
 stations, the portable units se't the cov- 
 erage limits for two-way communications. 
 
 OUTER COVER- 
 
 COAXIAL CABLE 
 
 CENTER 
 CONDUCTOR 
 
 ►^ H'* — SHIELD 
 
 INTERNAL FIELD STRENGTH 
 
 T 
 
 STRENGTH OF 
 SIGNAL FIELDS 
 
 EXTERNAL FIELD STRENGTH 
 
 DISTANCE FROM CABLE 
 
 FIGURE 2-17. - Coaxial cable, field strength. 
 2.3.2c. Waveguide Propagation 
 
 A waveguide is a hollow conductor, 
 through which electromagnetic waves 
 (radio waves) may propagate. Such a 
 waveguide may be made of copper (the 
 ideal), or other materials, such as coal 
 or shale (nonideal). Hence, a mine entry 
 is a waveguide. In order for a wave to 
 efficiently propagate in a rectangular 
 waveguide (mine entry or haulageway), the 
 wavelength must be equal to or less than 
 two times the greater dimension (a or b 
 of figure 2-19). Table 2-2 shows the 
 frequency spectrum designations with 
 their wavelength ranges. The dimension 
 of "a" that would be common in under- 
 ground communications is 3 meters. This 
 limits the lowest frequency range of 
 signals that will effectively propagate 
 within mine tunnels to the upper VHF and 
 the UHF range. A communication device 
 such as a CB radio has no application 
 since it operates at approximately 
 27 MHz, a frequency which is too low. 
 
 HOLES IN SHIELD 
 
 SHIELD-, 
 
 / ^CENTER CONDUCTOR 
 
 -INSULATION 
 
 FIGURE 2-18. - Cutaway view of leaky feeder cable. 
 
23 
 
 TABLE 2-2. - Frequency spectrum designation 
 
 Abreviation 
 
 Description 
 
 Frequency 
 
 Wavelength range 
 
 VF 
 
 Voice frequencies ........... 
 
 300-3,000 Hz 
 
 10&-10S m 
 
 VLF 
 
 Very low frequencies 
 
 Low frequencies ............. 
 
 3-30 kHz 
 
 105-10^ m 
 
 LF 
 
 30-300 kHz 
 
 10'+-103 m 
 
 MF 
 
 Medium frequencies •••••••••• 
 
 300-3,000 kHz 
 
 103-102 m 
 
 HF 
 
 High frequencies ............ 
 
 3,000-30,000 kHz 
 
 30-300 MHz 
 
 100 -10 m 
 
 VHF 
 
 Very high frequencies 
 
 Ultra high frequencies 
 
 Super high frequencies 
 
 Extremely high frequencies.. 
 
 10 - 1 m 
 
 UHF 
 
 300-3,000 MHz 
 
 1 - 0.1 m 
 
 SHF 
 
 EHF 
 
 3,000-30,000 MHz 
 
 30-300 GHz 
 
 10 - 1 cm 
 1 - 0.1 cm 
 
 Other factors that influence wave- 
 guide propagation are wall texture (the 
 smoother the wall, the better the propa- 
 gation) and tunnel straightness , and 
 electrical properties of the roof, walls, 
 and floor. 
 
 2. 3. 2d. Repeaters 
 
 Two general types of repeaters will 
 be considered for application in the mine 
 environment: Single-frequency (Fl-Fl) 
 
 HAULAGEWAY (NON-IDEAL) 
 
 COPPER WALLS (IDEALI 
 
 repeaters and frequency translation 
 (F1-F2) repeaters. In its simplest form 
 a repeater consists of two basic ele- 
 ments, a receiver unit and a transmitter 
 unit, as shown in figure 2-20. 
 
 2.3.2d.i Fl-Fl Repeater 
 
 Single-frequency repeaters are used 
 in most wired systems such as coaxial 
 systems where the transmission energy is 
 confined within the coax. These repeat- 
 ers function as signal amplifiers. The 
 attenuated input signal is detected, 
 amplified, and retransmitted. Since the 
 signals are confined to a separate coax, 
 isolation between transmitter and 
 receiver is maintained. 
 
 Single-frequency repeaters are also 
 used in some wireless repeater systems, 
 but extreme caution must be used 
 to prevent feedback between repeater 
 transmitter and receiver. Isolation must 
 be maintained between the transmitter 
 output and receiver input to prevent the 
 transmitted signal from being received 
 and amplified by the same unit. This 
 
 RECEIVE 
 ANTENNA 
 OR COAX 
 
 \ '/ 
 
 TRANS- 
 MITTER 
 
 TRANSMIT 
 ANTENNA 
 OR COAX 
 
 FIGURE 2-19. - Waveguides. 
 
 FIGURE 2-20. - Basic repeater block diagram. 
 
24 
 
 feedback can cause an oscillation or 
 squealing problem very similar to that 
 caused by placing a microphone in front 
 of its own speaker. Directional antennas 
 can be used to minimize this feedback 
 problem; however, the use of high -gain 
 directional antennas is not considered 
 practical in the mine environment owing 
 to installation problems and their sus- 
 ceptibility to mechanical damage. 
 
 Another method of overcoming these 
 problems is to use special repeaters that 
 do not depend on directional antennas. 
 In general terms , these units function 
 by separating the transmit and receive 
 signals with time division multiplex- 
 ing. The repeaters transmit pulses 
 of RF energy and receive between these 
 pulses. In this type of system all re- 
 peaters must be phase-locked with each 
 other to synchronize the time division 
 process. 
 
 2.3.2d.ii F1-F2 Repeater 
 
 F1-F2 repeaters receive signals from 
 a portable unit on one frequency (Fl) and 
 retransmit these signals on another fre- 
 quency (F2) to another portable unit. 
 The mobile radios transmit on Fl and 
 receive on F2. In this mode, all infor- 
 mation goes to the repeaters, then back 
 to the portable units. Some portable 
 units are also capable of transmitting on 
 F2 and, therefore, are able to talk to 
 one another without the repeaters on a 
 local simplex basis. With these systems, 
 the receive and transmit antennas at the 
 repeater are often covering the same 
 general frequency bands and they can be 
 combined so that only one antenna is 
 required. 
 
 Thus far, repeaters have been dis- 
 cussed only as a means to permit communi- 
 cation over greater distances than would 
 be possible using direct transmission 
 between portable radios. However, the 
 audio link between the transmitter and 
 receiver in the repeaters allows radio 
 access to and from other types of audio 
 circuits, such as specialized paging con- 
 soles or the telephone system. 
 
 One possible system configuration 
 which includes both a telephone link and 
 talk-through capability is shown in 
 figure 2-21. This configuration allows 
 for two modes of communications: F1-F2 
 would be used for a local mode, that is, 
 miner to miner within the working section 
 through the repeater; and the second mode 
 could support communications between a 
 miner located in a working section with 
 a second miner located somewhere else in 
 the mine. The audio link (fig. 2-21) 
 between the receiver and transmitter of a 
 repeater can be used to customize repeat- 
 ers to fit a variety of applications. An 
 audio wireline can also be used to link 
 a number of repeaters together to pro- 
 vide complete radio coverage of the mine 
 on a party line basis as shown in 
 figure 2-22. 
 
 2.3.3 Through-the-Earth Radio 
 
 VF (0.3- to 3-kHz) radio waves will 
 penatrate, to some extent, directly 
 through the earth. Although signal 
 strength is greatly attenuated, experi- 
 ments have shown that up to 1,000 feet 
 
 Fl 
 
 RECEIVE 
 
 F2 
 TRANSMIT 
 
 r' 
 
 REPEATER 
 
 AUDIO LINK 
 
 I I 
 
 TRANS- 
 MITTER 
 
 PHONE LINE 
 INTERFACE 
 
 TELEPHONE 
 WIRELINE 
 
 FIGURE 2-21i - Repeater linked to telephone system. 
 
25 
 
 v^i 
 
 REPEATER 
 
 V 
 
 V 
 
 FIGURE 2*22. - Minewide repeater system linked by 
 audio pair. 
 
 (305 meters) of 
 penetrated. 
 
 overburden may be 
 
 The transmitter may be a simple gen- 
 erator with a loop or grounded wire 
 antenna. The receiver may be a loop an- 
 tenna connected to a power amplifier with 
 a set of earphones or a meter. When the 
 transmitter is activated, it sets up a 
 magnetic field directly through the earth 
 (the overburden). 
 
 These characteristics of through- 
 the-earth radio can be utilized in emer- 
 gency situations to detect, locate, and 
 even communicate with miners trapped 
 underground. Once the position of an 
 underground transmitting antenna has been 
 determined using direction-finding tech- 
 niques, a loop antenna can be positioned 
 on the surface directly above the under- 
 ground position. The trapped miner has a 
 method of pulsing his transmitter off and 
 on, such that a coded message may be sent 
 to the surface. A high-power transmitter 
 attached to the surface loop may also be 
 utilized to establish down-link voice 
 communications with the underground 
 location. 
 
 2.3.4 Radio Pagers 
 
 Radio pagers are usually small FM 
 radio receivers. The simplest type of 
 radio pager is a one-way signal detector 
 or "beeper. " These devices emit an audi- 
 ble tone and/or blink a small light on 
 and off. The miner must then go to the 
 nearest phone to receive the message. 
 
 Another type of radio pager is a 
 one-way voice pager. These devices are 
 similar to the simple beeper-type pagers 
 except that the caller can deliver a 
 short verbal message to the person being 
 paged. A common type of one-way voice 
 pager sounds a tone to alert the miner 
 that a message will follow, and then 
 broadcasts the verbal message. A disad- 
 vantage of these pagers is that, although 
 it is possible to transmit instructions, 
 such as "turn off the power to number 4 
 left," it is not possible for the caller 
 to know for sure that the instruction was 
 even received, let alone carried out. 
 For this reason, one-way voice pagers 
 should not be used to instruct personnel 
 to perform specific tasks that may affect 
 safety. 
 
 Some one-way voice, and even beeper, 
 pager systems allow the caller to selec- 
 tively page a specific section or an 
 individual miner. The heart of these 
 systems is an encoder, which translates 
 the number of each pocket pager to a spe- 
 cific frequency or code that activates 
 only the designated pager. 
 
 2.4 Carrier Current Systems 
 
 Any underground wire or cable, when 
 fed an RF signal, tends to distribute 
 that signal throughout its length. Car- 
 rier current systems utilize this fact to 
 establish communication paths using 
 existing mine wiring. The wire used may 
 be ac or dc power lines, neutral lines 
 such as the hoist rope, existing phone 
 lines, or other wiring. 
 
 Carrier current devices are basic- 
 ally FM radio transceivers that transmit 
 
26 
 
 and receive over existing mine wiring 
 instead of using an antenna system. The 
 LF (low-frequency) and MF (medium- 
 frequency) RF ranges propagate best in 
 carrier current systems. A common exam- 
 ple of a carrier current system is the 
 trolley carrier phone systems presently 
 used in many mines using trolley or rail 
 haulage. Another example is the shaft 
 communication systems that utilize the 
 hoist rope itself to establish communica- 
 tions to and from the cage. The most 
 modern system, based on MF, promises to 
 be the most effective of all. 
 
 TROLLEYWIRE 
 
 MICROPHONE WITH 
 PUSH-TO-TALK 
 SWITCH 
 
 FIGURE 2-23. - Trolley carrier phone, block 
 diagram. 
 
 2.4.1 Trolley Carrier Phone 
 
 A simplified block diagram of a 
 typical trolley carrier phone is shown in 
 figure 2-23. As mentioned earlier, the 
 basic elements of any carrier cur- 
 rent phone are the FM receiver and 
 transmitter. 
 
 In a trolley carrier current phone 
 system, the receiver and transmitter are 
 connected to the trolley wire through a 
 coupler capacitor. The coupler capacitor 
 acts as a short circuit at the frequency 
 of the FM voice signals, but as an open 
 circuit to the trolley wire dc power 
 voltage. The high voltage levels on the 
 trolley wire are thus blocked from enter- 
 ing the receiver and transmitter sections 
 of the carrier phone, while the FM voice 
 signals pass freely through the coupler 
 capacitor. 
 
 The FM transceiver shown in 
 figure 2-23 contains a power supply that 
 converts trolley wire high voltage down 
 to low voltage levels to provide power to 
 the carrier phone circuits. The power 
 supply may also contain a battery for 
 backup power in case power on the trolley 
 wire is lost. Such a system operates 
 in the push-to-talk, release-to-listen 
 mode. 
 
 2.4.2 Hoist Rope Radio 
 
 Figure 2-24 shows a block diagram of 
 a hoist rope carrier current system. The 
 system consists of two signal couplers 
 and two transceivers. Each unit is 
 
 M, 
 
 ^ HOIST ROPE 
 
 TRANSCEIVER 
 
 COUPLER CABLI 
 
 CAGE COUPLER 
 
 FIGURE 2=24. '- Hoist radio hardware. 
 
 of the push-to-talk, release-to-listen 
 design. During transmission, the sending 
 unit feeds its coupler with a frequency- 
 modulated (FM) carrier. The coupler 
 induces a signal in the hoist rope, which 
 is then picked up by the coupler of the 
 second unit. Both couplers are elec- 
 trically identical, and each operates 
 both as a transmitting and as a receiving 
 element. Operation of the hoist radio is 
 the same as for a trolley carrier phone, 
 except that the hoist radio signal is 
 inductively coupled to the propagation 
 medium (hoist rope). Some hoist phones 
 
27 
 
 are simply modified trolley carrier 
 phones. Other hoist phone systems have 
 been specifically designed for operation 
 in a vertical shaft and usually provide 
 better coverage. 
 
 The transceivers of the hoist room 
 and cage are identical, except for the 
 battery required in the cage. The hoist 
 room power supply provides the power for 
 the surface equipment. Surface equipment 
 also may include a boom-type microphone 
 and a foot-actuated push-to-talk switch 
 to facilitate hands-free operation. 
 
 2.4.3 Medium Frequency (MF) Radio 
 
 Although radio transmission on 
 the surface of the earth is well under- 
 stood, transmission in an underground 
 environment generally is not. Complex 
 interactions occur between the radio wave 
 and the environment. Characteristics of 
 the geology (stratified layering, bound- 
 ary effects, conductivity, etc.) and the 
 mine complex (entry dimension, conduc- 
 tors, electromagnetic interferences, 
 etc. ) had to be measured and understood 
 before a practical mine radio system 
 could be built. To this end, consider- 
 able research has been directed. 
 
 In a confined area such as a mine, 
 radio waves can propagate useful dis- 
 tances only if the environment has the 
 necessary electrical and physical proper- 
 ties. The "environment" takes into 
 account the natural geology and manmade 
 perturbations such as the mine complex 
 itself. As an example, if the wavelength 
 (X) of a radio wave is small con^sared 
 with the entry dimensions, a waveguide 
 mode of propagation is possible. Attenu- 
 ation depends primarily upon the physical 
 properties of the entry such as cross- 
 sectional area, wall roughness, entry 
 tilts, and obstacles in the propagation 
 path. Secondary effects such as the 
 dielectric constants and earth conduc- 
 tivity also influence attenuation. 
 
 Mine radio systems based upon this 
 effect are available commercially. These 
 are UHF systems operating around 450 MHz 
 which provide useful but limited cover- 
 age. In high coal (6.5 feet), line- 
 of -sight ranges of 1,000 feet are often 
 possible. Range is reduced severely in 
 non-line-of -sight, such as when going 
 around a coal pillar. In lower coal, or 
 when obstacles exist in the propagation 
 path, range is reduced even more. For 
 this reason, conventional UHF radio sys- 
 tems require an extensive network of 
 leaky feeder transmission cables and 
 repeaters to become useful. Even so, 
 range from the cable is not usually in 
 excess of 30 to 50 feet, and equipment 
 cost is very high. Clearly another 
 approach is desirable. 
 
 An important contribution to under- 
 ground radio communications was made by 
 the Chamber of Mines of South Africa. As 
 early as 1948, programs were in place to 
 develop radio systems for deep mines, 
 primarily gold mines. The result was 
 that by 1973, an advanced 1-watt single 
 sideband (SSB) portable radio system had 
 been developed that apparently worked 
 well. The Bureau of Mines procured sev- 
 eral of these units for evaluation. Per- 
 formance in U.S. coal mines was not sat- 
 isfactory. There were several reasons 
 for this. First, U.S. mines are highly 
 electrified, producing considerable elec- 
 tromagnetic interference (EMI) not nor- 
 mally found in the South African mines, 
 which completely desensitized SSB radios. 
 Second, 1 watt was not enough power. 
 U.S. mines are mostly room and pillar, 
 which means that any radio system would 
 have to have reasonable range from local 
 conductors. Third, geological electrical 
 parameters were less favorable in the 
 United States. For these reasons, the 
 South African system was not acceptable. 
 
 The Bureau's approach was to first 
 determine the actual propagation charac- 
 teristics of MF in U.S. mines, and then 
 
28 
 
 to relate the propagation to the under- 
 ground environment such as the geology, 
 entry size, existing conductors, and EMI. 
 Several exhaustive in-mine measurement 
 and analysis programs were conducted. 
 These programs formed the foundation for 
 the first true understanding of how MF 
 propagates in a stratified medium of var- 
 ious electrical parameters, which are 
 often interlaced by manmade conducting 
 structures (rails and power lines) and 
 artificial voids (entryways). 
 
 Loop 
 
 transmitting 
 
 antenna 
 
 ^c 
 
 Coal 
 
 or 
 
 entry 
 
 Figure 2-25 is a simplified geometry 
 of an in-mine site that illustrates one 
 of the most important findings of the 
 measurement program, the "coal seam 
 mode." For this mode to exist, the coal 
 seam conductivity (Oc) must be several 
 orders of magnitude less than that of the 
 rock (Or). A loop antenna that is at 
 least partially vertically oriented pro- 
 duces a vertical electric field (E^) and 
 a horizontal magnetic field (H(t>). In the 
 rock, the fields diminish exponentially 
 in the Z-direction. In the coal seam, 
 the fields diminish exponentially at a 
 rate determined by the attenuation con- 
 stant (a), which in turn depends upon the 
 electrical properties of the coal. An 
 inverse square-root factor also exists 
 because of spreading. The effect is that 
 the wave propagates between the highly 
 conducting rock layers bounding the lower 
 conductivity coal seam. The fact that 
 the coal may have entries and crosscuts 
 is of minor consequence. 
 
 In the presence of conductors, the 
 picture changes considerably. In this 
 case, the effects of these conductors can 
 totally dominate the effects of the ge- 
 ology. In general, the presence of con- 
 ductors (rails, trolley lines, water 
 pipes, air lines, phone lines) is always 
 of advantage. 
 
 MF can couple into, and reradiate 
 from, continuous conductors in such a way 
 that these conductors become not only the 
 transmission lines, but also the antenna 
 system, for the signals. The most favor- 
 able frequency depends to some extent on 
 the relationship between the geology and 
 
 FIGURE 2-25. - The coal seam mode. 
 
 Local mine wiring 
 
 T/f, 
 R/f, 
 
 Global 
 repeater 
 
 T/f| or fz 
 R/f, 
 
 Base 
 station 
 
 T/f| or f2 
 
 Vest 1— ' 
 transceiver 
 
 T/f| or f 
 R/f, 
 
 Vehicula 
 
 
 Console 
 
 FIGURE 2-26. = Global repeater concept, 
 
 existing conductors. The frequency 
 effects are quite broad. Anything from 
 400 kHz to 800 kHz will usually be 
 adequate. 
 
 The MF system described here is 
 based upon vehicular and personnel 
 transceiver units, base stations, and 
 repeaters. It applies prior fundamental 
 research in the area of MF and utilizes 
 the existing mine wiring network (power 
 cables, trolley line, etc.) to achieve 
 whole-mine coverage. The basic system 
 configuration is shown in figures 2-26 
 and 2-27. 
 
 Figure 2-26 illustrates a minewide 
 repeater-base station concept known as 
 the global maintenance net. In this con- 
 figuration, mobile units (persons using 
 transceiver vests and/or vehicular trans- 
 ceivers) can maintain local communica- 
 tions by operating at frequency f i . The 
 range of communications in this case is 
 solely dependent on point-to-point radio 
 
29 
 
 propagation, aided by parasitic coupling. 
 A transmission on £2 causes repeater 
 action to occur, permitting the two mo- 
 bile units to be separated very large 
 distances. To achieve this repeater 
 action, it is only necessary for the 
 transmitting unit to reach the repeater, 
 either directly or by parasitic effects 
 to the repeater line coupler. Communica- 
 tions with a base station are also 
 possible. 
 
 Figure 2-27 illustrates a local 
 repeater concept constituting a local 
 cellular net. This local repeater is 
 known as a "cellular repeater" because it 
 illuminates a "cell" or area of the mine, 
 such as a working section, only. The 
 antenna for the cellular repeater is a 
 dual wire loop attached to timbers or 
 the rib. An interface to the mine tele- 
 phone system permits communications "off 
 section. " 
 
 The system design is distributed in 
 the sense that each net can be operated 
 independently of the other. In practice, 
 a net can be easily installed by coupling 
 a base station (at the portal) to elec- 
 trical conductors in the wire plant 
 (phone lines, power lines, etc.). Mobile 
 transceivers operating on the assigned 
 net frequency communicate with each other 
 and the base. Other nets use different 
 frequencies and are installed in the same 
 way. 
 
 Two types of mobile transceivers 
 have been developed for the system. 
 These transceivers consist of vest units 
 for individuals and vehicular units for 
 rolling stock. Functionally the two are 
 equivalent, differing only in power lev- 
 els and physical configuration. These 
 transceivers are shown in figures 2-28 
 and 2-29. 
 
 An important human factor problem 
 was solved by the vest design. By plac- 
 ing the radio circuit modules in pockets 
 on the vest, the weight and bulk of the 
 transceiver have been evenly distributed. 
 The loop antenna is sewn into the back of 
 the vest. The pockets are located where 
 medical records show less frequency of 
 injury. Sound is directed toward the 
 
 Pager phone line (existing) 
 
 T/f, 
 
 R/f2 
 
 Cellular 
 repeater 
 
 y. Transmitter 
 y Receiver 
 
 T/f| or fj 
 R/fi 
 
 T/f, or f. 
 
 Vest 
 transceiver 
 
 J 
 
 R/f, 
 
 Vehicu 
 transceiver 
 
 -J 
 
 lar —I 
 
 FIGURE 2-27. - Cellular repeater concept. 
 
 FIGURE 2=28.. - Vest transceiver. 
 
30 
 
 FIGURE 2-29, - Vehicular transceivero 
 
 ears from epaulet speakers. A hinged 
 control head is conveniently located on 
 the front. The design allows the miners 
 to maneuver in tight quarters and perform 
 normal mining tasks without catching the 
 radio on obstructions. 
 
 The vehicular unit can be conveni- 
 ently placed on any mine vehicle. It is 
 used in conjunction with a special loop 
 antenna of advanced design that produces 
 high magnetic moment. Mechanically, the 
 antenna is enclosed in high strength 
 lexan and is attached to the vehicle via 
 special brackets. The lexan will not 
 break even if severely flexed by impact. 
 
 Besides the mobile transceivers dis- 
 cussed above, the system also consists 
 of fixed transceivers such as repeaters 
 
 and base stations. (See figures 2-26 and 
 2-27.) For proper system operation it 
 is necessary that these fixed trans- 
 ceivers have very efficient antennas so 
 that the local wire plant can be prop- 
 erly illuminated and signals on the wire 
 plant are properly received. This 
 efficiency is paramount for whole-mine 
 coverage. 
 
 The cellular repeaters use dual-loop 
 antennas (for transmit and receiver) 
 attached to the rib or posts in such a 
 way that there is little danger of damage 
 in normal mine activities. The trans- 
 mit antenna produces a large magnetic 
 moment that provides the signal for local 
 cellular coverage, which is usually aided 
 by parasitic coupling and reradiation 
 effects. The receive antenna is similar. 
 
31 
 
 The global repeater and base station 
 use a newly designed RF line coupler (see 
 fig. 2-30) that permits very efficient 
 coupling to the mine wire plant. Like a 
 current probe, the coupler can be easily 
 clamped around local conductors. MF 
 signal current flowing through the 
 wire plant conductors produces a coupler 
 output signal (Vq), which is applied 
 to the input of the base station or 
 repeater. 
 
 The base station is intended to be 
 placed where mine management finds it 
 most advantageous, usually in the surface 
 office complex or with the dispatcher. 
 If desired, the base station can be 
 controlled remotely via signal lines that 
 allow the control console to be placed in 
 a surface building for convenience, while 
 the actual base transceiver is placed in 
 the mine where it can more efficiently 
 couple into the local wiring. Both the 
 global repeater and the base station uti- 
 lize the RF line coupler for maxi- 
 mum efficiency. The cellular repeater 
 is generally located in a working sec- 
 tion. It enables the vest to operate as 
 a mobile pager telephone by switchin; 
 voice signals between the local pager 
 telephone network and the vest. Vehic- 
 ular radios can also operate in this 
 mode. 
 
 The system was developed around an 
 interchangeable set of plug-in radio cir- 
 cuit modules. The same receiver, synthe- 
 sizer, and transmitter modules are used 
 in the vehicular transceiver, base sta- 
 tion, and repeaters. Servicing the 
 equipment only requires troubleshooting 
 to the board level. Since the equipment 
 uses the same radio circuit modules, the 
 performance specifications of all trans- 
 ceivers are similar. The signaling used 
 depends upon the specific network re- 
 quirements. All receivers are designed 
 with an adaptive noise-operated squelch 
 network that allows every transceiver on 
 the net to hear the same message (party 
 line) . 
 
 ACTUAL NETWORK 
 
 FIGURE 2-30. - RF line coupler for base sta- 
 tions and repeaters. 
 
 subaudible tone is used in the vest 
 transceiver to cause the cellular 
 repeater to switch the message (page) to 
 the pager telephone network. The re- 
 peater includes both a noise-operated 
 squelch and a subaudible tone squelch for 
 use in telephone switching. Subaudible 
 tone signaling is useful in identifying 
 "stuck on" transmitters that can block 
 the communications net. In-band signal- 
 ing is useful in emergency situations. 
 
 g 2.5 Hybrid Systems 
 
 Each of the communications systems 
 already discussed has some individual 
 shortcomings. However, one system may 
 complement another system to alleviate 
 certain problem areas. A hybrid is an 
 interconnection of two or more sub- 
 systems , taking advantage of the benefits 
 of each. 
 
 2.5.1 Improvements in System Versatility 
 
 As mines have grown and mining tech- 
 nology has improved, needs have arisen 
 for new and improved communication capa- 
 bilities that cannot adequately be pro- 
 vided by the traditional mine pager-type 
 phones or trolley wire carrier phones. 
 These needs include the following: 
 
 1. Ability to communicate when the 
 phone line or the trolley wire breaks. 
 
 The transmitters are designed with 
 both subaudible (100 Hz) and in- 
 band (1,000 Hz) tone oscillators. A 
 
 2. Ability to communicate with 
 personnel not in the vicinity of a 
 telephone. 
 
32 
 
 3. Ability to communicate over pri- 
 vate channels. 
 
 4. Ability to deliver important 
 messages during periods of heavy communi- 
 cations traffic during emergencies. 
 
 5. Ability to communicate with sur- 
 face public phones. 
 
 The following techniques are capable 
 of satisfying the foregoing needs using 
 hybrid systems: 
 
 1. Underground phones with manual 
 trunking or automatic switching can pro- 
 vide privacy and an interconnection to 
 the public telephone system on the sur- 
 face. Also, a larger number of simul- 
 taneous communications can take place 
 with multipair or multiplexed phone 
 systems . 
 
 2. Low-frequency radio offers a 
 means of paging and communicating 
 directly using the mine structure within 
 a working section, and through the mine 
 overburden in times of emergency. 
 
 interconnected with the public phone 
 system on a selective or temporary basis. 
 The intent of these systems is to provide 
 greater emergency communication capabil- 
 ity during off-hours. These systems 
 enable a person at a mine pager phone 
 to gain access to the public phone 
 system, or permit access to the mine 
 page or phone system from any public 
 phone. 
 
 Figure 2-31 shows one system in 
 which the interconnect between public 
 phone and mine phone is made automat- 
 ically. In this type of system small 
 hand-held tone-generators are required to 
 activate the automatic interconnect at 
 the mine office. 
 
 If a person in the mine wants to 
 reach a prearranged public telephone from 
 his mine phone, he sends a tone via the 
 tone generator and mine phone to a tele- 
 phone interconnect unit of the surface. 
 At this surface interconnect, the tone is 
 detected and activates a relay which, in 
 turn, automatically dials the preset tel- 
 ephone number. 
 
 3. Medium-frequency radio can be 
 used with power cables, trolley wires, 
 and roof bolts to provide haulageway and 
 section paging throughout the mine to key 
 mining personnel carrying pocket pagers. 
 
 4. Very-high-frequency radio can be 
 used with leaky feeder cable or coaxial 
 cable and antennas, as a technique for 
 guiding radio waves throughout the mine 
 haulageways and entries. This technique 
 can be used to provide whole-mine com- 
 munications with hand-held radios carried 
 by key mining personnel. 
 
 5. Ultra-high-frequency radio can 
 provide wireless communication between 
 key roving miners carrying radios within 
 a working section, without the aid of 
 additional wiring. 
 
 2.5.2 Dial Phone-Pager Phone Systems 
 
 Interconnect devices are avilable 
 that permit mine paging telephones to be 
 
 r" 
 
 TELEPHONE OFFICE | 
 I J 
 
 PUBLIC PHONE 
 
 
 / n ^ Tt 
 
 CENTRAL 
 EXCHANGE 
 
 1 H^X 
 
 
 1 
 
 AUTO ANSWER 
 
 MAIN OFFICE 
 
 r' 
 
 J 
 
 MINE PHONE LINE 
 
 LOUDSPEAKING PHONE 
 
 o D 
 
 I 
 
 I SECTION I 
 
 FIGURE 2-31.- Dial phone to pager phone 
 hybrid. 
 
33 
 
 When a person calls the "auto 
 answer" telephone number from a public 
 phone, the interconnect unit automat- 
 ically answers the phone, and upon recep- 
 tion of the audio tone from the outside 
 party, connects the incoming call 
 directly to the mine pager phone line, 
 thereby enabling the calling person to 
 page and talk to the desired person in 
 the mine. 
 
 Systems also exist where the inter- 
 connection between the mine phone system 
 and the public telephone system is made 
 manually, such as by a person on the 
 surface. 
 
 2.6 Other Systems 
 
 2.6.1 Seismic Systems 
 
 A seismic system can be used for 
 trapped miner location. If a miner 
 strikes a roof bolt, floor, or rib of the 
 mine with a heavy object, the vibrations 
 travel through the earth to the surface 
 and can be converted into electric 
 signals by seismic transducers called 
 geophones. These signals can be ampli- 
 fied, filtered, and recorded. Because 
 the shock waves reach individual geo- 
 phones at different times, the seismic 
 recordings can be analyzed and the loca- 
 tion of the miner can be determined. 
 Analysis of seismic signals is a highly 
 specialized field and beyond the scope of 
 this manual. This method requires the 
 assistance of an individual trained in 
 seismic methods. However, the seismic 
 system is the only trapped-miner system 
 presently in operation and accepted by 
 MSHA. Every miner should have a sticker 
 (fig. 2-32) affixed to the inside of his 
 helmet that he can refer to if entrapment 
 should ever occur. 
 
 2.6.2 Stench System 
 
 Stench is used primarily as an evac- 
 uation warning. It should be introduced 
 into the underground system at as many 
 locations as possible, with the intake 
 
 WHEN ESCAPE IS CUT OFF 
 
 1. BARRICADE 
 
 2. LISTEN for 
 
 3 shots, then... 
 
 3. SIGNAL by 
 
 pounding hard 
 10 times 
 
 4. REST 15 minutes 
 then REPEAT signal unti 
 
 5. YOU HEAR 5 shots, which 
 means you are located fS 
 and help is on the way. Is 
 
 (■|M|LI^!J21^ 
 
 FIGURE 2-32. - MSHAsignaling sticker. 
 
 air and the compressed air as priority. 
 Wherever miners may be in the mine, 
 driven air is required and eventually the 
 driven air and stench will arrive at 
 their location. Stench may be any 
 clearly distinguishable odor. 
 
 The delay time in a stench warning 
 system is one of its most important 
 drawbacks. Another very important 
 negative point is that stench warning 
 cannot inform the miner what has hap- 
 pened, where it has happened, or what he 
 should do. Many times this type of in- 
 formation can be worse than no informa- 
 tion at all. 
 
 2.6.3 Hoist Bell Signaling 
 
 Much of the communication between 
 the various levels of a mine and the 
 hoist room consists of hoist bell signal- 
 ing. This is a one-way communication 
 system by which miners can request a cage 
 and/or desired level. 
 
 In the hoist room, there is a power 
 source for the system and a buzzer. Each 
 shaft station has a buzzer and a pull 
 bottle. The bottle, when pulled, closes 
 a switch that sounds the buzzer in 
 the hoist room and at all other levels 
 (fig. 2-33). The number of times the 
 bottle is pulled corresponds to a command 
 code. 
 
34 
 
 TWISTED 
 PAIR 
 
 TJimu, 
 
 SWITCHED 
 LINE 
 
 HOISTROOM 
 BUZZER 
 
 '///////////////////, 
 
 /JP 
 
 POWER 
 SOURCE 
 
 110 VAC LINE 
 
 {^ 
 
 PULL 
 BOTTLE 
 
 FIGURE 2-33. » Hoist bell operation. 
 
 2.6.4 Visual Pagers 
 
 One of the most common type of mine 
 communication devices is the pager phone. 
 However, with these systems, many calls 
 are lost because the phone is too far 
 from a working face or the page is 
 not heard owing to high ambient noise. 
 Visual pager systems are being developed 
 that may eventually alleviate this 
 problem. 
 
 The most common type of visual pager 
 system consists of strobe lights located 
 at strategic locations in the mine. 
 These lights are controlled by a dis- 
 patcher who can set or reset them as 
 required. They are usually used in con- 
 junction with pager phones. Some modern 
 multichannel phones have a light on the 
 face of the phone that provides the vis- 
 ual paging function. 
 
 2.7 Summary 
 
 There are three basic types of com- 
 munication systems used in underground 
 mines: wired phone systems, radio sys- 
 tem, and carrier current systems. Wired 
 
 phone systems include common dial 
 telephones, pager phones, dial-type pager 
 phones, magneto phones, intercoms, and 
 sound-powered phones. These may be con- 
 nected in party line fashion using a two- 
 wire pair, or in selective calling 
 fashion using multipair or multiplex 
 techniques. Wired phone systems may have 
 no switchboard (party line) for small 
 systems, a manual switchboard, an auto- 
 matic exchange, or a more sophisticated 
 computer-controlled switch. A major dis- 
 advantage of any wired phone system is 
 that a roof fall could disrupt communica- 
 tions between miners and the surface. 
 
 Radio systems include all wireless 
 communication systems. Coverage is lim- 
 ited in radio systems because of poor 
 propagation of radio waves underground. 
 Voice frequency ranges can be used 
 for through-the-earth radio. Ultra-high- 
 frequency radio can be used when it is 
 combined with leaky feeder cables, anten- 
 nas, and repeaters to extend coverage. 
 Personal radio pagers can be used to sum- 
 mon an individual to a wired phone. 
 
 Carrier current systems utilize 
 existing mine wiring to propagate RF sig- 
 nals. An RF signal from a carrier phone 
 is induced onto a cable and transmitted 
 throughout the length of that cable. A 
 transceiver, inductive or capacitive 
 coupled to the carrier cable, receives 
 the RF signal, strips off the carrier, 
 and lets the electromagnetic voice signal 
 activate a speaker or earphone. 
 
 Modern MF radio systems are being 
 developed that combine the best features 
 of radio systems and carrier current sys- 
 tems. In these systems, no physical con- 
 tact to existing mine wiring is required. 
 
 Since one system cannot usually sat- 
 isfy all the communication requirements 
 in a mine, interfaces have been developed 
 to make hybrid systems. Hybrids (two or 
 more systems interconnected) take advan- 
 .tage of the beneficial qualities of one 
 system to alleviate the deficiencies of 
 another system. 
 
35 
 
 BIBLIOGRAPHY 
 
 1. Aldridge, M. D. Analysis of Com- 
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 available from NTIS PB 225 862. 
 
 2. Alton, J. E. A Dial-and-Page Tel- 
 ephone System. Paper in Underground Mine 
 Communications (in Four Parts). 1. Mine 
 Telephone Systems. BuMines IC 8742, 
 1977, pp. 18-26. 
 
 3. Anderson, D. W. Stout, and H. E. 
 Parkinson. Interconnecting New Communi- 
 cations to Existing Systems. Paper in 
 Mine Communications, Proceedings: Bureau 
 of Mines Technology Transfer Seminar, 
 Bruceton, Pa., March 21-22, 1973. Bu- 
 Mines IC 8635, 1974, pp. 73-86. 
 
 4. Bradburn, R. A., and J. D. 
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 Communications. BuMines IC 8745, 1977, 
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 5. Bradburn, R. A. , and R. L. Lagace. 
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 6. Bradburn, R. A., and H. E. Parkin- 
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 7. Chufo, R. L. , R. L. Lagace, and 
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 Mine Communications (in Four Parts). 
 4. Section-to-Place Communications. Bu- 
 Mines IC 8745, 1977, pp. 65-72. 
 
 8. Chufo, R. L., and R. G. Long. 
 Pager-Phone Guidelines and Test Equip- 
 ment. Paper in Underground Mine Communi- 
 cations (in Four Parts). 2. Paging Sys- 
 tems. BuMines IC 8743, 1977, pp. 7-15. 
 
 9. Dobroski, H. , Jr. A Coaxial-Cable 
 Telephone System. Paper in Underground 
 
 Mine Communications (in Four Parts). 
 1. Mine Telephone Systems. BuMines 
 IC 8742, 1977, pp. 42-57. 
 
 10. 
 
 MCM-101 Call-Alert Pag- 
 
 ing. Paper in Underground Mine Communi- 
 cations (in Four Parts). 2. Paging Sys- 
 tems. BuMines IC 8743, 1977, pp. 3-6. 
 
 11. 
 
 Radio Paging. 
 
 Underground Mine Communications 
 Parts). 2. Paging Systems. 
 IC 8743, 1977, pp. 29-33. 
 
 Paper in 
 
 (in Four 
 
 BuMines 
 
 12. Dobroski, H. , Jr., S. J. Lipoff, 
 and J. E. Trombly. Call-Alert Paging for 
 Pager-Phone Systems, Paper in Under- 
 ground Mine Communications (in Four 
 Parts). 2. Paging Systems. BuMines 
 IC 8743, 1977, pp. 24-28. 
 
 13. Dobroski, H. , Jr., and J. E. 
 Trombly. Visual Paging System. Paper in 
 Underground Mine Communications (in Four 
 Parts). 2. Paging Systems. BuMines 
 IC 8743, 1977, pp. 16-23. 
 
 14. Ginty, J. J., R. L. Lagace, and 
 P. G. Martin. Applicability of State- 
 of-the-Art Repeaters for Wireless Mine 
 Communications. Arthur D. Little, Inc., 
 Cambridge, Mass., July 1975. 
 
 15. Lagace, R. L. Report Highlights 
 of the Working Group on Electromagnetic 
 Through-the-Earth Mine Communication 
 Links. Proc. 2d WVU Conf. on Coal Mine 
 Electrotechnology , Morgantown, W. Va. , 
 June 12-14, 1974, pp. 12-1—12-18. 
 
 16. Lagace, R. L. , W. G. Bender, 
 J. D. Foulkes, and P. F. O'Brien. Appli- 
 cability of Available Multiplex Carrier 
 Equipment for Mine Telephone Systems. 
 Arthur D. Little, Inc., Cambridge, Mass., 
 June 1975. 
 
 17. Lagace, R. L. , and H. E, Parkin- 
 son. Two-Way Communications With Roving 
 Miners. Paper in Mine Communications, 
 March 21-22, 1973. BuMines IC 8635, 
 1974, pp. 46-72. 
 
36 
 
 18. Long, R. G. Technical Guidelines 
 for Installation, Maintenance and Inspec- 
 tion of Underground Telephone Systems. 
 Arthur D. Little, Inc., Cambridge, Mass., 
 June 1975. 
 
 19. Murphy, J. N. , and H. E. Parkin- 
 son. Underground Mine Communications. 
 Proc. IEEE, V. 66, No. 1, January 1978, 
 p. 26. 
 
 20. Parkinson, H. E. Emergency/Non- 
 Emergency Mine Communications. Paper in 
 Mine Communications, March 21-22, 1973. 
 BuMine IC 8635, 1974, pp. 3-16. 
 
 21. Parkinson, H. E., and J. D. 
 Foulkes. Conventional Telephone Equip- 
 ment. Paper in Underground Mine Com- 
 munications (in Four Parts). 1. Mine 
 
 Telephone Systems, 
 1977, pp. 3-17. 
 
 BuMines IC 8742, 
 
 22. Powell, J. A., and R. A. Watson. 
 Seismic Detection of Trapped Miners Using 
 In-Mine Geophones. BuMines RI 8158, 
 1976, 8 pp. 
 
 23. Sacks, H. K. Trapped-Miner Loca- 
 tion and Communication Systems. Paper 
 in Underground Mine Communications 
 (in Four Parts). 4. Section-to-Place 
 Communications. BuMines IC 8745, 1977, 
 pp. 31-43. 
 
 24. Spencer, R. H. , and H. E. Parkin- 
 son. Roving Miner Paging. Paper in Mine 
 Communications, March 21-22, 1973. Bu- 
 Mines IC 8635, 1974, pp. 17-35. 
 
CHAPTER 3. —SOLUTIONS TO THE COMMUNICATION REQUIREMENTS 
 
 37 
 
 3.1 Introduction 
 
 Three types of communication systems 
 have become popular in solving communica- 
 tion requirements underground: Loud- 
 speaking pager phones, carrier current 
 phones, and magneto ringing phones. 
 Basically, all three are simply single- 
 channel party line systems. Although 
 these systems are quite reliable, the 
 single channel creates a variety of prob- 
 lems. For example — 
 
 1. Since no call is confidential, 
 messages are sometimes purposely made 
 vague, especially if accidents or safety 
 topics are being discussed. 
 
 wired phone system by using a remote 
 portable radio. 
 
 3. Remote monitors that alert per- 
 sonnel when there is a toxic or explosive 
 gas buildup. 
 
 4. Control interfaces that allow 
 remote control of fans, pumps, or other 
 devices. 
 
 5. Transmitters and receivers that 
 can serve as emergency links. 
 
 6. Loopback that allows an alter- 
 nate path of communications if the main 
 path is cut. 
 
 2. A potential user must literally 
 "wait in line" until the channel becomes 
 clear for his use; thus, when foremen 
 have to wait to call in reports or supply 
 requests, this single chan-nel creates a 
 serious productivity bottleneck. 
 
 This chapter focuses on equipment 
 and methods to meet the special communi- 
 cation needs of individuals in various 
 places of the underground mine. The com- 
 munication requirements can be broken 
 down into four categories: 
 
 3. In many large mines, there are 
 independent branch lines that naist be 
 tied together by a dispatcher, adding 
 further delays to the system. 
 
 To solve these problems , some mines 
 have . installed other phone systems — 
 mostly commercial dial phones in indus- 
 trial enclosures that offer extra channel 
 capacity and private line features. 
 Others have installed a system that com- 
 bines both dial- and page-phone features 
 in a single unit. 
 
 1 . The mine 
 communication) . 
 
 entrance (shaft 
 
 2. Permanent and semipermanent lo- 
 cations (shop areas, lunchroom, crusher 
 stations, etc.). 
 
 3. Mining areas (the room-and- 
 pillar sections, longwall faces, block 
 caving areas, etc.). 
 
 4. Haulageways (tracked 
 haulage, diesel, belt haulage). 
 
 trolley 
 
 Although these do represent improve- 
 ments, they do not truly solve the over- 
 all problems that face modern mines. 
 Besides extra channels, a communica- 
 tion system should have the following 
 features to enhance productivity and 
 safety: 
 
 1. A means of paging a roving miner 
 to alert him that he is wanted on the 
 phone. 
 
 Methods of implementing systems to 
 meet the communication needs of these 
 areas are described in sections 3.2 
 through 3.5. 
 
 Section 3.6 discusses methods of 
 satisfying special communication require- 
 ments that exist. Major topics in this 
 category include communications with rov- 
 ing personnel, the isolated miner, and 
 motorman-to-snapper communications . 
 
 2. Wireless-to-wired system inter- 
 connects by which a miner can talk on the 
 
 Emergency communication systems are 
 described in section 3.7. Detecting and 
 
38 
 
 locating the trapped miner, rescue team 
 communications , and emergency warning 
 systems are discussed. 
 
 Although the methods of establishing 
 communications throughout a mine are 
 broken down and described in separate 
 sections, as outlined above, it is impor- 
 tant to realize that these systems should 
 be tied together or interconnected in 
 some way. The overall design plan must 
 include provision for integrating the 
 various communication subsystems together 
 into a minewide system. Such a system, 
 designed with the total mine operating 
 plan in mind, will be the most effective. 
 In a like manner, a judicious choice of 
 monitored parameters in the underground 
 environment and selected machinery will 
 yield a cost savings in production and 
 augment safety. Many man-hours and dol- 
 lars can be saved by knowing conditions 
 before they become a problem. Situations 
 that could become disastrous can be pre- 
 dicted and proper solutions implemented 
 before the disaster occurs. Because 
 proper environmental and machine monitor- 
 ing and control is another key to safer 
 and more productive underground mining, 
 these factors should also be considered 
 in the overall plan of any communication 
 system. 
 
 3.2 The Mine Entrance 
 
 The entrances to underground mines 
 are either vertical shafts, slope en- 
 trances, or horizontal drifts. Slope and 
 horizontal drift entrances can be consid- 
 ered as a continuation of a haulageway 
 and are treated in section 3.5. This 
 section is devoted primarily to shaft 
 communications. 
 
 In the past, operators of single- 
 level mines with overburdens less than 
 1,200 feet have felt that communications 
 between the top of the shaft, the bot- 
 tom, the hoistroom, and possibly a com- 
 munications center were adequate. Many 
 mines in this category (which includes 
 most underground coal mining operations) 
 did not have the capability of two-way 
 voice communication with personnel in the 
 cage. 
 
 One of the biggest reasons for this 
 deficiency in communications to and from 
 the cage has been that reliable equipment 
 simply was not available for establishing 
 this vital two-way voice communication 
 link. This reason is no longer valid. 
 Today, equipment is commercially availa- 
 ble to implement effective two-way voice 
 communication, even while the cage is 
 moving, down to depths in excess of 
 10,000 feet. 
 
 A useful hoist-shaft communication 
 system must satisfy the requirements for 
 communication throughout the full travel 
 of the cage, providing voice communica- 
 tion between the cage and the hoistman, 
 as well as to underground shaft stations. 
 An effective system should also provide 
 for shaft-inspection communication be- 
 tween the inspector and the hoistman, and 
 should have a slack-rope indication. For 
 the modern, automated shaft, signals are 
 also required to permit selection of 
 level, enable interface with interlocks, 
 and permit jogging for exact position at 
 any level. 
 
 The limited space within the cage 
 places an operational restraint on equip- 
 ment. Equipment must be small and should 
 be located so that it cannot be damaged 
 by any of the various uses of the cage 
 such as transporting supplies and machin- 
 ery. An additional microphone-speaker 
 station may also be desired for multi- 
 level cages or when several cages are 
 joined together. 
 
 A reliable hoist-shaft communication 
 system should be considered as a vital 
 part of any overall communication system. 
 Shaft communication is especially impor- 
 tant during or following an accident or 
 disaster situation. Experience has 
 taught that the hoist often becomes a 
 bottleneck during rescue or evacuation 
 operations, and good communication to and 
 from the cage is essential. 
 
 Traditionally, bell signals have 
 been used between those requesting the 
 cage and the hoistman, and until re- 
 cently^ bell signaling was usually the 
 only form of hoist-shaft communication. 
 
39 
 
 Today, however, equipment is available 
 that allows two-way voice communication 
 between persons in the hoist cage and the 
 hoistman or other locations at the shaft 
 top or bottom. 
 
 Presently available methods of im- 
 plementing two-way voice communication 
 with the hoist cage include trailing ca- 
 ble systems, radio systems, and hoist 
 rope carrier current systems. 
 
 3.2.1 Bell Signaling Systems 
 
 There was a time when bell signaling 
 was the only form of communication 
 between those requesting a cage and the 
 hoist operator. Because of this, bell 
 signaling systems have gained widespread 
 acceptance and are used on many hoists. 
 
 Figure 3-1 shows a simplified sche- 
 matic diagram of a typical shaft buzzer 
 signaling system. In the system de- 
 picted, a single twisted pair wire is run 
 down the mine shaft and "tapped off" at 
 those shaft stations where signaling is 
 required. Figure 3-1 shows a system with 
 the twisted pair tapped at three levels 
 
 UNDERGROUND 
 
 SWITCHED LINE- 
 
 ^ 
 
 HOISTROOM 
 BUZZER 
 
 77777777777777777777777777777, 
 
 j_ « TWISTED PAIR 
 
 — 110 VAC LINE 
 
 ^ 
 
 ^ 
 
 PULL BOTTLE 
 
 PULL BOTTLE 
 
 ^ 
 
 -A'" 
 
 PULL BOTTLE 
 
 FIGURE 3-1. = Bell signaling syster 
 
 (level A, level B, and level C). To sig- 
 nal the hoist operator, a miner at any 
 level pulls the pull bottle, causing the 
 associated switch to close. This applies 
 voltage to the buzzer at that level, and 
 also to the hoistroom buzzer and all oth- 
 er buzzers, through the switched line of 
 the twisted pair. 
 
 Bell signaling systems, although 
 proven to be reliable, do have some 
 severe shortcomings. First, there is no 
 way to convey special messages to the 
 hoistman. Special equipment or assist- 
 ance or unusual cage movements cannot be 
 requested unless a signaling code has 
 been defined for that specific request. 
 A second disadvantage, especially for 
 mines with many shaft stations, is that 
 the bell codes required can become quite 
 long. Long or complicated bell codes are 
 obviously more difficult to remember and 
 can become a source of confusion, espe- 
 cially during an emergency or disaster 
 situation. During these critical periods 
 of high emotional stress, mistakes are 
 easy to make even when signaling codes 
 are posted. Some mines with many levels 
 and/or shaft stations have partially 
 overcome this disadvantage by assigning 
 an employee to the hoist cage. Because 
 this miner, called a eager, is perma- 
 nently assigned to act as the hoist cage 
 operator, the bell signaling code has 
 become "second nature" to him. 
 
 Another disadvantage of the bell 
 signaling system is that communication 
 with the cage is impossible when it is 
 between levels. This deficiency is 
 especially crucial during shaft inspec- 
 tion or repair. Some mines have par- 
 tially overcome this problem by running a 
 pull cord down the shaft. This cord is 
 kept in a position next to the shaft tim- 
 bers by staples and can be used for emer- 
 gency stops and signaling between shaft 
 stations. This system does provide some 
 degree of emergency communication from 
 the cage while it is between shaft sta- 
 tions; however, operation of the system 
 can be extremely dangerous since it 
 requires the operator to reach out of the 
 cage and grab a cord, which may be moving 
 relative to the cage. 
 
40 
 
 3.2.2 Trailing Cable Systems 
 
 One method of establishing two-way 
 voice communication with the hoist cage 
 is by using a trailing cable. In this 
 type of system. A communications cable 
 is physically attached to the bottom of 
 the cage and allowed to hang down the 
 shaft below the cage. 
 
 Figure 3-2 shows a typical trailing 
 cable system. In the figure, three 
 phones (one in the hoistroom and one at 
 each of the two shaft stations) are con- 
 nected by a phone line that has been 
 strung down the shaft to a junction box 
 located about halfway down the shaft. 
 Connections are made within the junction 
 box to the trailing cable which provides 
 the link to the phone mounted in the 
 cage. The trailing cable system can be 
 
 ///////// 
 
 ■/////////// 
 
 '/ 
 
 tied into the existing wired communica- 
 tion system in the mine, or it can be 
 implemented as an independent, shaft- 
 only, communication system. 
 
 In addition to the disadvantages 
 associated with any wired communication 
 system (normal cable maintenance and line 
 breaks), the trailing cable system has 
 limitations in terms of depth because of 
 the amount of cable that can be trailed 
 from the cage. 
 
 3.2.3 Radio Systems 
 
 Another approach to satisfy the 
 voice communication requirements with 
 personnel in the cage is by using two-way 
 radio systems. Some recently installed 
 radio systems are meeting the hoist com- 
 munication requirements. One radio sys- 
 tem currently being used at an iron ore 
 mine is illustrated in figure 3-3. In 
 this system, portable police-type 150-MHz 
 FM radios were used in 19-foot-diameter 
 shafts. The surface antenna is a dipole 
 mounted on plywood bolted to the steel 
 collar at the top of the shaft. A coax 
 runs from the dipole to the hoist room, a 
 distance of about 500 feet. In the cage, 
 the radio and 12-volt battery are mounted 
 in a plywood box. 
 
 Results of studies indicate that the 
 attenuation of radio signals increases 
 
 //////// ///T/ 
 
 rrjTTT 
 
 FIGURE 3-2. - Trailing cable system. 
 
 § 
 
 HOISTMAN'S RADIO- 
 CONNECTED TO 
 ANTENNA AT HEADFRAME 
 
 RADIO SIGNAL 
 
 FIGURE 3-3. - Hoist cage radio. 
 
41 
 
 sharply as shaft size is decreased. For 
 straight, unobstructed shafts with a 
 diameter in the neighborhood of 12 feet, 
 radios opetating in the frequency range 
 of 500 to 1,000 MHz should provide commu- 
 nication to a depth of approximately 
 1,500 feet. For smaller diameter shafts, 
 radio communications will only be possi- 
 ble over shorter distances. 
 
 3.2.4 Hoist Rope Carrier Current System 
 
 A carrier current system using the 
 hoist rope as the carrier has been devel- 
 oped that provides reliable two-way voice 
 communication between the cage (even 
 while in motion) and the hoistman to cage 
 depths in excess of 10,000 feet. The 
 principle of operation of the hoist rope 
 carrier current system is similar to that 
 of the carrier current commonly used in 
 trolley carrier phone systems. Both 
 systems transmit and receive RF energy 
 over a transmission line. In a trolley 
 system, the transmission line is the 
 trolley wire. In the hoist system, the 
 carrier signal is transmitted on the 
 hoist rope. Both systems utilize trans- 
 mitters and receivers (transceivers) that 
 communicate with each other by RF cur- 
 rents superimposed on a cable. 
 
 SHEAVE 
 
 The principal diff 
 trolley carrier system 
 carrier system is the 
 energy is transferred 
 from, the transmission 
 hoistman' s transceiver 
 cannot be physically at 
 touch) the hoist rope, 
 of superimposing RF ene 
 must be used. 
 
 erence between the 
 
 and the hoist rope 
 
 way in which RF 
 
 to, and received 
 
 line. Because the 
 
 at the headframe 
 
 tached to (or even 
 
 a different method 
 
 rgy onto the rope 
 
 The solution is to inductively 
 couple the hoistman' s transceiver to the 
 hoist rope. Figure 3-4 shows a block 
 diagram of a hoist rope carrier current 
 system. The system consists of two sig- 
 nal couplers and two transceivers. Each 
 transceiver is of the push-to-talk, 
 release-to-listen design. During trans- 
 mission, the sending transceiver feeds 
 its coupler with an FM carrier. The 
 coupler induces a signal into the hoist 
 rope, which travels up and down the hoist 
 
 HOIST 
 
 SHEAVE 
 COUPLER 
 
 CAGE 
 
 FIGURE 3-4. - Hoist rope carrier current system, 
 
 rope and is picked up by the coupler at 
 the other transceiver. Each coupler 
 operates as both a transmitting and 
 receiving element. The cage coupler is 
 clamped to the hoist rope at a point just 
 above the cage. The coupler for the sur- 
 face transceiver should be permanently 
 mounted below the sheave wheel and about 
 6 inches from the rope. Coaxial cable 
 should be used to connect each coupler to 
 its transceiver. 
 
 3.2.5 Hoist Signaling Summary 
 
 The pull-bottle shaft bell has been 
 the universally accepted means of cage 
 signaling. More than 60% of the hoists, 
 notably those in bedded deposit mines, 
 have only one underground level; hence 
 this type of signaling system is simple 
 and effective. In multilevel mines, sig- 
 naling codes become complex to the point 
 where a full-time cageman may be required 
 to control the cage during all man and 
 equipment lifts. Depending on the size 
 
42 
 
 and nature of the shaft, commercial radio 
 equipment operating at 150 or 450 MHz can 
 provide a voice link down to 2,000 or 
 3,000 feet. 
 
 For a very deep (2,500 feet or 
 greater) or narrow shaft (less than 
 10 feet in diameter) , comiminication sys- 
 tems are available that inductively 
 couple RF signals to the hoist cable. 
 These carrier current systems provide 
 two-way communication with the cage in 
 even the smallest shafts down to depths 
 in excess of 10,000 feet. 
 
 3.3 Permanent and Semipermanent In-Mine 
 Locations 
 
 Looking only at the permanence of a 
 telephone installation, phone locations 
 can be divided into the following three 
 categories: 
 
 Permanent (life of the mine). 
 
 Semipermanent (more than a year be- 
 tween moves) . 
 
 Frequently 
 monthly). 
 
 moved (weekly 
 
 to 
 
 Permanent locations include surface 
 sites, the dispatcher's station, under- 
 ground offices and shop areas, lunch- 
 rooms, rail or belt heads, storage areas, 
 the crusher operator, and along main 
 haulageways. 
 
 Semipermanent phones would be found 
 mostly in the submains of a mine. After 
 panels have been fully developed, most of 
 the phones in the submain would be re- 
 located to more active sections. One or 
 two phones would remain for use by roving 
 personnel. If a submain became part of 
 the haulage system, in all likelihood 
 more phones would remain in use to meet 
 the operating practices of the mine. 
 
 Frequently moved phones are pri- 
 marily located near the working faces of 
 the mine, typically in working sections 
 off submains. These phones are moved 
 with the section in order to maintain 
 close communication with the dispatcher, 
 
 maintenance, and management personnel. 
 Communications equipment associated with 
 advancing or frequently moved face areas 
 is treated in section 3.4. 
 
 The single-pair wired phone system 
 is the communication system commonly 
 employed to satisfy the requirements of 
 permanent locations. Magneto phones were 
 first used, but although many are still 
 in use, they have been largely replaced 
 by loudspeaking pager phones. 
 
 In a few mines the conventional tel- 
 ephone with a rotary dial and ringer 
 (mounted in an explosion-proof housing) 
 has been used. Systems using these dial 
 phones are usually an extension of an 
 aboveground private automatic branch ex- 
 change (PABX) or a single-party indepen- 
 dent system with a small switchboard. 
 Recently multipair cable and even multi- 
 plex systems have been used to intercon- 
 nect phones and to connect individual 
 phones to an aboveground PABX. 
 
 3.3.1 Single-Pair Pager Phones 
 
 Pager phones were specifically 
 designed to meet the requirements of per- 
 manent and semipermanent locations for 
 underground mining operations. They dif- 
 fer from a conventional telephone in that 
 instead of a ringer, a loudspeaker is 
 used in each phone to alert the person 
 being called, and each phone has its own 
 batteries for power instead of being cen- 
 trally powered. In a single-pair instal- 
 lation, the pager phones are inter- 
 connected by a single twisted pair of 
 wires and all phones are on a single 
 party line. A 14- to 18-gage copper pair 
 with a neoprene jacket is most often 
 used. 
 
 Figure 3-5 shows a hypothetical, 
 moderate-sized room-and-pillar coal mine 
 with an average working section size of 
 300 feet by 400 feet, and an average 
 panel size of 800 feet by 1,200 feet. 
 
 The upper half of figure 3-5 shows 
 the main haulageway, the operational sub- 
 mains, and the working sections and 
 illustrates how a single-pair pager phone 
 
43 
 
 y^m^oL 
 
 g SPLICE CASE 
 
 9 FREQUENTLY MOVED PHONE 
 
 SEMIPERMANENT PHONE 
 
 PERMANENT PHONE 
 
 SEMIPE.RMANENT _^-__j»r 
 
 PHONE IN TTTj 
 
 SUBMAIN ' 1 J-"^ 
 
 oDDoa 
 
 J — ^oa 
 
 nnoQ 
 
 QODCpo 
 0)0 0000 
 
 DCao 
 DDQO 
 qQOD 
 
 
 EQUENTLY MOVED 
 PHONE IS KEPT 
 NEAR THE WORKING 
 SECTION 
 
 PERMANENT PHONE IN MAIN 
 
 O QZDO C3C>D|C>O^Da OO O 9 O 
 
 bP^DOQOQ 
 
 
 FIGURE 3-5. • Single-pair pager phone installation. 
 
 system would be wired. The solid black 
 line represents the single pair. The 
 black squares represent splicing points, 
 and the circles represent the telephones. 
 Telephones can be seen at the dis- 
 patcher's office, in the underground shop 
 area, and along the main haulageways; 
 there, is one at each butt entry and one 
 near each working section, and a twisted 
 pair is shown running to the pager phone 
 at each working section. When a panel is 
 worked out, the phone at that butt entry 
 may be removed and installed at another 
 location. Usually, as shown in fig- 
 ure 3-5, a few phones are left behind 
 along the submains in worked-out sec- 
 tions. These phones may be used peri- 
 odically by roving maintenance personnel 
 or inspectors. 
 
 A mine is not static; its architec- 
 ture or layout changes, but fortunately 
 these changes are usually known well in 
 advance so that cable selection and the 
 phone line layout in the mine can be 
 planned to accommodate future growth. As 
 far as changes are concerned, the phones 
 shown in the example of figure 3-5 fall 
 into the three basic categories as 
 follows: 
 
 1. Telephones in the dispatcher's 
 office, shop area, and main haulageway , 
 and opposite each submain, would rarely, 
 if ever, be moved (permanent). 
 
 2. Telephones opposite each butt 
 entry would remain in place for one year 
 or so until more panels in the submain 
 have been developed (semipermanent). 
 
 3. Mine safety regulations require 
 that a communication link must be estab- 
 lished within 500 feet of the working 
 face; hence, telephones at the working 
 sections are required to be moved fre- 
 quently (perhaps once a week). 
 
 3.3.2 Multlpair Systems 
 
 For a multlpair access installation 
 (fig. 3-6) , planning for future mine 
 growth becomes important. The figure 
 shows an example of how a multlpair sys- 
 tem may have grown in our hypothetical 
 mine, which has four working sections (A, 
 B, C, and D) . In this example, when the 
 system was installed, working section A 
 did not exist, so three-pair cable was 
 used to give sections B, C, and D private 
 lines. (The telephone at the working 
 face is an extension of the butt entry 
 phone, which may not be reasonable in low 
 coal.) When section A came into opera- 
 tion, either more cable had to be in- 
 stalled or more telephones had to be con- 
 verted into extensions without private 
 lines. Figure 3-6 shows that six-pair 
 cable was run along the main haulageway, 
 so that at this stage of development, 
 several telephones were forced to share a 
 pair. 
 
 Several telephones are extensions, 
 and as long as that is a satisfactory 
 condition, six-pair cable in the main 
 haulageway Is sufficient. However, if 
 the objective is to provide every tele- 
 phone with its own pair (which really is 
 the point of a multlpair dial access sys- 
 tem) , additional cables have to be run 
 down the main haulageway. The lesson, of 
 course, is to keep future needs in mind 
 when planning cable layouts, particularly 
 in areas like main haulageways and main- 
 tenance areas where telephone locations 
 are unlikely to change for many years. 
 
44 
 
 isssst^^s?^;:^^^ 
 
 lA 2A 3A 
 
 
 FIGURE 3-6. - Multipair installation. 
 
 Figure 3-7 shows details of the 
 multipair system; once again, the 
 three categories of telephones — perma- 
 nent, semipermanent, and frequently moved 
 telephones — can be seen. 
 
 3.3.3 Multiplexed Systems 
 
 Various types of multiplexed systems 
 can also be implemented to satisfy commu- 
 nication requirements of permanent and 
 semipermanent locations underground. 
 
 One system using multiplex equipment 
 and a small PABX has been installed in a 
 deep, multilevel, metal mine in the West- 
 ern United States. This system utilized 
 an existing twisted pair already strung 
 through the mine to establish two seven- 
 channel private communication links. A 
 simplified diagram of the system is shown 
 in figure 3-8. 
 
 The single twisted pair utilized by 
 the systems extended from the surface, 
 down shaft A to the 3,700-foot level, 
 horizontally through a 5,000-f oot-long 
 
 ^^^^^^iii;^^; O SEMIPERMANENT TELEPHONE 
 
 •^^ ® FHEQUENTLV MOVED TELEPHONES 
 ■ SPLICE CASE 
 
 TELEPHONES 
 
 FIGURE 3-7. . Detail of multipair. 
 
 FIGURE 3.8„ - Multiplex system with PABX. 
 
 drift to the underground headframe of 
 shaft B, and then down shaft B to the 
 5,600-foot level. An air-conditioned 
 room was available in the shaft B area at 
 the 3,700-foot level that met all envi- 
 ronmental requirements of commercially 
 available PABX systems. Additionally, 
 this location was approximately centered 
 with respect to the physical locations of 
 the desired phones. The single twisted 
 pair was opened at this point, thereby 
 forming two independent wire pairs (one 
 running back to the surface, the other 
 running down shaft B). A carrier system 
 was then installed on each pair, and 
 these two independent carrier systems 
 were then connected to the PABX line cir- 
 cuits. This provided 14 private channels 
 (lA through 7A and IB through 7B) for 
 communication within the mine. This sys- 
 tem (described in more detail in appendix 
 A), not being intrinsically safe, is not 
 suitable for use in coal mines. 
 
 Presently, there is no intrinsically 
 safe multiplexed telephone system de- 
 signed for mine use that is commercially 
 
45 
 
 available. However, the Bureau of Mines 
 is developing such a system. This system 
 provides eight full duplex channels, some 
 of which can be dedicated to monitor and 
 control functions. The system uses in- 
 expensive twisted shielded pair and is 
 not under control of any central switch- 
 ing or control center. Because of this, 
 the system will not be made inoperative 
 because of a^ cable break or a central 
 PABX failure. Other features include a 
 message-leaving light on each phone, low- 
 battery indicators, and compatibility 
 with standard loudspeaking pager phone 
 systems. 
 
 3.4 Mining Area 
 
 Safety regulations require that a 
 communication link must be established 
 within 500 feet of the working face. In 
 coal mines a butt entry portable phone 
 meets that requirement at the beginning 
 of a panel's development, but a frequent- 
 ly moved section phone must be installed 
 once the face has moved 500 feet from the 
 butt entry phone. Weekly movement of the 
 section phone might be necessary to keep 
 the section foreman within range. 
 
 communication with other parts of the 
 mine. 
 
 Most existing mine communication 
 systems stop at the last open crosscut of 
 the section. Present mine communication 
 systems are aimed at satisfying the need 
 that the mine section foreman be able to 
 communicate with the mine shift foreman. 
 However, in some mines there may be addi- 
 tional communication needs within the 
 mine section. Needs that are not ade- 
 quately met include communications be- 
 tween the continuous miner operator and 
 the shuttle buggy operators, between the 
 shuttle buggy operators and the "gather- 
 ing" locomotive operator, and between the 
 general maintenance foreman and the sec- 
 tion maintenance man repairing a machine. 
 By satisfying these needs, both the safe- 
 ty and efficiency of mining operations 
 can be improved. The existing power 
 trailing cables to the face machine pro- 
 vide one means to achieve these communi- 
 cations capabilities in a reliable and 
 economic manner. Another method of es- 
 tablishing voice communications between 
 miners working at the face is by a radio 
 system. 
 
 Under normal operating conditions 
 the section foreman communicates by fixed 
 phone to the shift foreman to request 
 supplies and maintenance services, and to 
 file his periodic productivity reports. 
 Under emergency conditions he requests 
 medical aid for personnel and reports 
 hazardous conditions in his area. His 
 primary concern is the safety and produc- 
 tivity of his crew. 
 
 The high acoustic noise level cre- 
 ated by the mining machinery greatly 
 reduces the effective communications 
 between the foreman and his crew. This 
 noise also interferes with the foreman 
 receiving calls. Often a motorman must 
 deliver a call-in message to the foreman 
 when he is transferring haulage cars in 
 his section. A standard procedure in 
 some belt haulage mines is to turn off 
 the conveyor system, thereby causing all 
 the section foremen to call in. The 
 working section crew primarily depends on 
 the fixed pager phone system for direct 
 
 The operational and safety advan- 
 tages of communication capabilities are 
 several and diverse. The shuttle buggy 
 operator will be able to alert the con- 
 tinuous mining machine operator of an 
 impending roof fall. The shuttle buggy 
 operators will be better able to coordi- 
 nate their activities as they go in to 
 dump on the belt or into the cars. The 
 maintenance mechanic will be able to com- 
 municate with the surface while working 
 at a face machine. When maintenance on a 
 face machine is required, the maintenance 
 mechanic can be called directly from the 
 troubled machine. 
 
 3.4.1 Radio Systems 
 
 The use of two-way radios can result 
 in better coordination of section ac- 
 tivities, especially during the movement 
 of mobile machines that must work in 
 concert with each other at the face area. 
 In many cases the operators cannot see 
 one another, but with a system of 
 
46 
 
 communications they can still effectively 
 work together. Safety will also be 
 improved by better communication with 
 isolated workers; for example, fan-hole 
 drill operators in iron ore mines. 
 
 Improved management can be realized 
 by means of effective section communica- 
 tion. The foreman can exercise better 
 supervisory control, resulting in more 
 efficient utilization of available per- 
 sonnel. Another benefit is the reduction 
 of unnecessary travel, an extreme burden 
 when mining low coal or on longwall sec- 
 tions. Repairmen, mechanics, and util- 
 itymen can be quickly reached and dis- 
 patched to their place of need at the 
 time of need. In spite of these advan- 
 tages, two-way voice communication using 
 portable radios is only now becoming 
 practical for use in underground working 
 sections. This has been due to the lim- 
 ited range that could be attained using 
 the small handheld units. 
 
 Almost anyone who has ridden in an 
 automobile is familiar with the radio 
 fade that occurs when a car enters a tun- 
 nel. One might expect, then, that radio 
 wave propagation would be very poor in 
 mines, and it is still not feasible to 
 design practical "wireless" portable ra- 
 dios capable of full mine coverage, ex- 
 cept possibly for the smallest mines. 
 However, both theory and experience show 
 that the propagation characteristics of 
 radio waves in mine tunnels improve as 
 the frequency increases into the UHF 
 band. This is attributable to a wave- 
 guide effect that is prominent when the 
 wavelength of the radio wave becomes 
 small compared with the cross-sectional 
 dimensions of the tunnel. In the UHF 
 band from 400 to 1,500 MHz, tunnel propa- 
 gation is adequate to provide sectionwide 
 radio coverage. 
 
 Probably the most important factor 
 that determines the ability of UHF radio 
 waves to propagate in underground mine 
 tunnels is the cross-sectional dimension 
 of the tunnels. In general, a high, wide 
 
 opening favors better radio wave propaga- 
 tion. Figure 3-9 shows a comparison in 
 the ability of 450-MHz UHF radio waves to 
 propagate in high coal (7 feet) as 
 opposed to low coal (3.5 feet), assuming 
 a 16-foot-wide entry. The comparison 
 also assumes that 2-watt UHF walkie- 
 talkies are the source of signal. As 
 indicated in figure 3-9, communication is 
 possible for ranges up to 1,500 feet, 
 along a straight entry in high coal, but 
 the range drops to 400 feet in low coal. 
 Of course the same principles apply to 
 tunnels in noncoal mines. 
 
 Corners also present obstacles to 
 the propagation of UHF radio waves. For 
 a path that includes one corner, ranges 
 are reduced but improve if one of the 
 radios can be moved closer to the corner. 
 However, propagation around a second 
 corner is usually poor. To help offset 
 this corner effect, it is good practice 
 to transmit from intersections when pos- 
 sible, thus reducing the number of 
 corners that have to be negotiated. Some 
 other obstacles to radio wave propagation 
 at ultrahigh frequencies are equipment 
 such as shuttle cars and machines that 
 reduce the cross-sectional area of the 
 tunnels or entries. Table 3-1 shows that 
 when shuttle cars are present, the range 
 is typically reduced by 200 feet in high 
 coal and by 50 feet in low coal. 
 
 Portable radio 
 
 Ui^ 
 
 .J 
 
 ::5 
 
 
 FIGURE 3-9. - UHF radio wave propagation in 
 high and low coal. 
 
47 
 
 TABLE 3-1. - Typical range reduction due to tunnel obstructions at 450 kHz 
 
 Shuttle car ft. . 
 
 Bends 
 
 Permanent stoppings .... 
 Longwalls ft. . 
 
 High coal (7 by 16 ft) 
 
 200 
 
 Moderate to severe.... 
 
 ...do 
 
 1,200 
 
 Low coal (3-1/2 by 16 feet) 
 
 50. 
 
 Moderate to severe. 
 
 Do. 
 250. 
 
 The range of effective conmunication 
 can be substantially increased by the 
 use, and judicious placement, of a re- 
 peater, and in some applications, a radi- 
 ating cable. When this is done, good 
 communication can be established even un- 
 der some of the worst conditions encoun- 
 tered on working sections. Referring to 
 figure 3-10, suppose the two radios la- 
 beled A and B are out of direct radio 
 range of each other. The repeater can 
 function to bring the radios within range 
 in the following manner. When radio A 
 transmits on frequency Fl , the signal is 
 picked up by the repeater, which ampli- 
 fies and converts it to a different fre- 
 quency (F2) and retransmits it at a 
 higher power level. Radio B receives the 
 retransmitted signal. In this way, com- 
 munications from radio A to radio B and B 
 to A are established. 
 
 When the tunnels between the porta- 
 ble radios are heavily obstructed by 
 machinery or metal roof -support struc- 
 tures, radiating (leaky coax) cable may 
 also ■ be installed in the tunnel to pick 
 up and carry the signals to and from the 
 
 FIGURE 3=10. 
 repeater. 
 
 Extending range with radio 
 
 repeater and portable radios. Figure 3- 
 11 shows a sample cable installation for 
 use with the repeater. In this case, the 
 signals from radio A are picked up by the 
 radiating cable itself, carried to the 
 repeater, retransmitted as F2 signals on- 
 to the cable, carried along by the cable, 
 and leaked into tunnels where they are 
 received by radio B. The reverse occurs 
 for transmission from B to A. 
 
 Even though a radio repeater such as 
 shown in figure 3-10 can extend the oper- 
 ating range of radios A and B, this still 
 provides only local coverage such as a 
 working section. However, a radiating 
 cable-repeater system such as shown in 
 figure 3-11 can extend the operating 
 range for many miles. The limiting fac- 
 tor in this case is the ability of a 
 radio to transmit to and receive from the 
 radiating cable. 
 
 The implementation of a UHF radio 
 system for a mine working section can be 
 approached from the standpoint of a basic 
 
 %x 
 
 RADIATING CABLE 
 
 fl- 
 
 s 
 
 RADIATING 
 
 CABLE 
 
 REPEATER 
 
 ^ 
 
 FIGURE 3.11. 
 cable. 
 
 Extending range with radiating 
 
48 
 
 "building block" philosophy as shown in 
 figure 3-12. The fundamental building 
 block is the UHF walkie-talkie radio 
 itself. Several of these are sometimes 
 all that is required for an effective 
 sectionwide communication system. Usu- 
 ally, however, certain portable accesso- 
 ries are helpful to some miners; namely, 
 speaker-microphone headsets, carrying 
 vests, and remote handheld microphones. 
 
 In some situations, it may be neces- 
 sary to extend the range of communica- 
 tions beyond that achievable when trans- 
 mitting directly between portable units. 
 This can be accomplished by adding a 
 radio repeater to the system. A re- 
 peater, which can effectively double the 
 area of coverage, is essentially a signal 
 booster that receives weak signals from 
 distant radios and retransmits them at 
 full power. Further enhancement is pos- 
 sible by connecting the repeater to its 
 antenna by means of a long length of 
 special "radiating" cable that can be run 
 through areas of poor coverage, such as 
 the area along a longwall chockway. 
 Radiating cable, also known as leaky coax 
 or leaky feeder, allows signals to leak 
 out of or into itself at a controlled 
 rate. It effectively behaves as a long 
 antenna that can guide radio waves around 
 corners and bends. 
 
 For a more comprehensive system, an 
 interconnect may be installed to inter- 
 face the radio system with other communi- 
 cation systems , such as a pager phone or 
 carrier phone system. This would be use- 
 ful for paging key personnel in the 
 section who are out of audible range of 
 the section pager phone. However, the 
 interconnect should operate only on a 
 selective basis to avoid interference to, 
 or by, the section radios. Hardware for 
 implementing this radio interconnect is 
 commercially available. 
 
 UHF section radio has been used suc- 
 cessfully on room-and-pillar working sec- 
 tions at several mines. One such mine 
 had a single conventional room-and-pillar 
 section as shown in figure 3-13. The 
 seam height was medium low (42 to 48 
 inches). The section radio system con- 
 sisted of walkie-talkie radios carried by 
 various miners and a radio repeater lo- 
 cated at a communications center (known 
 as the communication sled) , which was 
 placed near the power sled. The foreman, 
 mechanic, shot firer, and a utility 
 cleanup man were equipped with 2-watt ra- 
 dios operating on two channels, 454 and 
 457 MHz. The purpose of the repeater was 
 twofold: (1) To extend coverage beyond 
 the direct portable-to-portable range, 
 
 To mine 
 phone system 
 
 ^ 
 
 
 
 T 
 
 
 
 1 
 
 
 
 1 
 
 
 
 
 
 R 
 
 
 
 
 
 
 
 
 
 ^ ,o 
 
 Interconnect 
 Antennas 
 
 Radiating cable 
 Radio repeater 
 
 Portable accessories 
 Portable radios 
 
 FIGURE 3-12. - UHF radio system "building 
 blocks." 
 
 Seam height; 
 42 — 48 inches 
 
 FIGURE 3-13. - Section layout of room-and- 
 pillar section. 
 
49 
 
 and (2) to provide an interconnect 
 between the radio system and a system of 
 carrier phones, which were mounted on 
 mobile machines and interconnected by 
 means of the trailing cable conductors to 
 the machines. It was thus possible to 
 communicate between roving miners 
 equipped with radios and machine opera- 
 tors equipped with powerline carrier 
 phones. Paging into the section radio 
 system from a surface point was also pos- 
 sible via a surface-to-section carrier 
 phone link and a special interface in the 
 communication sled. A low-frequency 
 through-the-earth radio link between the 
 surface and communication sled was also 
 provided, as shown in figure 3-14. At 
 this mine the portable radios by them- 
 selves were usable over an area encom- 
 passing more than half of the working 
 section. With the repeater, sectionwide 
 radio coverage was possible. 
 
 A similar system was used at another 
 room-and-pillar mine utilizing continuous 
 mining machines. This section radio com- 
 munication system also included machine- 
 mounted carrier phones and a surface- 
 to-section interface at a communication 
 sled. Conditions at this mine were much 
 more favorable for radio communication. 
 
 CARRIER SIGNAL 
 
 THRU - THE - EARTH SIGNAL 
 
 UHF RADIO REPEATER 
 
 SECTION 
 
 PORTABLE 
 
 RADIOS 
 
 
 FOREMAN (PAGE) 
 
 DRILLER CUTTER / BOLTER LOADER 
 
 UTILITY CLEANUP 
 
 SHOT FIRER 
 
 WIRED LINKS 
 
 -RADIO LINKS 
 
 FIGURE 3-14. - Radio link between surface 
 and communication sled. 
 
 mainly because the seam height was 6 to 
 7 feet. Direct portable-to-portable com- 
 munication was generally good over an 
 area encompassing up to three-quarters of 
 the working section, although some dead 
 zones were encountered where several 
 corners had to be traversed. When the 
 face was at maximum advance from the 
 power center, the repeater located in the 
 communication sled near the power center 
 was out of reach of some portable radios; 
 however, this could be rectified by 
 extending the repeater antenna toward the 
 face area by means of a coaxial cable. 
 
 3.4.2 Longwall Mining 
 
 With investment in a longwall face 
 being in the millions of dollars, and 
 production delays amounting to hundreds 
 of dollars a minute, positive control and 
 communication must be obtained. A repre- 
 sentative longwall face crew might com- 
 prise a foreman, two shearer operators, 
 three chock advance miners , one or two 
 mechanics, a headgate operator, and one 
 miner at the tailgate. Voice communica- 
 tion is frequently required between each 
 of these crew members and between the 
 headgate and tailgate. Since miners at 
 the face must work under rather crowded 
 conditions, starting and stopping the 
 conveyor and mining machines are particu- 
 larly crucial operations. It is essen- 
 tial that everyone on the face knows what 
 is happening. During maintenance opera- 
 tions, frequent interchange of informa- 
 tion between miners working at various 
 points along the face is required. Good 
 communication will improve the capability 
 of describing and locating problems and 
 coordinating maintenance efforts to 
 reduce downtime. 
 
 Figure 3-15, a longwall installation 
 in low coal, dramatically illustrates the 
 limited working space in longwall mining. 
 The area consists of a long lateral tun- 
 nel in which equipment may be easily 
 damaged. Moreover, it is fatiguing to 
 travel any appreciable distance to get to 
 a phone placed along the face. Acoustic 
 
50 
 
 FIGURE 3-15. = Typical conditions encountered 
 in longwall mining. 
 
 noise is also very high. Therefore, a 
 communication system designed specifi- 
 cally for longwall mining applications 
 should meet the following requirements: 
 
 1. Minimum size. 
 
 2. Rugged. 
 
 3. Direct acoustic sound along the 
 face. 
 
 4. Rugged cable and connector de- 
 sign to survive in the harsh environment. 
 
 5. Sufficient power to permit oper- 
 ation along the maximum length of the 
 longwall. 
 
 6. Certain control and signaling 
 features that can be incorporated into 
 the phone system. 
 
 U.S. longwall faces commonly use 
 standard U.S. pager phones as a means of 
 implementing interface communication. 
 However, these systems do not provide an 
 adequate face communication system. 
 Major problems are as follows: 
 
 1. The phones and cables are easily 
 damaged owing to the close quarters and 
 severe environment. 
 
 2. Miners on the face may have to 
 travel 50 to 100 feet to use phones; 
 sometimes phones can survive only at the 
 headgate or tailgate, which is marginally 
 
 acceptable on a short face, say 250 feet, 
 but unsatisfactory on faces as long as 
 400 feet; in contrast, phones are placed 
 40 to 50 feet apart in West Germany. 
 
 3. The conveyor creates a high- 
 noise environment, and shearer noise 
 often makes it impossible for shearer 
 operators to hear a page. 
 
 4. Communication is required later- 
 ally along the face, and U.S. pager 
 phones have not been designed with this 
 in mind. 
 
 Several European systems, however, 
 are available that have been specifically 
 designed for longwall applications. Fig- 
 ure 3-16 shows one type of phone, which 
 has already been installed in a few U.S. 
 longwalls and is reportedly well accept- 
 ed. Figure 3-17 shows the main control 
 unit, which is installed at the headgate. 
 Some of the features of European longwall 
 pager phone systems are pull-wire signal- 
 ing, machinery lockout buttons, prestart 
 warning, fault detectors (in some cases) 
 which stop the machinery, blast-proof de- 
 sign, and a central power supply at the 
 headgate with standby batteries in the 
 individual phone units. 
 
 For the potential U.S. user, there 
 are, of course, problems associated with 
 this equipment. The first is expense. A 
 10-phone system incorporating all the 
 desired features may cost around $25,000. 
 A 10-phone system with voice-only capa- 
 bilities might cost only about $6,000 to 
 $7,000, but this is still at least twice 
 
 FIGURE 3-16. - Longwall phone. 
 
51 
 
 ^ 
 
 FIGURE 3-17, - Main control unit. 
 
 as much as a U.S. pager phone system. 
 Secondly, there are a limited number of 
 suppliers. Thirdly, a mine may have to 
 either carry its own inventory or expect 
 long lead times in getting spare parts. 
 Finally, in-house maintenance skills have 
 to be developed. However, given the high 
 cost of a longwall system ($1 to $2 mil- 
 lion) , a proper understanding of the 
 value of a good phone system in reducing 
 downtime indicates that these systems are 
 still worth considering. 
 
 With any system, certain individuals 
 should be able to communicate from any 
 location along the chockway without the 
 fatiguing ordeal of crawling to a phone. 
 This requirement cannot be totally met by 
 any wired phone system, and with some 
 exceptions wireless radio for longwalls 
 
 
 - 300 TO 600 FT TYPICAL 
 
 LONGWALL FACE 
 
 ^... 
 
 d 
 
 FIGURE 3-18. - Repeater-based UHF radio sys- 
 tem layout for longwalls. 
 
 is not feasible at ultrahigh frequencies. 
 Table 3-2 summarizes the important points 
 regarding the design and implementation 
 of a longwall UHF radio system. However, 
 cable-aided UHF radio is feasible and may 
 be another choice for obtaining the 
 linear tunnel coverage required on a 
 longwall section. 
 
 On shorter faces , a radiating cable 
 extending along the length of the long- 
 wall and passively terminated at each end 
 with a suitable antenna can provide face 
 coverage without a repeater. A radio 
 repeater, connected to the cable at one 
 end, may be needed on longer faces, or 
 when coverage to the head entry outby the 
 headgate is required. A repeater-based 
 configuration for a longwall UHF radio 
 system is shown in figure 3-18. In this 
 system, good radio coverage can be ex- 
 pected along the face area and into the 
 head entry for several hundred feet. If 
 the repeater should fail, direct communi- 
 cation between portable radios is still 
 possible at reduced range. This system 
 can be implemented using commercially 
 available battery-operated hardware that 
 is also MSHA approved (fig. 3-19). 
 
 TABLE 3-2. - Ranges of completely wireless communication system 
 for longwalls at 450 MHz 
 
 Type of roof 
 
 Range with no 
 
 Range with shearer machine, 
 
 ft 
 
 support 
 
 machine, ft 
 
 High coal 
 
 Low coal 
 
 
 
 High coal 
 
 Low coal 
 
 
 Chocks 
 
 Shield 
 
 300 
 1,000 
 
 150 
 150 
 
 100 
 300 
 
 50 
 50 
 
52 
 
 The reason for this is because the solu- 
 tions to the communication requirements 
 in these systems are similar. Nontrolley 
 haulage systems include the following: 
 
 Rail vehicles with self-contained 
 power sources (battery or diesel 
 
 powered) . 
 
 All rubber-tired vehicles. 
 
 FIGURE 3=19. . MSHA=approved UHF repeater 
 and battery unit, with 'hardhat" antenna and 
 portable radios. 
 
 3 . 5 Hau 1 ageway s 
 
 Operators of vehicles in underground 
 haulageways must be able to communicate 
 with one another and with other areas in 
 the mine to improve safety and increase 
 production. 
 
 The type of systems that can be 
 implemented to meet the communication 
 requirements of underground haulageways 
 depends upon the method of haulage it- 
 self. From the standpoint of communica- 
 tions, haulage systems can be considered 
 as either trolley or nontrolley. 
 
 Trolley haulage, as used in this 
 manual, means vehicles that are tracked 
 (ride on rails) and are electrically 
 powered from an overhead trolley wire. 
 In mines using this type of haulage, a 
 carrier phone system using the trolley 
 wire is almost always used to satisfy the 
 communication requirements. The trolley 
 wire and tracks serve as the carrier 
 current path. Methods, techniques, and 
 ways of improving carrier phone systems 
 are given in section 3.5.1. Special con- 
 trol, monitoring, and communications 
 requirements involved when moving off- 
 track equipment under an energized 
 trolley wire are described in paragraph 
 4.4.3d of chapter 4. 
 
 All other forms of haulage systems 
 are grouped into the nontrolley category. 
 
 Communication systems applicable to 
 nontrolley haulage systems are described 
 in section 3.5.2. 
 
 3.5.1 Trolley Haulage 
 
 As previously mentioned, carrier 
 phones are usually used to satisfy the 
 communication requirements in haulage- 
 ways where rail vehicles that draw power 
 from an overhead trolley wire are used. 
 One reason carrier phones have become so 
 popular is that they operate over the 
 existing trolley wire dc power circuits 
 to provide two-way voice communication 
 between the tracked vehicles and with 
 fixed stations in the mine. No addi- 
 tional wires or cables must be installed 
 in the mine. In underground mining, 
 these carrier systems are used exten- 
 sively for traffic control of the tracked 
 haulage equipment and personnel carriers. 
 These phones are FM push-to-talk 
 transmitter-receiver units designed for 
 common talk (party line) operation. 
 
 Carrier frequency couplers consist- 
 ing of bypass capacitors are used to pro- 
 vide continuity of the RF signal path 
 between sections of trolley served by 
 different dc power centers. These car- 
 rier systems typically operate in the 
 60- to 200-kHz range. (See section 2.4 
 for basic theory of operation of carrier 
 current phone systems.) 
 
 The carrier phone located on each 
 tracked vehicle is primarily used for 
 control of vehicle traffic. All vehi- 
 cles are kept in communication with each 
 other and the dispatcher over the single- 
 channel (party line) carrier phone 
 system. This single-channel network 
 keeps the dispatcher and all motormen in 
 
53 
 
 continuous contact with one another so 
 that right-of-way and the disposition of 
 haulage cars will be known to all. One 
 inherent advantage of the trolley carrier 
 phone system is that it is a party line 
 system. In certain applications, this 
 would be a disadvantage since private 
 communication channels are not available. 
 For haulageway traffic control, however, 
 it is beneficial if each motorman does 
 hear conversations between other motormen 
 and the dispatcher. This phone system 
 also allows the dispatcher to notify all 
 motormen of any mine emergency. The two 
 drawbacks to this system follow: 
 
 Trolley wire power failures, which 
 cause the carrier communication system to 
 go dead unless backup batteries are 
 installed. 
 
 Dead zones, which are sections of 
 track where the phone is inoperative due 
 to excess electrical noise, excess atten- 
 uation of signal strength, or standing 
 wave effects. 
 
 The first drawback, loss of communi- 
 cation due to power outage, can be cor- 
 rected by the use of backup batteries in 
 each vehicle (required by law if the 
 carrier current system is the only com- 
 munication system in the mine) . The 
 backup batteries would normally be 
 trickle-charged to full capacity and then 
 maintained at full charge. In the event 
 of a power failure on the trolley wire, 
 the backup batteries would automatically 
 power the carrier phones and allow for 
 voice conmiunications for many hours. 
 Because the haulage system is vital to 
 mine operations, extended power outages 
 on the trolley line are not tolerated. 
 Any trolley power failure is immediately 
 recognized and corrected as soon as pos- 
 sible. Thus, communication outages due 
 to power failures are minimal. 
 
 The second problem associated with 
 some carrier phone systems is that of 
 "dead zones." There are areas where two- 
 way communication between a vehicle and a 
 dispatcher or between vehicles is not 
 possible. Dead zones are caused by 
 extreme attenuation of signals, excess 
 noise, standing waves, and/or inadequate 
 
 squelch control. The most significant 
 of these causes is the extreme attenua- 
 tion of the carrier phone signals on the 
 trolley wire-rail. The trolley wire- 
 rail is a poor radio frequency trans- 
 mission line for several reasons, the 
 most dominant of which is the presence 
 of many bridging loads between the trol- 
 ley wire and rail. Branches and the lack 
 of good electrical terminations contrib- 
 ute to the problem as well. The bridg- 
 ing loads, which both absorb and reflect 
 power, comprise such items as personnel 
 heaters, rectifiers, pumps, haulage ve- 
 hicles (motors) , locomotive and jeep 
 lights, insulators, signal and illumina- 
 tion lights, and even the carrier phones 
 themselves. 
 
 Because of the importance of good 
 communications in the haulageway s and 
 because a large number of mines use car- 
 rier phones to meet these requirements, 
 many programs have been sponsored to 
 improve trolley carrier phone systems. 
 
 One program was designed to (1) 
 identify poor performance of trolley car- 
 rier phone systems, (2) assess the causes 
 of poor performance and classify them on 
 the basis of equipment, coupling, or 
 transmission problems, and (3) propose 
 and verify the means to overcome these 
 problems. 
 
 Figure 3-20 illustrates the signal- 
 attenuation rate for an "unloaded" 
 trolley wire-rail transmission line; a 
 band of rates is shown because the actual 
 rate depends on the conductivity of the 
 surrounding medium. If an attenuation 
 rate of 1 dB/km is used, a trolley car- 
 rier phone line having an allowable 
 transmission loss of 70 dB (from 25 volts 
 to 8 millivolts) yields a communication 
 range of 70 km (43 miles). This perform- 
 ance, in the absence of bridging loads, 
 can be compared with that of a sample 
 trolley wire-rail loaded as illustrated 
 in figure 3-21. Here, just three bridg- 
 ing loads of modest value (typical of 
 vehicles and personnel heaters) reduce 
 the signal 55 dB over a distance of just 
 4,500 feet. The figure also shows the 
 signal level that would exist over the 
 same distance on a properly terminated 
 
54 
 
 2 — 
 
 
 ^ 
 
 100 200 
 
 FREQUENCY (kHz) 
 
 400 
 
 800 
 
 FIGURE 3-20, - Signal-attenuation rate for an 
 'unloaded' trolley wire-rail transmission line. 
 
 trolley wire-rail without bridging loads. 
 With such signal reductions, it is easy 
 to see why it is difficult to obtain 
 long-range transmission of carrier sig- 
 nals using the trolley wire. 
 
 There is one approach that appears 
 to have merit in overcoming the excep- 
 tionally high attenuation rates that can 
 be expected on the trolley wire-rail — the 
 dedicated wire technique. 
 
 3.5.1a Dedicated Wire 
 
 The best approach to overcoming the 
 extremely high attenuation rates imposed 
 by bridging loads is a single-purpose, or 
 "dedicated," wire. The characteristics 
 of an unloaded trolley wire-rail are such 
 that it forms a low-loss transmission 
 line. Therefore, a separate wire strung 
 in an entryway, with the same rail return 
 
 -40 
 -50 
 -60 
 -70 
 
 • DATA 
 — THEORY 
 
 DISTANCE ALONG LINE - FEET 
 
 SIMULATED LENGTH = 4460 FEET 
 
 ^ 
 
 -19 
 2000 pF 
 CAPACITORS 
 
 SHUNT LOADS 
 
 • DATA 
 THEORY 
 
 DISTANCE ALONG LINE - FEET 
 
 FIGURE 3-21, - Signal level on simulated trolley 
 wire-rail, 
 
 path as the trolley wire but unloaded by 
 any bridging loads, would similarly serve 
 as a low-loss line. Such a configuration 
 forms a three-wire transmission line. 
 
 Studies have shown that the primary 
 mode of propagation for such a configura- 
 tion is a low-loss mode supported by the 
 dedicated wire, with the rails serving as 
 the return signal path. The signal im- 
 provements that can be expected from such 
 a configuration are illustrated in figure 
 3-22, which shows the voltage signal 
 strength versus the distance along a 
 heavily loaded trolley wire-rail with a 
 parallel dedicated wire. 
 
 Four separate conditions of trans- 
 mission and reception are shown in 
 
55 
 
 high-voltage trolley wire and is 
 influenced by its loads. 
 
 not 
 
 TRANSMIT ON TROLLEY OR 
 DEDICATED WIRE AND 
 RECEIVE ON THE OTHER 
 
 TRANSMtT AND RECEIVE ON 
 TROLLEY WIRE (WITH A 
 
 DEDICATED WIRE)- 
 
 FIGURE 3-22. - Voltage signal strength versus 
 distance along heavily loaded trolley wire^rail 
 v/ith a parallel dedicated wire. 
 
 figure 3-22. For example, if the dis- 
 patcher transmits on the dedicated wire 
 and the motor operators receive on the 
 trolley wire, then the dispatcher's 
 transmission will produce a curve of 
 trolley signal level versus distance like 
 curve B. At a distance of approximately 
 12.5 miles the dispatcher's signal shows 
 only a loss of 50 dB. The remaining sig- 
 nal level is entirely adequate for opera- 
 tion of the carrier phones, since the 
 allowable transmission loss is about 70 
 dB. 
 
 The crosshatched area between 
 curves C and D illustrates the improve- 
 ment that can be obtained when both the 
 dispatcher and motor operators still 
 transmit and receive on the trolley wire 
 but when a dedicated wire has been in- 
 stalled along the haulageway. 
 
 Curve A shows the signal loss be- 
 tween fixed base stations, both of which 
 can transmit and receive over the dedi- 
 cated wire. 
 
 The dedicated wire is installed with 
 no direct connection to the trolley wire- 
 rail; however, a strong electromagnetic 
 coupling exists between the dedicated 
 wire-rail and the trolley wire-rail, 
 simply because of their physical proxim- 
 ity. Thus, the dedicated wire is not 
 jeopardized by direct coupling to the 
 
 Tests indicate that excellent re- 
 sults can be obtained with a dedicated 
 wire. The dedicated-wire concept permits 
 installation of a transmission line with 
 controlled branching that can be termi- 
 nated to avoid standing-wave patterns. 
 The price paid is that the wire has to be 
 installed and maintained in a haulageway. 
 
 A recent study (26)^ recommends that 
 the dedicated-wire method is usually the 
 most effective and practical way of up- 
 grading trolley carrier phone systems. 
 
 . Another research program provided a 
 set of five guidelines for operat- 
 ing personnel to improve their carrier 
 phone systems. These guidelines give 
 detailed instructions for installing 
 trolley carrier phone equipment onboard 
 mine vehicles and at the dispatcher's 
 room, converting a rail haulage trolley 
 wire-rail and feeder system into a 
 functional carrier-frequency-transmission 
 line, checking the performance of the 
 trolley carrier phone system, and using 
 portable test equipment to aid in system 
 maintenance. A detailed description of 
 the conclusions and the recommendations 
 set forth in these guidelines are pre- 
 sented in chapter 6 of this manual. 
 
 Rather than offering detailed com- 
 ments on the contents of each of these 
 guidelines here, we focus on just one 
 aspect of the guideline concerned with 
 converting the trolley wire-rail into an 
 efficient transmission line. In the pre- 
 ceding discussion on the causes of poor 
 performance, the extremely poor propaga- 
 tion characteristic of the trolley wire- 
 rail was cited. Apparently, this poor 
 propagation dominates in determining the 
 performance of trolley carrier phone sys- 
 tems. Thus, it is appropriate that seri- 
 ous consideration be given to determining 
 the signal and noise levels on each 
 trolley wire system. Signal strength and 
 
 ^ Underlined numbers in parentheses 
 refer to items in the bibliography at the 
 end of t±iis chapter. 
 
56 
 
 electromagnetic noise level measurements 
 should be made at points along the 
 trolley and noted on a mine map. The 
 procedure is simple. The dispatcher's 
 transmitter is used as the signal source, 
 and both the strength of the signal along 
 the haulageway and the corresponding 
 noise level are measured. This measure- 
 ment is conveniently made by equipping a 
 jeep with a tuned voltmeter. The jeep 
 moves along the haulageway, stopping at 
 intervals of about 2,000 feet. The opera- 
 tor calls the dispatcher and asks for a 
 10-second keying-on of his transmitter. 
 The received voltage on the tuned volt- 
 meter is noted on a mine map, and the 
 noise level is also noted. This map then 
 identifies regions of the mine where ex- 
 cess noise may be the problem, as well as 
 regions where weak signal levels cause 
 problems. The map also aids in identify- 
 ing the key bridging loads branches, or 
 unterminated lines that can cause prob- 
 lems. This signal- and noise-mapping 
 process is the key to identifying the ma- 
 jor causes of poor signal reception in a 
 particular mine. 
 
 Once the probable source of diffi- 
 culty has been identified, the remaining 
 part of the guidelines can be consulted 
 to determine possible ways of treating 
 the problem. For example, if a rectifier 
 is affecting signal propagation, the 
 guidelines provide three different ways 
 to treat the rectifier to reduce the 
 problem. 
 
 means of dispatcher-vehicular communica- 
 tions. Most problems associated with 
 these systems are transmission line re- 
 lated; a trolley line was never intended 
 to be a good communications line, and it 
 certainly is not. However, techniques do 
 exist for improving overall communica- 
 tions. These techniques can be easily 
 implemented, and the results are often 
 excellent. 
 
 3.5.2 Nontrolley Haulage 
 
 An increasing number of both newly 
 developed and older mines have been aban- 
 doning tracked trolley vehicles and are 
 conducting their haulage, maintenance, 
 and personnel transport operations with 
 other types of vehicles. Obviously, com- 
 munications to and from vehicles operat- 
 ing independently of a trolley wire can- 
 not be implemented by the trolley carrier 
 phones discussed in the previous section. 
 
 Communication systems required for 
 battery- or diesel-powered rail vehicles 
 or rubber-tired vehicles have one common 
 characteristic. Because these vehicles 
 are not physically attached or connected 
 to any wiring or other conductor in the 
 mine, some form of radio link must be 
 utilized to establish the final voice 
 link with the vehicle. If voice communi- 
 cation exists from a nontrolley vehicle, 
 then an antenna-radio link of some form 
 must be used to replace the direct con- 
 nection provided by the trolley wire. 
 
 3.5.1b Summary 
 
 Communications with moving tracked 
 vehicles in a rail haulage mine pose a 
 difficult problem. These communications 
 take place from dispatcher to vehicles or 
 from vehicle to vehicle via the trolley 
 line, which is a very poor communications 
 line. As a result, dead spots and high- 
 noise areas can occur anywhere along the 
 line; also, signal strength can decrease 
 simply as a function of distance. 
 
 Although trolley carrier phone sys- 
 tems leave much to be desired for haul- 
 ageway communications, the fact remains 
 that they do represent one practical 
 
 Several methods exist for providing 
 communication between nontrolley mining 
 vehicles. Studies have been conducted of 
 high-frequency systems utilizing the so- 
 called leaky-coax cable to carry signals 
 throughout a mine. Other studies in the 
 wireless radio area have shown that at 
 medium frequencies, signals follow the 
 existing mine wiring for great distances. 
 
 3.5.2a Leaky-Coax Systems 
 
 A leaky coax is a special type of 
 coaxial cable that allows radio frequency 
 signals to leak into and out of itself. 
 With this type of cable, signals can be 
 transmitted to and received from mobile 
 
57 
 
 radio units near the cable. Leaky coax 
 is therefore ideally suited for haulage 
 applications. In effect, the cable 
 guides the radio signals down the tunnel 
 (fig. 3-23). Although signal strength 
 does attenuate along a cable run, re- 
 peaters or in-line amplifiers can be used 
 to extend the range of coverage. Several 
 techniques have been used: 
 
 1. Borrowing from conventional 
 mobile radio communications practice, 
 individual fixed-base stations can be 
 installed at intervals as necessary to 
 provide the total range, all stations 
 being under a common remote control with 
 the first. Such a system has been in use 
 at a British mine since 1970. 
 
 an audio return line is required and, 
 when branches are required, the system 
 can become complex. 
 
 3. Multiple-frequency repeater 
 schemes (fig. 3-25) have also been used 
 successfully; the simplest uses one 
 transmitter and one receiver. 
 
 Communication benefits of a leaky- 
 coax system are typified by one system 
 developed for an iron ore mine (block- 
 caving operation) using rubber-tired, 
 diesel-powered vehicles. The system 
 chosen to satisfy the communication 
 requirements at this mine consisted of a 
 UHF leaky-coax system. Figure 3-26 is a 
 simplified diagram of the system. 
 
 2. A series of one-way in-line 
 repeaters, such as the daisy-chain system 
 shown in figure 3-24, is effective; it 
 does have a slight disadvantage in that 
 
 AREA OF RADIO COVERAGE 
 FREQUENCY - 420 MHz 
 SIGNAL STRENGTH (VOLTAGE OR ELECTRIC FIELD! 
 
 . " 2000 FEET 
 
 LIMIT OF COMMUNICATIONS IN HAULAGEWAV 
 
 FIGURE 3-23. - Cable "guiding" radio signal 
 down a tunnel. 
 
 t> — — > 
 
 LEAKY FEEDER WITH ONE-WAY REPEATERS 
 
 i 
 
 FIGURE 3-24. - Blockdiagramof a daisy-chain 
 repeater system. 
 
 In addition to providing communica- 
 tion to personnel carriers, maintenance 
 and production vehicles, and the 
 ambulance (fig. 3-27) , the system pro- 
 vides communications for roving miners, 
 foremen, fan-hole-drill operators, and 
 supervisors. Communication requirements 
 were satisfied by using (1) UHF wire- 
 less radio, (2) a radiating coaxial cable 
 or "leaky" transmission line to carry 
 the signal throughout the haulage and 
 subdrifts of the mine, (3) interconnected 
 VHF and UHF repeaters, (4) portable 
 transceivers, and (5) vehicle-mounted 
 transceivers. 
 
 I- 
 
 REQUENCY, 420 ^ 
 
 REPEATER OR BASE 
 STATION WITH 
 SURFACE INTER- 
 CONNECT 
 
 ANTENNA TERMINATION 
 
 SECTION eRANCH OFF 
 
 ^. 
 
 HAULAGEWA' 
 1.200 METERS 
 
 ROVING HANDE TALKIE 
 
 FIGURE 3-25. - Two-frequency repeater concept. 
 
58 
 
 FAN HOLE 
 
 DRILL 
 
 OPERATOR 
 
 AMBULANCE 
 
 MAINTENANCE 
 VEHICLE 
 
 FIGURE 3-26. 
 system. 
 
 RF leaky-coax communication 
 
 FIGURE 3-27. - UHF mobile radio mounted on 
 underground ambulance. 
 
 The mine was divided into two RF re- 
 gions, with each region (zone) containing 
 one UHF-VHF repeater station and associ- 
 ated runs of leaky coax. The system of 
 cables effectively wires the mine for UHF 
 signals between portable and mobile un- 
 its. Each repeater station can receive 
 and transmit signals on the cable at both 
 UHF and VHF. VHF signals are used on 
 the cable as a communication link between 
 the stations, while the UHF is used for 
 the communication link to and from the 
 portable and mobile units. The two UHF 
 
 repeaters transmit on F2 and receive on 
 Fl as shown in figure 3-28. The VHF re- 
 peaters use frequencies F3 and F4 to in- 
 terconnect the two UHF zones. Each UHF- 
 VHF repeater station can simultaneously 
 transmit and receive on both UHF and VHF. 
 
 The mobile radios transmit on Fl and 
 receive on F2. All information therefore 
 goes to the repeaters, then back to all 
 other units. The portable radios are 
 also capable of transmitting on F2 and 
 therefore are able to talk to one another 
 without the repeaters on a local simplex 
 basis. Audio control lines are provided 
 from the crusher console to repeater 
 station A and from the surface guardhouse 
 to the shaft bottom station, thus provid- 
 ing system access from two hardwired 
 locations as well as an important emer- 
 gency link to the surface. 
 
 As an example, suppose that a mobile 
 radio in zone A wants to talk to a roving 
 miner equipped with a portable radio in 
 zone B. The operator in zone A keys 
 his radio and talks into his microphone 
 to transmit his message on UHF Fl . The 
 UHF signal is coupled to the leaky coax 
 and travels to the UHF-VHF repeater in 
 zone A. Repeater A rebroadcasts the mes- 
 sage back to zone A on UHF F2 and also 
 sends the message to the repeater in 
 zone B on VHF F4. This signal travels on 
 the coaxial cable to UHF-VHF repeater B 
 where it is picked up by the VHF re- 
 ceiver. The signal is then converted to 
 UHF F2 and routed onto the leaky coax for 
 distribution in zone B. 
 
 V 
 
 a — ^ 
 
 '^ 
 
 ri 
 
 d 
 
 
 REPEATER A 
 
 REPEATER B 
 
 FIGURE 3-28. . UHF-VHF repeoler system. 
 
59 
 
 For this type of Installation, 
 specifications recommended that the cable 
 be supported every 5 feet. To avoid 
 installing a large number of anchors in 
 the rocks, a 3/16-inch steel messenger 
 wire was attached at 20-foot intervals to 
 roof-bolt-supported T-bars (fig. 3-29) . 
 The cable was then strapped to the mes- 
 senger wire with standard cable ties. 
 
 A vehicle-mounted work platform, 
 which could be mechanically raised or 
 lowered and which was equipped with a 
 frame for supporting cable reels, facili- 
 tated cable installation. The factory 
 cut the cable to predetermined lengths, 
 installed connectors, and tagged the 
 cable with location identifiers. In mine 
 areas that were so far removed from the 
 main cable that radio transmissions could 
 not be established, a stub cable was 
 installed with one end connected through 
 a power divider to the main cable and the 
 other end terminated with an antenna. 
 
 3.5.2b UHF Reflective Techniques in 
 Underground Mines. 
 
 UHF radio (300 mHz to 3 gHz) is the 
 only way of achieving true radio propaga- 
 tion in an underground mine. Propagation 
 is possible because the mine entries 
 function as waveguides that confine the 
 transmitted energy. Several thousand 
 feet of range, line-of -sight, is often 
 possible without leaky feeder cables if 
 the entries are large enough. 
 
 FIGURE 3=29o 
 
 messenger wire. 
 
 Radiax cable installation on 
 
 However, the nature of UHF is such 
 that propagation around bends and corners 
 introduces tremendous signal losses. In 
 this regard, it is similar to the trans- 
 mission of light and, like light, it can 
 be reflected by flat metallic surfaces. 
 These characteristics of UHF make possi- 
 ble a whole-mine communication system 
 that does not rely on leaky feeder ca- 
 bles. The Bureau of Mines evaluated such 
 a system in an underground limestone mine 
 that had large dimension haulageways. A 
 UHF reflective radio system was designed 
 to allow communication between super- 
 visory personnel, maintenance personnel, 
 haulage operators, and surface opera- 
 tions. Communication was also provided 
 between the hoist operator and slope car 
 occupants. A closed circuit television 
 (CCTV) system allowed continuous, remote 
 visual monitoring of critical belt trans- 
 fer points and underground dust disposal 
 operations. 
 
 The Black River Mine was selected as 
 a typical metal-nonmetal room and pillar 
 mine. It is nearly 4,000 feet in diam- 
 eter, 650 feet deep, and has essentially 
 straight crosscuts approximately 30 feet 
 wide and 24 to 40 feet high with pillars 
 approximately 35 feet square. Entry is 
 through a 2,200-foot slope by means of a 
 single drum, hoist-powered flat car and 
 enclosed man carrier. Rubber tired, die- 
 sel powered mine vehicles travel along 
 designated haulage and travel roads from 
 the active faces on the mine's perimeter 
 to two rock crushers, the shop area, and 
 the base of the slope. 
 
 Tests of communication between hand- 
 held, 2-watt UHF transceivers in the room 
 and pillar limestone mine were satisfac- 
 tory for approximately 2,000 feet through 
 straight haulageways but the range of 
 comminication at right angles to haulage- 
 ways into Intersecting crosscuts was 
 quite limited. It was evident that the 
 radiation from the transceivers was not 
 being reflected by the limestone pillars 
 into the intersecting crosscuts. 
 
 To improve communication in inter- 
 secting haulage roads, 27 passive re- 
 flectors were designed and installed at 
 
60 
 
 major intersections. The reflectors were 
 formed from 4- by 8-foot sheets of No. 16 
 gage soft aluminum sheet that were sus- 
 pended from wires attached to roof plates 
 and bolt anchors. The roof height was 
 sufficient to allow haulage vehicle 
 clearance at each installation. Two dis- 
 tributed antenna systems were designed 
 to provide either an antenna or reflec- 
 tor at the intersection of principal 
 haulage and travel roads. Each antenna 
 system consisted of approximately 1,200 
 feet of 7/8-inch low loss foam dielectric 
 transmission line which fed, through 2:1 
 power dividers, four 5-dB gain mobile 
 whip antennas that were suspended at 
 intersections. 
 
 A leaky coaxial cable anatenna sys- 
 tem along the principal haulage and 
 travel roads was considered but rejected 
 because the range of communication at 
 right angles to the leaky cable into in- 
 tersecting crosscuts would have been much 
 less than the range of communication from 
 antennas. The leaky cable system is 
 appropriate for long tunnels but not for 
 intersecting roads in a room and pillar 
 geometry. Also, a leaky cable would be 
 more expensive. 
 
 One central, or "backbone" coaxial 
 cable carried 60 Hz power, radio signals, 
 and CCTV signals for the entire system. 
 Redundant routing of the backbone cable 
 insured continued system operation in the 
 event of a cable break. 
 
 Fourteen 11-watt mobile radios 
 equipped with automatic identification 
 and emergency alarm encoders were in- 
 stalled on vehicles in the mine. The en- 
 coders are used on mine haulage trucks to 
 automatically send three status signals; 
 truck bed up (dumping) , truck bed down, 
 and hot engine. This information is dis- 
 played by number codes along with the 
 truck's identification number on display 
 units in the engineering office above 
 ground and the mine foreman's office un- 
 derground. A record of all calls, sta- 
 tus, and alarm messages is automatically 
 printed in the engineering office. 
 
 Fifteen 2-watt portable transceivers 
 equipped with automatic identification 
 and emergency alarm encoders are used by 
 mine department heads, foremen, and per- 
 sonnel in the mine. 
 
 Signal margin measurements of the 
 base-repeater station signals along 
 haulage and travel roads were made after 
 both distributed antenna systems had 
 been completed, which demonstrated that 
 approximately 75% of the mine area re- 
 ceived satisfactory signals, but active 
 mining areas along the perimeter of the 
 mine were not adequately served. The 
 distributed antenna system would have 
 to be extended to serve additional anten- 
 nas near the mine faces. However, the 
 cable attenuation would drastically re- 
 duce the power radiated from the antennas 
 and the signals received from the mobiles 
 and portables so that very little im- 
 provement would be realized. Additional 
 base-repeater stations were considered; 
 however, the added complexity and cost of 
 multiplexing equipment and for extending 
 the backbone cable control system stim- 
 ulated the development of a low-cost, 
 two-way multichannel signal booster sys- 
 tem. A prototype signal booster was con- 
 structed and tested. Six amplifier sig- 
 nal boosters, 10,000 feet of cable, and 
 16 additional antennas were installed in 
 the mine. Subsequent signal measurements 
 showed adequate coverage of all desired 
 areas. 
 
 3.5.2c Dedicated-Wire Radio Systems 
 
 It is possible to use trolley car- 
 rier current techniques and hardware to 
 communicate with vehicles that do not use 
 a trolley line, such as battery-powered 
 railed or rubber-tired vehicles. How- 
 ever, in this case, a "dedicated wire" is 
 essential for proper operation. Such a 
 system is shown in figure 3-30. 
 
 The dedicated wire takes the place 
 of the trolley line. However, since the 
 carrier phone on the jeep communicates 
 with the dedicated wire by a loop 
 antenna, instead of touching it like it 
 
61 
 
 DISPATCHER 
 
 y 
 
 LOOP ANTENNA 
 
 ( ) 
 
 
 
 
 
 
 
 
 CARRIER 
 PHONE 
 
 
 
 
 
 "^ 
 
 ^S7 
 
 ^S^ 
 
 BATTERY POWERED JEEP 
 
 FIGURE 3-30. - Dedicated-wire radio system. 
 
 would a trolley line, this system is rel- 
 atively inefficient. In general, it is 
 usually necessary for the loop antenna to 
 be rather close to the dedicated wire for 
 communications. This problem is caused 
 in part by the fact that carrier phones 
 were never intended to use antennas, and 
 cannot operate at high enough frequencies 
 to make this approach efficient. 
 
 However, research has shown that 
 such a system operates well if medium 
 frequencies are used. These frequencies, 
 usually around 500 to 900 kHz (as opposed 
 to 100 kHz typical of trolley carrier 
 phones) can operate with loop antennas 
 very efficiently. Considerable research 
 is being done by industry and the Bureau 
 of Mines to develop whole-mine medium- 
 frequency systems. 
 
 3. 5. 2d Wireless Radio System 
 
 discussing the advantages and disadvan- 
 tages of each technique, the subject of 
 radio interference and signal attenuation 
 in underground mines must be considered. 
 
 3.5.2d.l Interference 
 
 During normal operation, the machin- 
 ery used underground creates a wide range 
 of many types of intense electromagnetic 
 interference (EMI), which is a major lim- 
 iting factor in the range of a radio com- 
 munication system. EMI generated in 
 mines is generally a random process. 
 Therefore, the most meaningful parameters 
 for EMI are statistical ones. In work by 
 the National Bureau of Standards, time 
 and amplitude statistics have been used 
 in order to unravel the complexities of 
 EMI noise in mines. Without going into 
 the details of data collection techniques 
 or advanced statistical analysis, we will 
 summarize the findings and conclusions on 
 EMI affecting haulageway radio communica- 
 tions. Figure 3-31 shows interference 
 levels measured along haulageways in four 
 different mines. 
 
 The EMI noise levels shown for 
 mine 1 are based on measurements made in 
 a mine located in southwestern Pennsyl- 
 vania. Room-and-pillar techniques were 
 used with mining accomplished using a 
 continuous miner, shuttle cars, and elec- 
 tric trolley rail haulage. 
 
 An obvious advantage of any true ra- 
 dio system is that the system requires no 
 transmission lines or cables. These sys- 
 tems are immune to communication outages 
 caused by line breaks due to roof falls 
 or damage from machinery. However, the 
 underground mining industry cannot take 
 for granted the utilization of wireless 
 communications as can their counterparts 
 on the surface. As an example, at CB ra- 
 dio frequencies, reliable communication 
 in a mine entry is limited to about 100 
 feet. Two options are available to the 
 underground mine operator: (1) To use 
 frequencies that are high enough to uti- 
 lize the entries as waveguides, or (2) to 
 use frequencies that are low enough that 
 propagation through the earth, or by par- 
 asitic coupling, can be insured. Before 
 
 ^ HIGH 
 
 INTERFERANCE 
 AREA 
 
 LOW 
 
 INTER 
 
 AREA 
 
 ERANCE 
 
 
 
 i 
 
 
 
 
 
 \ 
 
 L 
 
 
 MINEl 
 
 HIGH COAL, RAIL HAULAGE 
 
 
 
 
 T 
 
 S^ 
 
 / 
 
 
 -BLOCK CAVING 
 
 
 MINE 2 
 
 LOW COAL. BELT HAULAGE 
 
 MINE 3 
 
 IRON MINE 
 
 
 MINE 4 
 HIGH COAL 
 
 1 
 
 BELTH 
 
 <> 
 
 / 
 
 
 _ 
 
 AULAGE 
 
 1 1 1 
 
 1 
 
 1 1 
 
 - 
 
 100 120 
 
 FREQUENCY^Hz 
 
 FIGURE 3-31. - Interference levels measured 
 along haulageways in four mines. 
 
62 
 
 The majority of noise measurements 
 were made in an area where the overburden 
 ranged from 600 to 900 feet. The entire 
 mine, including all machinery, is powered 
 by 600 volts dc. All conversion from 
 alternating to direct current is done on 
 the surface, with the result that no ac 
 power is brought into this mine. 
 
 The EMI noise levels shown for 
 mine 2 are based on measurements made in 
 West Virginia. The coal in this mine 
 occurs in a narrow seam, approximately 
 3 feet thick, and is called low coal. 
 The measurements were made in the two 
 sections of the mine using the longwall 
 mining technique where overburden was 
 between approximately 900 and 1,500 feet. 
 The mine also had seven conventional 
 room-and-pillar sections. This mine used 
 250-volt dc trolley haulage to carry coal 
 out of the mine, and ac-powered conveyor 
 belt haulage from the section to the 
 trolley. All of the section longwall 
 mining equipment was ac powered, with the 
 exception of a dc-powered cable winch 
 which was used occasionally to advance 
 portions of the longwall equipment. The 
 face and associated longwall equipment 
 were 450 feet long. There were a total 
 of six electric motors in the section 
 ranging from 15 to 300 hp. The shear and 
 face conveyor were powered by 950 volts, 
 and the stage loader and hydraulic pumps 
 operated from 550 volts. The stepdown 
 transformer supplying these voltages was 
 kept approximately 150 to 700 feet back 
 from the face and was supplied with 
 13,200 volts. 
 
 The EMI noise levels shown for 
 mine 3 are based on measurements made in 
 a Pennsylvania iron mine. The level 
 where measurements were taken was approx- 
 imately 2,300 feet below the surface. 
 The ore body is a large, flat, oval de- 
 posit about 300 feet thick, mined by un- 
 dercutting and allowing the ore to cave 
 into drawpoints called entries. Air- 
 cooled, V-8 diesel-powered, rubber-tired, 
 load-haul dump (LHD) vehicles were used 
 to haul the ore to the underground crush- 
 er and dump it into the ore crusher; it 
 
 was transported by conveyor belt horizon- 
 tally 825 meters, then lifted to the 
 surface by a skip. The other types of 
 haulage equipment used in this mine also 
 were diesel powered and rubber tired. 
 All haulageways were through reliable 
 rock or were heavily reinforced with con- 
 crete and steel. The mine used a mixture 
 of incandescent, mercury-arc, and fluo- 
 rescent lighting. 
 
 The noise levels for mine 4 were 
 made in a West Virginia mine where room- 
 and-pillar mining techniques were used. 
 The measurements were made primarily in a 
 section where overburden was approxi- 
 mately 600 to 900 feet. Mining was 
 accomplished using a continuous miner, 
 head-loader, shuttle cars (buggies), con- 
 veyor belt, and electric trolley haulage. 
 The electric trolley and the shuttle cars 
 were powered by 300 volts dc. All other 
 equipment, including fans and rock dust- 
 ing machines, was ac powered. 
 
 The noise measurements taken in 
 haulageways of these mines tended to show 
 magnetic field strengths typically 60 to 
 70 dB pA/m up to a few kilohertz, which 
 then decreases sharply above 8 to 12 
 kHz. 
 
 As seen in figure 3-32, the EM noise 
 amplitude decreases with increasing fre- 
 quency; however, three propagation mech- 
 anisms must be considered: (1) Through 
 the earth, (2) through the entries sup- 
 ported by metallic structures and con- 
 ductors, and (3) through the entries 
 where they serve as a "waveguide." For 
 propagation through the entries, it would 
 appear, from the data presented, that se- 
 lection of frequencies much greater than 
 100 kHz would be desirable. 
 
 For situations in which the propaga- 
 tion is directly through the earth, at- 
 tenuation (signal loss) increases as fre- 
 quency is increased. Because of lower 
 attenuation at lower frequencies, better 
 signal-to-noise ratios exist at low fre- 
 quency despite the higher noise levels. 
 
63 
 
 GENERAL 
 
 LOSS 
 
 IN SIGNAL 
 
 STRENGTH 
 
 FIGURE 3-32. = EM noise amplitude decrease 
 with increasing frequency. 
 
 3.5.2d.ii Signal Attenuation in the 
 Haulageway 
 
 As a radio signal travels dovm 
 a haulageway or tunnel, its strength 
 decreases. Typical signal attenuation 
 along a straight tunnel, for three dif- 
 ferent radio frequencies, is shown in 
 figure 3-32. Transmission loss may be 
 combined directly with transmitter power 
 and antenna gains to determine the re- 
 ceived signal for any candidate UHF sys- 
 tem. In terms of transmission loss, a 
 pair of 1-watt UHF walkie-talkies has a 
 range of 143 to 146 dB. 
 
 Significant propagation characteris- 
 tics apparent from figure 3-32 are — 
 
 Attenuation (in decibels) Increases 
 nearly linearly with increasing distance. 
 
 Transmission loss decreases signif- 
 icantly at a given distance as the fre- 
 quency is increased. 
 
 3. 5. 2d. ill Signal Attenuation Around 
 Corners 
 
 Observed signal attenuation around 
 a corner is also shown in figure 3-32. 
 Corner attenuation is plotted in deci- 
 bels relative to the signal level ob- 
 served in the center of the main tunnel. 
 
 Figure 3-32 shows that signal attenuation 
 around a corner is considerable. Because 
 of the high attenuation of a single cor- 
 ner, propagation around multiple corners 
 is even more severely attenuated. 
 
 Although it is an advantage to oper- 
 ate at a higher frequency in a straight 
 tunnel, the higher frequencies suffer 
 the greatest loss in turning a corner. 
 Therefore, the choice of frequency is 
 often dictated by the type of coverage 
 desired. 
 
 Based on the interference and signal 
 attenuation rates observed, the effective 
 communication range for UHF radios can be 
 predicted. Figure 3-33 shows the pre- 
 dicted range for a 1,000-MHz, 1-watt 
 portable transceiver. 
 
 The presence of stoppings for direc- 
 tion of airflow, passages blocked by 
 machinery, or blockage caused by a roof 
 fall seriously limits the communication 
 range of a UHF system. Obstructions 
 highly attenuate all UHF signal transfer, 
 thus making the same systems impractical 
 for some mine applications. 
 
 3300 FT 
 
 XDOL 
 
 JJLL 
 
 pjcm 
 
 UJJJDODnnpZLLi 
 
 3300FT -.ii^ 
 
 LZiuuuuun^nGMuu! iL_\_} 
 
 — innnnnnrTTininnnn rnn — 
 rpTT nnnn rtT"^ i • ' ' 
 i I I i i n.nrr-i 1 i i i 
 
 JJC£ 
 
 TjCL 
 
 ES 
 
 jnrr 
 
 a 
 
 i I I M I n-rr 
 
 } \ i 1 I I I I I 1 I I t L 
 
 i 1 I I I I J J I V 1 ; 1 I I ; J 
 
 FIGURE 3-33. - Predicted range for 1,000-MHz, 
 1-watt portable transceiver. 
 
64 
 
 3.5.3 Belt Haulage 
 
 Mines using conveyor belts to move 
 coal or ore underground usually have a 
 secondary transportation system for the 
 movement of men and materials. When the 
 man-material transportation system is 
 tracked-trolley , the obvious solution to 
 haulageway communication requirements is 
 the trolley phone system described in 
 section 3.5.1. If the man-material 
 transportation system is nontrolley, then 
 some form of radio, leaky feeder, or 
 wired phone system is required. 
 
 A coimnon practice in mines using 
 belt haulage is to locate telephones at 
 the intersections of all mains and sub- 
 mains, and at the head and tail of all 
 working conveyor belts. (Belt fires most 
 often occur at these points.) In the 
 absence of trolley phones, belt haulage 
 mines also usually locate phones approxi- 
 mately every 600 feet along the belts. 
 These phones are installed for the life 
 of the mine and are seldom moved. 
 
 Although a fully developed submain 
 might have butt entry ports every 
 600 feet along its length, telephones are 
 required only at the active or working 
 butt entry ports. This usually limits 
 the maximum number of phones per submain 
 to six, owing to the capacity of most 
 haulage systems. These phones are moved 
 about every year or so until all panels 
 in the submain have been developed. If a 
 feeder belt is used in the submain, addi- 
 tional phones are recommended at the head 
 and tail of these belts. 
 
 Phones permanently installed at the 
 head and tail, and at other strategic 
 locations along the belt, usually meet 
 the communication requirements during 
 normal day-to-day operations. The draw- 
 back to any wired phone system is that a 
 miner must be at a phone to make or 
 receive a call. Communication with belt 
 maintenance or inspection personnel mov- 
 ing along the haulageway can only be 
 accomplished by some form of radio link. 
 
 The same systems described in sec- 
 tion 3.5.2 (nontrolley haulage) can be 
 utilized to meet the communication 
 requirements in belt haulageways. 
 
 3.6 Special Requirements 
 
 This section describes ways of meet- 
 ing those special communication require- 
 ments not directly related to the mine 
 entrance (section 3.2), permanent and 
 semipermanent locations (section 3.3), 
 mining areas (section 3.4), and haulage- 
 ways (section 3.5). Major topics 
 included in this area of special require- 
 ments include communications with roving 
 or isolated personnel and motorman- 
 to-snapper communications. 
 
 3.6.1 The Roving or Isolated Miner 
 
 A modern mine is a vast underground 
 complex of working sections, haulageways, 
 and repair shops , which extends for sev- 
 eral square miles underground. Key per- 
 sonnel may not work in fixed locations; 
 for instance, a section foreman may be 
 assigned to a single section, but that 
 section could embrace a vast area, or 
 maintenance personnel or electricians 
 could be anywhere in the mine at any 
 time. Because such personnel are impor- 
 tant to the smooth operation and high 
 productivity of a mine, considerable pro- 
 duction losses can occur if they cannot 
 be located when they are needed. 
 
 Inspectors and other management per- 
 sonnel may also be anywhere in the under- 
 ground complex. These people need to 
 stay in continuous contact with the com- 
 munication center so that they can be 
 informed of any emergencies that might 
 arise and/or make management decisions. 
 
 The maintenance crew is also spread 
 throughout the mine. To receive repair 
 requests and dispatch his crews for emer- 
 gency or nonscheduled repair work, a 
 maintenance foreman must be able to con- 
 tact individual crew members dispersed 
 throughout the mine. 
 
65 
 
 Communication requirements to and 
 from these key individuals can only be 
 completely satisfied by a wireless 
 (radio) paging or walkie-talkie system. 
 Several paging systems are presently 
 available to meet these requirements. 
 The small lightweight pagers that can be 
 carried by roving personnel are classi- 
 fied as one of three types: 
 
 Beepers (call alert). 
 
 One-way-voice (pocket pagers). 
 
 Two-way-voice (walkie-talkies). 
 
 One shortcoming of the first two 
 types of systems (beepers and pocket 
 pagers) is that the person initiating the 
 page has no way of knowing if the page 
 has been received. This can be espe- 
 cially critical in the case of the pocket 
 pager systems where voice messages can be 
 transmitted to the person being paged. 
 Because the pocket pager is a receive- 
 only device, the person being paged can- 
 not directly notify the dispatcher or 
 person making the page that he has 
 received the message. Therefore, one- 
 way-voice (pocket) pagers should only be 
 used for paging messages ("call the dis- 
 patcher," "report to the maintenance 
 area," etc.). Instructions such as "shut 
 off the number 2 pump" should not be 
 given using one-way communication devices 
 unless it can be verified that the mes- 
 sage was received and acted upon. The 
 advantages gained by any of the three 
 types of paging systems are directly 
 related to the reduced time required to 
 contact key individuals when 
 tion underground is unknown, 
 the simplest beeper systems , 
 being paged can, within a few seconds, be 
 headed for a section phone to take a 
 message. 
 
 3.6.1a One-Way-Voice (Pocket) Pagers 
 
 In a mine that uses rail haulage 
 vehicles powered from an overhead trolley 
 wire locomotive or jeep, carrier phones 
 allow the vehicle operators to com- 
 municate with each other and with a 
 
 their loca- 
 
 Even with 
 
 the person 
 
 dispatcher who controls the flow of 
 traffic. As explained in section 2.4, 
 the trolley line itself is the communi- 
 cation link between all the vehicles and 
 the dispatcher. 
 
 However, communication need not be 
 limited to phones connected to the 
 trolley line. A special carrier-current 
 tone signal can also be impressed on the 
 trolley line, which will function as a 
 long-line antenna, broadcasting the tone 
 signal into the mine where it can be 
 received by special pocket 
 (fig. 3-34). Hardware is 
 cially available that allows a dispatcher 
 to voice-page selected individuals , 
 deliver short messages, or inform them 
 where to go to receive detailed instruc- 
 tions. Figure 3-35 is a block diagram of 
 a general radio paging system based on 
 carrier-current techniques. A carrier 
 phone, located at some central loction 
 such as a dispatcher's room, is equipped 
 with a small pushbutton-encoder unit. 
 This unit causes the carrier phone to 
 transmit short tone bursts whose frequen- 
 cy depends on which pushbutton was 
 pushed. These tone bursts are transmit- 
 ted from the carrier phone in exactly the 
 same way that a voice signal would be 
 sent out. 
 
 radio pagers 
 now commer- 
 
 FIGURE 3-34. 
 radiopager. 
 
 Miner equipped with pocket 
 
66 
 
 
 
 
 TROLLEY LINE 
 
 II DOES NOT TURN 
 ON 2 
 
 
 r • 
 
 SELECTIVE PAGING 
 SIGNALS TO PAGER 1 
 
 
 
 CARRIER 
 PHONE 
 
 
 
 
 
 
 
 
 BEEP PLUS \ 
 
 VOICE 
 
 MESSAGE ^ 
 
 mil 
 
 
 
 MANUAL 
 PUSH BUTTON 
 
 POCKET PAGER 
 I TURNS ON 
 
 
 ENCC 
 
 DED 
 
 
 
 DISPATCHER'S OFFICE 
 
 FIGURE 3-35. - Block diagramof general radio- 
 paging system. 
 
 The pocket receivers that have been 
 developed to respond to these tones are 
 really small FM radio receivers that are 
 activated by the tones and remain on for 
 about 15 seconds. Once the tones have 
 been sent, the dispatcher then talks into 
 his carrier phone in the usual manner. 
 Only the pocket pager activated by the 
 tones will receive the message, so that 
 the dispatcher can selectively radio-page 
 any individual. In an emergency, a spe- 
 cial tone can activate all pagers at 
 once. The pocket pager is a receiver 
 only and cannot be used to talk back to 
 the dispatcher. Therefore, the system 
 should be used only for paging, not for 
 giving instructions. 
 
 The system shown in figure 3-35 is 
 designed so that only the dispatcher can 
 initiate a page, because he is the only 
 one who has a carrier phone equipped with 
 an encoder. However, other encoders 
 could be used with other carrier phones, 
 if necessary. Figure 3-36 shows a system 
 in which the encoder is remotely accessed 
 by a dial telephone line. Thus, any dial 
 telephone associated with the mine 
 switchboard (PBX) could be used to initi- 
 ate a page without ever being near the 
 encoder. Such a system offers an advan- 
 tage should many people have to page into 
 the mine from several surface locations. 
 To operate the system, a user goes to a 
 telephone and dials the number assigned 
 to the pager he or she wishes to call. 
 The encoder converts the telephone dial 
 pulses into tones and transmits them via 
 the carrier phone. The tones turn on the 
 
 eo3 — ^ 
 c°3 
 
 ///////////////////// 
 
 UNDERGROUND 
 
 TROLLEY LINE 
 
 V77/ 
 
 >\ 
 
 1 1 
 
 
 M 
 
 /^ 
 
 FIGURE 3-36. - Block diagram of system with 
 remotely accessed encoder. 
 
 desired pager, at which time the user can 
 speak into the mouthpiece to deliver the 
 voice message. 
 
 Existing pager receivers are 
 equipped with a small internal timer that 
 automatically turns the device off after 
 a preselected time, usually 15 seconds. 
 A continuous "On" mode is usually not de- 
 sirable because it wastes battery power. 
 With the automatic time-out feature, bat- 
 teries last for months. However, there 
 are times when the continuous monitoring 
 of the radio paging system is important 
 to certain maintenance personnel. 
 
 A radio paging system can be oper- 
 ated on a special channel (frequency), 
 or on the regular channel used by the 
 locomotives. The only difference is that 
 if both are included on the same regular 
 channel, all the carrier phones will 
 hear the paging traffic, but the pagers 
 will hear only what is sent to them 
 directly. 
 
 A radio paging system can incor- 
 porate both the automatic encoded system 
 (fig. 3-36) and a roof-bolt antenna sys- 
 tem (fig. 3-37). The automatic encoder 
 and carrier phone can be located on the 
 surface; all else is underground. The 
 in-mine roof bolts are separated by about 
 300 feet and connected to the carrier 
 
67 
 
 POCKET PAGER 
 
 
 
 MOTOnwAN 
 
 TROLLEY LINE 
 (GIVES HAULAGEWAY COVERAGE) 
 
 (GIVES REMOTE COVERAGE) 
 
 
 11 
 
 I POCKET PAGER 
 
 CARRIER 
 
 PHONE 
 REPEATER 
 
 III II 
 
 POCKET PAGERS 
 
 FIGURE 3-37. - Block diagram of system using 
 roof-bolt-type antenna. 
 
 phone by No. 12 wire. With this system, 
 paging can be accomplished from as far as 
 500 feet from a roof bolt antenna. 
 
 3.6.2 Motorman-to-Snapper 
 
 Many mines use loading track loops 
 for loading mined coal or ore at the sec- 
 tions before transporting it to the sur- 
 face. This type of operation involves 
 the coordinated activities of two indi- 
 viduals: a "snapper" or "swamper" who 
 couples and uncouples the cars; and a lo- 
 comotive operator, or motorman, who moves 
 the train backward and forward at pre- 
 scribed times. If the snapper is not in 
 the clear when the train is moved, its 
 sudden motion can injure or kill him. 
 Thus, effective communication between the 
 motorman and snapper is vital. 
 
 Because of the curvature of the loop 
 track (fig. 3-38) and the location of the 
 locomotive on the main haulage track, the 
 two individuals are not usually within 
 sight or hearing of each other. Without 
 communication or at least some system of 
 signaling, coordination is difficult un- 
 less other workers are stationed along- 
 side the track to relay information. 
 However, this wastes time and manpower. 
 It is clear that the safety and efficien- 
 cy of the loading operation would be 
 vastly improved if there were a reliable 
 communication link between the motorman 
 and snapper. 
 
 DECOUPLES CARS 
 
 FIGURE 3-38. - Diagram showing need for com- 
 munication in haulage loop-around. 
 
 The design of any practical system to 
 meet the communication needs between mo- 
 torman and snapper requires that it does 
 not interfere with other communications, 
 is convenient, has a restricted range so 
 that similar systems can be used else- 
 where in the mine, and can be built with 
 commercially available hardware. Typi- 
 cally, a range of 1,500 feet or less is 
 all that is necessary to assure adequate 
 coverage for the maximum separation be- 
 tween the snapper and motorman. Two sys- 
 tems that can presently be implemented 
 using commmercially available hardware 
 are the telephone and trolley-carrier 
 phone system and the walkie-talkie radio 
 system. 
 
 3.6.2a Telephone and Trolley-Carrier 
 Phone System 
 
 In the telephone and trolley-carrier 
 phone system (fig. 3-39), the snapper 
 communicates by means of a belt-carried, 
 
 TROLLEYLINE 
 
 EXTENSION CORD 
 
 FIGURE 3-39, - Telephone and trolley-carrier 
 phone system. 
 
68 
 
 miniature mine telephone known as a belt 
 phone. A phone line is installed on the 
 rib or roof of the mine along one side of 
 the loading track. The belt phone can be 
 connected to this line by an extension 
 cord that has insulation-piercing clips 
 at one end. Alternatively, receptacles 
 that allow the belt phone to be plugged 
 in at convenient points can be provided 
 on the line. 
 
 A pager phone to carrier phone cou- 
 pler connects the phone line and the 
 trolley line. Phone line signals are 
 converted to trolley line signals and 
 vice versa by this coupler. The motorman 
 communicates by a trolley phone, which 
 operates on a frequency different from 
 that of the haulage communications. 
 
 CAUTION.— It should also be noted 
 that phones connected to the trolley 
 line in this manner are not permissi- 
 ble, and should be separate from the 
 main phone system. Any such inter- 
 connect musts be coordinated with 
 MSHA. 
 
 If duplicate systems are used in a 
 mine, the range of the trolley line sig- 
 nals has to be restricted by appropriate- 
 ly attenuating the transmitter output. 
 This system can be implemented using 
 standard trolley phones and phone-line- 
 to-trolley-line couplers. In addition, a 
 belt phone (fig. 3-40) is now commercial- 
 ly available. Equipped with a hardhat- 
 mounted speaker and an adjustable-boom- 
 type microphone, it has outgoing paging 
 capability and will operate compatibly 
 with available phone-line-to-trolley-line 
 couplers. 
 
 At least one mine has successfully 
 used an interface system between the 
 phone and trolley lines to provide 
 motorman-snapper communication. The sys- 
 tem is diagrammed in figure 3-41. A re- 
 mote interface, fabricated by technicians 
 at the mine, acts as a coupler between 
 the trolley line and a dedicated phone 
 line. The motorman can communicate via 
 the existing carrier phone system, where- 
 as the snapper mast communicate via 
 
 FIGURE 3-40. = Miner wearing belt phone„ 
 
 CARRIER PHONE 
 '^ ON MOTOR 
 
 TROLLEY 
 
 LINE 
 
 
 
 ^^/ 
 
 PHONELINE 
 
 
 
 1 MOTOR 1 
 
 
 O U 
 
 
 FH ) 
 
 I 
 
 
 
 ^ l^ 
 
 INTERFACE 
 
 
 
 rVs. vV> 
 
 
 
 ^JACKBOXES 
 
 FIGURE 3-41. - Diagram of interface system 
 between phone and trolley lines. 
 
 the phone line using a modified telephone 
 handset. A twisted-pair phone line, with 
 jackboxes connected at 50-foot intervals, 
 is strung up in the loop-track area and 
 connects to the dedicated phone line. 
 The snapper plugs his handset into a 
 nearby jackbox to establish communication 
 to the motorman. 
 
 3.6.2b Walkie-Talkie System 
 
 The walkie-talkie radio system uses 
 UHF portable radio equipment. Both the 
 motorman and snapper are equipped with 
 walkie-talkies (fig. 3-42). 
 
69 
 
 FIGURE 3-42. - Motorman and snapper walkie-talkie 
 system. 
 
 Because of the curvature of the loop 
 tunnel, propagation of radio waves at UHF 
 is severely restricted. In fact, direct 
 radio communication between the two in- 
 dividuals may not be possible in some 
 cases. However, this deficiency can be 
 overcome with a dual-frequency radio re- 
 peater connected to a radiating cable. 
 The cable carries the radio signals, and 
 the repeater effectively boosts them to 
 a higher power level. The coaxial cable 
 extends along the loading track and down 
 the main haulageway far enough to assure 
 communication coverage to the motorman. 
 Cables several hundred feet shorter can 
 be used if an approprate antenna is con- 
 nected at the end. Commercially availa- 
 ble portable radio transceivers and re- 
 peaters can be used to implement this 
 system. 
 
 A medium-frequency radio transceiver 
 (520 kHz) with sufficient range has been 
 developed that makes snapper-motorman 
 communications possible 
 ing additional cables, 
 aided by the conductors 
 in the loop-around. 
 
 without install- 
 
 Transmission is 
 
 normally present 
 
 Effective communication between the 
 snapper and motorman can provide 
 the coordination needed to eliminate 
 
 uncertainties regarding train movement in 
 the mine. This results in improved effi- 
 ciency and a reduction in the num-ber of 
 accidents related to the loading opera- 
 tion. Systems can be custom made from 
 available telephone and carrier phone 
 equipment. Leaky-feeder UHF equipment is 
 similarly available for custom systems. 
 
 3.7 Emergency Communications 
 
 There are two conditions under which 
 a communication system should operate. 
 These are normal operations (regular day- 
 to-day operation) and emergency condi- 
 tions. The need for reliable underground 
 communications following a disaster is 
 obvious. Two major requirements for any 
 emergency communication system follow: 
 
 1. The system must work following 
 the disaster. (This implies that the 
 system worked before the disaster and 
 that the system is protected from, or 
 immune to, fire, explosion, roof fall, 
 etc. ) 
 
 2. Miners must be familiar with 
 operation of the system. (Mistakes are 
 easy to make during periods of high emo- 
 tional stress.) 
 
 It should be recognized that there 
 are advantages in combining any emergency 
 communication system into the system used 
 for normal day-to-day operations. In 
 this way, miners can become familiar 
 enough with the system to operate it dur- 
 ing disaster conditions. Daily use of 
 the system also provides a mechanism of 
 regular testing, thus insuring that the 
 system will be operational. 
 
 3.7.1 Detecting and Locating the Trapped 
 Miner 
 
 The history of coal mine disasters 
 has established a need for a simple, 
 reliable system for locating and communi- 
 cating with miners trapped underground. 
 Such a system will not only increase the 
 chances of a successful rescue, but will 
 also reduce the risks to the rescue team 
 by keeping them from searching the wrong 
 locations. 
 
70 
 
 The problems of finding miners 
 trapped underground can be illustrated by 
 a disaster that occurred in 1945, in 
 which 24 men were killed by an explosion. 
 Figure 3-43 shows the location where nine 
 men barricaded themselves for 53 hours in 
 that particular incident. Rescue crews 
 tried for 2 days to reach the active area 
 of the mine in 5 and 6 Lefts while being 
 hampered by caved workings, fires, smoke, 
 gas, and loose roof. Three days later, 
 while exploring 9 Right, they found foot- 
 prints. After investigating, they found 
 a chalk-marked board indicating that five 
 men were in 4 Left entry. In 5 Left, 
 another mark was found directing search- 
 ers to second Left off of 5 Left. Seven 
 of the nine men survived the ordeal. All 
 might have lived if their location had 
 been known so they could have been 
 reached sooner. The time required to 
 rescue barricaded miners is critical. In 
 the recorded cases of barricading, 75 
 percent of the survivors were rescued 
 within 10 hours. 
 
 'I I 
 
 UthVJ 
 
 "< ^ <;' t. ^ Cef Booster fan j jj ) 
 
 !>9M., '■■,-;-.o..v->,. \ :,■ , , 
 
 7 2lxxlln 
 
 After a disaster, miners who manage 
 to escape can direct rescue teams to 
 those parts of the mine where others may 
 remain trapped. The nature of the mine 
 workings and the circumstances of the 
 disaster can also be used in locating 
 survivors, but all of these techniques 
 are based on guesswork. Accurate knowl- 
 edge of the location of trapped men is 
 required to increase their chances for 
 survival and to reduce the hazards to the 
 rescue team that might otherwise conduct 
 an unnecessary, futile search in danger- 
 ous, incorrect areas. 
 
 It is obvious that any information 
 that could be exchanged between the 
 trapped miners and the rescuers during a 
 rescue effort would be advantageous. In- 
 formation such as unusual conditions 
 known to the miners trapped, 
 advice for them to follow 
 arrived, are two examples, 
 words , a system that would 
 location of trapped miners 
 communication with them would 
 the probability of their rescue 
 
 FIGURE 3-43. - Part of mine showing areato which 
 miners retreated and erected imperfect barricade. 
 
 or medical 
 
 until aid 
 
 In other 
 
 provide the 
 
 and permit 
 
 increase 
 
 and also 
 
 reduce hazards to the rescue and recov- 
 ery team. Two systems for locating and 
 communicating with trapped miners have 
 been developed: a seismic system and an 
 electromagnetic system. 
 
 The seismic system relies on detec- 
 tion of small ground vibrations resulting 
 from a miner(s) banging on the roof or 
 ribs with some heavy object. This system 
 is presently operational and is being im- 
 proved continuously. In this system, the 
 trapped miner signals on the mine floor 
 or roof with any heavy object and seismic 
 detectors (geophones) on the surface are 
 used to detect these signals. Computa- 
 tion of the location of the trapped miner 
 by using the difference in the arrival 
 time of the signals at various geophone 
 positions on the surface has been quite 
 successful. A seismic location system 
 has the advantage that the miners do 
 not require any special equipment and 
 need only to be trained in how and when 
 to signal. The disadvantage is that 
 discontinuities in the overburden can 
 significantly affect rescue signal propa- 
 gation relative to both detection and 
 computation of location of the signal. 
 
71 
 
 Additionally, in a rescue and recovery 
 operation, the time required to deploy 
 and relocate, if necessary, a massive 
 geophone array may hamper the progress 
 desired. However, the seismic system 
 does provide the trapped miner with an 
 additional degree of protection when no 
 other method of communication can be es- 
 tablished. The Mine Safety and Health 
 Administration maintains a seismic rescue 
 system as part of its Mine Emergency 
 Operations group. All miners should ob- 
 tain MSHA stickers for their hard hats 
 (fig. 2-26) in case they should become 
 entrapped. 
 
 The electromagnetic system relies on 
 a small voice frequency (VF) transmitter 
 that can be carried by the miner, and 
 surface receivers that "listen" for the 
 signals broadcast directly through the 
 earth or through the mine workings by the 
 miner's transmitter. Basic development 
 of VF EM systems is completed, and proto- 
 type hardware is in the testing phase. 
 
 A typical trapped-miner transmitter 
 (fig. 3-44) weighs one-half pound and can 
 be worn on the belt. Cap lamp battery 
 
 units also exist. In an emergency, 
 and when it is decided that all routes of 
 escape are closed, the antenna wire is 
 uncoiled, laid out in as large a loop as 
 possible, and connected to the trans- 
 mitter. The transmitter and loop antenna 
 produce a magnetic field, as shown in 
 figure 3-45. The direction of these 
 signal-field lines can be used to pin- 
 point the location of the underground 
 loop antenna. By measurements taken on 
 the surface, the location of the antenna 
 can be determined within a few feet. 
 
 After detecting and locating a 
 trapped miner, the surface search team 
 can establish a voice down-link communi- 
 cations path to the men underground. 
 This voice link is established by deploy- 
 ing a large loop antenna directly above 
 the trapped miners and connected it to a 
 very powerful amplifier and voice system 
 (fig. 3-46). By speaking into the micro- 
 phone associated with the system, strong 
 electromagnetic signals are generated and 
 transmitted by the loop antenna. These 
 signals penetrate the earth, and the 
 trapped miners can hear actual voice from 
 the surface on their transceiver. The 
 surface can then ask key questions to 
 
 MAGNETIC FIELD LINES 
 (VERTICAL DIRECTLY 
 OVER LOOP) 
 
 VF TRANSMITTER 
 
 FIGURE 3=44. = Underground=miner=carried VF 
 equipment for signaling surface rescue crew. 
 
 II 1 \ 
 
 FIGURE 3-45. - Production of a magnetic field 
 by transmitter and loop antenna. 
 
72 
 
 ^' 
 
 LOOP ANT EN 
 
 '^^^>N [~^;^r:^7"[ x::z::> 
 
 ^^;;j;;j5.;;5^^?5755^^^,,^^^ 
 
 LOCATION 
 AND CODE 
 SIGNALS 
 
 A 
 
 N 
 
 FIGURE 3-46. 
 system. 
 
 Through=the-earth transmission 
 
 ascertain the conditions underground. As 
 an example, they can ask the trapped 
 miners to key three pulses of signal for 
 a "yes" answer and two pulses for a "no" 
 answer. This type of down-link voice and 
 up-link code signaling system allows the 
 surface team to learn anything they wish 
 about the situation underground and also 
 allows them to give instructions or 
 information concerning escape routes and 
 rescue attempts. 
 
 One advantage of an electromagnetic 
 system over a seismic system is that 
 the EM transmitter operates continuously 
 once deployed and will function for many 
 hours, or even days, from one cap lamp 
 battery. Besides operating continuous- 
 ly, its electrical signal is a known 
 rhythmic "beep," which is much easier 
 to detect than the random thumps of a 
 miner pounding on the ribs or roof. An- 
 other advantage is that the detection re- 
 ceiver can be readily carried by one min- 
 er (fig. 3-47) and can be used to cover a 
 reasonably large area. It can also be 
 used by underground rescue teams since it 
 is permissible. A version of the surface 
 receiver has been adapted for use in hel- 
 icopters. With this unit, large areas 
 can be scanned quickly. Once a signal is 
 detected, portable surface-carried units 
 can obtain an exact fix. The surface 
 gear for a seismic system, on the other 
 hand, is complex and stationary. Its de- 
 ployment site must be carefully selected. 
 If it is not within 2,000 feet of the 
 
 FIGURE 3-47. - Surface VF receiver and loop 
 antenna in use at simulated mine disaster. 
 
 signal source, it probably will not work. 
 In a large mine, this limitation is a se- 
 rious handicap. In mountainous terrain, 
 setting up the seismic geophones can pre- 
 sent especially difficult problems. 
 
 3.7.2 Refuge Shelter 
 
 When it appears to be impossible to 
 escape, or imprudent to attempt escape, 
 following a mine fire or explosion, 
 miners are trained to isolate themselves 
 from toxic gases and smoke by erecting 
 barricades. Although many miners have 
 been rescued from behind barricades, some 
 have died behind inadequately constructed 
 barricades. As a solution to this prob- 
 lem, sectional or central refuge chambers 
 have been established by some companies. 
 If a chamber is constructed, some form of 
 communication to the surface should be 
 included to inform rescue crews that the 
 chamber is being used and of the condi- 
 tion of its occupants. 
 
 Communication to a refuge shelter 
 could be provided by means of a borehole 
 equipped with a telephone pair connect- 
 ing to the surface, by existing wiring 
 within the mine, or by some form of 
 through-the-earth system. The in-mine 
 telephone system would be the least reli- 
 able after an explosion unless the cable 
 
73 
 
 installation had been specifically hard- 
 ened. Boreholes would be highly reliable 
 but would require a new borehole for each 
 refuge chamber or whenever a refuge cham- 
 ber was moved. 
 
 3.7.3 Rescue Team Communications 
 
 Even though searching a mine after a 
 fire or explosion is a slow and often 
 dangerous job, the rescue team must reach 
 any trapped or barricaded miners as soon 
 as possible. Effective communication 
 between the rescue team and the surface 
 or base camp, as well as communication 
 between individual members of the team, 
 is an essential element in any successful 
 rescue attempt. 
 
 The primary advantage of this type 
 of system is that it is simple and yet 
 usually provides good-quality voice com- 
 munication. Also the phone wire trailed 
 behind the rescue team provides a physi- 
 cal link back out of the search area. 
 This link can become an important factor 
 if the team must retreat under conditions 
 of poor visibility, or if a second rescue 
 team wishes to "follow" the first team. 
 The disadvantages of this type of system 
 are (1) the wire spool, which may be 
 heavy, must be transported by the rescue 
 team, and (2) the wire strung behind the 
 rescue team is susceptible to damage from 
 secondary explosions or roof falls. 
 
 3.7.4 Medium -Frequency Rescue Systems 
 
 One method that has proven effective 
 in maintaining communication to and from 
 the rescue team is illustrated in figure 
 3-48. In this relatively simple system 
 the rescue team splices into a good phone 
 line and then unrolls line from a spool 
 as they advance into the mine. During 
 a recent rescue, this type of system 
 provided good communication even after 
 the rescue team had traveled approximate- 
 ly a mile along a haulageway and then 
 descended another 1,200 feet down a shaft 
 from an underground headframe. 
 
 PERMANENT PHONE LINE 
 
 PHONE LINE 
 WOUND 
 ON SPOOL 
 
 FIGURE 3-48. - Effective method for maintain- 
 ing communication to and from rescue team. 
 
 Considerable research has been con- 
 ducted within the last 8 years in the ar- 
 ea of underground MF transmissions. This 
 research showed that MF signals could 
 propagate for great distances in most ge- 
 ologies and offered the hope of a whole- 
 mine radio system. The Bureau of Mines 
 and the South African Chamber of Mines 
 (SACM) pursued research independently. 
 
 Around 1974, SACM introduced a new 
 single-sideband system and followed up 
 later with another designed especially 
 for rescue team use. Performance in 
 South Africa was reported to be good. 
 The evaluation of these units in U.S. 
 mines showed them to be inadequate. The 
 type of modulation used [single sideband 
 (SSB)] made them sensitive to electromag- 
 netic interference (EMI). In addition, 
 power level was far too low and ineffi- 
 ciencies in both circuit and antenna de- 
 signs produced short-range performance. 
 
 The Bureau's approach to the problem 
 was more fundamental. A program was de- 
 signed and executed to study in-mine MF 
 propagation and learn how it interacted 
 with the complex environment. This envi- 
 ronment consists of various geological 
 factors such as stratified layers of dif- 
 ferent electrical parameters, entry size, 
 local conductors, EMI, etc. 
 
74 
 
 Figure 3-49 is a simplified geometry 
 of an in-mine site that illustrates one 
 of the most important findings of the 
 measurement program — the "coal seam 
 mode." For this mode to exist, the coal 
 seam conductivity (0c) must be several 
 orders of magnitude less than that of the 
 rock (Op). A loop antenna that is at 
 least partially vertically oriented, pro- 
 duces a vertical electric field (E^) and 
 horizontal magnetic field (H<j)). In the 
 rock, the fields diminish exponentially 
 in the Z-direction. In the coal seam, 
 the fields diminish exponentially at a 
 rate deteirmined by the attenuation con- 
 stant (a) which in turn depends upon the 
 electrical properties of the coal. An 
 inverse square-foot factor also exists 
 because of spreading. The effect is that 
 the wave, to a large degree, is trapped 
 between the highly conducting rock layers 
 and propagates long distances within the 
 lower conducting coal seam. The fact 
 that the coal may have entries and cross- 
 cuts is of minor consequence. 
 
 In the presence of conductors, the 
 picture changes considerably. In this 
 case, the effects of these conductors can 
 totally dominate over the effects of the 
 geology. In general, the presence of 
 conductors (rails, trolley lines, phone 
 lines) is advantageous. 
 
 MF signals can couple into, and re- 
 radiate from, continuous conductors in 
 such a way that these conductors become 
 not only the transmission medium but 
 also the antenna system for the signals. 
 Figure 3-50 illustrates this concept. 
 The most favorable frequency depends to 
 some extent on the relationship between 
 the geology and existing conductors. 
 The frequency effects are quite broad. 
 Anything from 400 to 800 kHz is usually 
 adequate. 
 
 3.7.4a Specific Application of MF 
 
 Communications to Rescue Teams 
 
 The low attenuation of MF signals in 
 many stratified geologies, such as coal 
 mines, can be of great benefit to rescue 
 teams. If existing mine wiring (like 
 
 ^c 
 
 Coal 
 
 or 
 
 entry 
 
 Loop 
 
 transmitting 
 
 antenna 
 
 FIGURE 3-49. - Coal seom mode. 
 
 Local mine conductor 
 
 Signal reradiating 
 from conductor 
 
 FIGURE 3-50. - MF parasitic coupling and 
 reradiation. 
 
 powerlines or belt lines) are present, 
 the range is even greater. This permits 
 a rescue team member to stay in commu- 
 nication with other members, the fresh 
 air base, and outside disaster control 
 centers. 
 
 To date, MF technology has not been 
 specifically applied to rescue team 
 communications. Such application is the 
 second step in the Bureau's overall MF 
 communications program. However, there 
 is no basic difference between opera- 
 tional MF systems and postdisaster MF 
 systems. By October 1982, the Bureau's 
 operational MF systems was in place in 
 several cooperating underground mines. 
 By October 1983, performance evaluation 
 of the systems will be completed. As the 
 performance proceeds, emphasis will be 
 directed to specific postdisaster-rescue 
 applications. 
 
75 
 
 3.7.4b System Concepts 
 
 The main advantage of MF communica- 
 tion is simplicity. Figure 3-51 shows a 
 rescue team member equipped with a pro- 
 totype MF vest radio. This vest radio 
 permits rescue team members to maintain 
 local communications (fig. 3-52). 
 
 In most cases, rescue teams will 
 utilize a lifeline for rapid retreat in 
 case of smoke when visibility is limited. 
 The lifeline offers interesting possibil- 
 ities for MF radio communications. Some 
 rescue teams actually use the line al- 
 ready to carry communications via sound- 
 powered telephones. Such a scheme is 
 both archaic and often ineffective. 
 
 Since this line is a continuous con- 
 ductor back to the fresh air base, it 
 provides a convenient parasitic path for 
 MF communication as shown in figure 3-53. 
 To assure even more reliable communica- 
 tions, physical audio links could be made 
 with the lifeline as shown in figure 3- 
 54. Such an approach provides redundancy 
 via simultaneous audio and radio links. 
 
 Figure 3-55 illustrates a total MF 
 base station for rescue team use. At the 
 fresh air base, the briefing officer (as 
 the individual is sometimes called) is 
 equipped with a standard intrinsically 
 safe base station or repeater; the offi- 
 cer could also be equipped with a vest. 
 With such an arrangement, communications 
 are possible not only between rescue team 
 members, but also with the surface and 
 with other distant rescue teams. In ad- 
 dition, it also provides a possible link 
 to the trapped miners. 
 
 Since existing mine wiring is exten- 
 sive and minewide, it is easily seen that 
 it provides yet another redundant link 
 for the rescue team members. Since other 
 rescue teams are also in the vicinity of 
 mine wiring, interteam communications are 
 possible if desired. This concept of in- 
 terteam communications is a radical de- 
 parture from existing procedures. It 
 will permit one rescue team, in one part 
 of the mine, to modify the ventilation in 
 such a manner that it does not degrade 
 
 FIGURE 3-51. = Rescue team member with 
 MF vest radio. 
 
 FIGURE 3-52. - Basic MF communications 
 among rescue team members. 
 
76 
 
 Life line and/or local mine conductors 
 
 Vest |_v 
 radio 
 
 Vest 
 radio 
 
 Advancing 
 rescue team 
 
 Fresh 
 air base 
 
 FIGURE 3-53. • LifelineasaparasiticMF path. 
 
 Plug 
 
 I Audio link 
 
 Audio plus 
 MF 
 
 Vest 
 radio 
 
 ^f> 
 
 Y 
 
 Vest 
 radio 
 
 team communications 
 
 Fresh air 
 base 
 
 FIGURE 3-54. - Lifeline as a redundant commu- 
 nications line for MF and audio communication. 
 
 Local mine wiring 
 
 Life line 
 
 Base 
 station or 
 repeater 
 
 Signal coupler 
 
 Microphone 
 
 FIGURE 3-55. - Total MF base station for 
 rescue teams. 
 
 the ventilation in the vicinity of an- 
 other rescue team. Equally important is 
 the fact that trapped miners are also 
 probably in the vicinity of existing mine 
 wiring. 
 
 3.7.4c Location and Communications 
 Systems for the Rescue of 
 Trapped Miners 
 
 So far this section has primarily 
 addressed the application of MF com- 
 munication to rescue teams. However, 
 the ultimate objective of the rescue 
 
 operation is to reach trapped miners in a 
 timely manner before they succumb to the 
 effects of injury, exposure, or toxic at- 
 mospheres. To this end, rescue team com- 
 munications is but a part. The key to 
 successful rescue lies in the rapid loca- 
 tion of the trapped miners. Without 
 this, valuable time can be wasted in 
 diverting rescue efforts to the wrong 
 area, often with tragic results. 
 
 Bureau research in the area of lo- 
 cation has been addressed by through- 
 the-earth seismic and EM systems. In 
 the seismic system, trapped miners pound 
 on the roof or ribs of the mine to gen- 
 erate seismic vibrations. These vibra- 
 tions travel through the overburden to 
 the surface where they can be detected 
 by sensitive transducers called geo- 
 phones. Computer analysis of the arrival 
 times of the seismic signals at the 
 various geophones permits the source to 
 be accurately located. This system is 
 operational and is kept in readiness by 
 MSHA Mine Emergency Operations. Present 
 Bureau research in EM means to locate 
 and communicate with trapped miners is 
 shown in figure 3-56. The system con- 
 sists of two parts, a transceiver that 
 is normally carried on the miner's belt 
 and a surface system for detection and 
 communications . 
 
 Loop antennas 
 
 rf^ ^ K b 
 
 /Microphone 
 
 Transmitter 
 
 Location signal 
 
 A 
 
 Transceiver 
 
 Battery with 
 
 power 
 take-off 
 
 FIGURE 3-56. - Voice frequency electromag- 
 netic system for location and communication 
 with trapped miners. 
 
77 
 
 In operation, the trapped miner re- 
 moves the transceiver from the belt, de- 
 ploys a self-contained loop antenna, and 
 attaches the transceiver to a special cap 
 lamp battery. This antenna consists of 
 300 feet of No. 18 wire that must be de- 
 ployed in the largest area possible to be 
 effective. A location signal is trans- 
 mitted directly through the earth. 
 
 On the surface, sensitive receivers 
 detect the signal and locate the source. 
 Once detection and location are made, a 
 large surface transmitter is deployed 
 above the trapped miner. This transmit- 
 ter is powerful enough to send voice mes- 
 sages by radio, directly down through the 
 earth. 
 
 The trapped miner's transceiver re- 
 ceives this voice. The surface personnel 
 then ask the miner "yes-no" questions 
 concerning his or her condition and that 
 of the mine. The miner responds by sim- 
 ple on-off keying of the transceiver. In 
 this manner a two-way communications link 
 is established, entirely through the 
 earth, and rescue operations can start in 
 the most efficient manner. 
 
 Details of this EM system can be 
 found in numerous reports. This is known 
 as a voice frequency (VF) system because 
 all communications take place in the VF 
 band of 300 to 3,000 Hz. 
 
 The seismic system is very effective 
 in mines up to 2,200 feet deep, and does 
 not require the miner to be equipped with 
 any special devices. However, it does 
 require the miner to be able to pound. 
 Injury or lack of a sufficiently heavy 
 object with which to pound may render the 
 system ineffective. The most serious 
 drawback is that of time. The surface 
 receiver station (geophones, field truck 
 with computer, etc.) may take too long 
 to set up. Bad weather and terrain can 
 further delay the surface station deploy- 
 ment. 
 
 The EM-VF receiver system is less 
 affected by adverse conditions on the 
 surface because it is lighter and more 
 easily transportable. However, it has 
 
 its own disadvantages. The trapped miner 
 must be equipped with a special trans- 
 ceiver, and must be able to deploy the 
 antenna in a sufficiently large area. 
 Injury or confined quarters may prevent 
 deployment. In addition, under the best 
 of conditions, the system has a range 
 limit of about 1,000 feet. Although a 
 new system is under development that will 
 increase the range to 3,000 feet, this 
 improvement comes about only with com- 
 plex, slowly deployed surface equipment. 
 Therefore, it will be subject to the same 
 delays as the seismic system. 
 
 MF communication offers advantages 
 over through-the-earth approaches by 
 permitting in-mlne communications to 
 the trapped miners. This could be in 
 addition to, or in place of, through- 
 the-earth schemes that may fail because 
 of excessive overburden or the inability 
 of the trapped miner to deploy his or her 
 end of the system successfully. Fig- 
 ure 3-57 illustrates this concept. 
 
 In this illustration, the trapped 
 miner is equipped with a small MF trans- 
 ceiver built into the top of the cap lamp 
 battery or worn on the belt. Note that 
 this is exactly the same packaging con- 
 cept used for the VF through-the-earth 
 system shown in figure 3-56. The intent, 
 however, is not to send a signal through 
 the earth, but rather to induce a signal 
 onto local mine wiring. If this is ac- 
 complished, the in-mine rescue team also 
 
 (A C power line) 
 Local conductor No. 2 
 
 (Trolley line) 
 Local conductor No I 
 
 Life line 
 
 ( Parasitic ] 
 I coupling I 
 
 Base station 
 or repeater 
 
 J 
 
 Trapped miner 
 MF transceiver 
 
 Vest 
 transceiver 
 
 Distance can be miles 
 
 -»■ [Trapped minerj 
 
 [Rescue team] 
 
 FIGURE 3-57. - MF in-mine location and com- 
 munication system. 
 
78 
 
 is likely to be in the vicinity of mine 
 wiring and can receive the signal. It 
 must be pointed out very clearly that 
 mine wiring does not mean that one con- 
 tinuous assembly of wiring is involved. 
 If the trapped miner is near a power ca- 
 ble and not near a trolley line, and the 
 rescue team is near a trolley line and 
 not near a power cable, this does not 
 mean that a communications link between 
 the two cannot exist. An induced MF sig- 
 nal on one type of conductor will para- 
 sitically couple to all others, even if 
 there is no physical connection. This is 
 the uniqueness of MF communication. 
 
 signals of narrow bandwidth that parasit- 
 ically couple onto mine wiring, and are 
 widely distributed. This can be received 
 by the in-mine rescue team. If this oc- 
 curs, they will use their more powerful 
 MF equipment (vests or base stations) to 
 establish a voice link to the trapped 
 miner. By asking the trapped miner yes 
 or no questions, his or her location can 
 be learned. However, direct location via 
 MF communication is impossible. The par- 
 asitic coupling characteristics of MF 
 signals do not permit the through- 
 the-earth VF type of location; the signal 
 could be on many conductors. 
 
 In operation, the trapped miner Obviously VF and MF systems could be 
 
 would deploy an MF loop antenna or cou- combined such that the benefits of both 
 
 pier, preferably onto available local VF (fig. 3-56) and MF (fig. 3-57) could 
 
 wiring. The coupler could be a small de- be obtained. Equally important is the 
 
 vice of small volume similar to a current fact that the MF trapped miner device 
 
 transformer. The loop could be a coupler could be used in nonemergency situations 
 
 that was unwound. In either case, the as a page receiver and thereby be a cost 
 
 antenna is small. If nearby wiring does effective addition to a general mine com- 
 
 not exist, the loop could be deployed in munication system. Table 3-3 lists MF 
 
 hope of coupling to distant wiring. When communication system specifications, 
 so deployed, the transmitter sends out MF 
 
 TABLE 3-3. - MF communication system specifications 
 
 Emissions, narrowband FM: 
 
 Occupied bandwidth kHz . . 10 
 
 Rf frequency kHz.. 60-1,000 
 
 Peak deviation kHz.. ±2.5 
 
 Modulated frequency Hz.. 200-2,500 
 
 Receiver, superheterodyne: 
 
 Sensitivity 1.0 uV (12-db sinad) 
 
 Selectivity 8-pole crystal filter 
 
 IF 3-dB bandwidth (minimum) kHz.. 12 
 
 IF 70-dB bandwidth (maximum) kHz.. 22 
 
 RF bandwidth kHz.. 60-1,000 
 
 Squelch Noise operated and tone 
 
 Transmitter, push-pull, class B: 
 Output power, W: 
 
 Vest 4.0 
 
 Vehicular 20.0 
 
 Antenna magnetic moment (ATm^): 
 
 Vest 2.1 
 
 Vehicular 6.3 
 
 RF line coupler, transfer impedance (Z-p): 
 1-in coupler, ohms: 
 
 350 kHz 10.0 
 
 520 kHz 11.2 
 
 820 kHz 17.8 
 
 4-in coupler, ohms: 
 
 520 kHz 10.6 
 
79 
 
 3.7.4d Performance Data 
 
 In order to evaluate the potential 
 of MF signals as a means to locate and 
 communicate with trapped miners, and to 
 provide communications for the actual 
 rescue team operation, a test was con- 
 ducted at the York Canyon Mine near 
 Raton, N. Mex. , in June 1982. This mine 
 is a coal mine located in the York seam 
 of the Raton Basin. The terrain is hilly 
 such that the mine overburden varies from 
 about 200 to 800 feet. 
 
 The mine has four main drift entries 
 that are about 7,000 feet long. Off 
 these entries , submains were driven and 
 longwall mining occurs. A borehole is 
 located at about the 7,000-foot mark. 
 This borehole contains a twisted pair ca- 
 ble that is associated with the fire mon- 
 itoring system on the longwall panels. 
 
 This is an ac mine that transports 
 the coal by belt. Rubber-tired vehicles 
 provide transportation for personnel and 
 supplies. The distance from the portal, 
 down the main entries to the longwall 
 faces, can be nearly 15,000 feet). 
 
 At the mine portal, a MF signal cou- 
 pler was attached to the mine telephone 
 lines. This coupler was controlled by a 
 standard MF base station. A second cou- 
 pler and base station were placed at the 
 top of the borehole. The coupler was 
 clamped around the cable that went down 
 the borehole. 
 
 Two personnel entered the mine and, 
 by vehicle, traveled down the main en- 
 tries to the vicinity of the borehole 
 (7,000 feet). These personnel were 
 equipped with MF vest transceivers that 
 had a magnetic moment of 2.1 ATm^ and a 
 sensitivity of 1 V at 520 kHz. The in- 
 tent of the test was to ascertain whether 
 or not these personnel could communicate 
 with the base at the portal, or the base 
 at the top of the borehole. If so, it 
 would demonstrate that MF-equipped rescue 
 teams could communicate with the outside 
 command center without deploying their 
 own communications line, or relying on 
 the integrity of the mine phone line that 
 may, or may not, be intact. In addition. 
 
 it would demonstrate that if a trapped 
 miner was equipped with a MF transceiver 
 of similar specifications, he or she 
 could directly communicate with rescue 
 teams in the mine, or search crews on the 
 surface who were monitoring any conduc- 
 tors egressing the mine. 
 
 The result of the test showed that 
 communications were possible from almost 
 anywhere in the haulage and belt entries 
 to either base station. It was even pos- 
 sible for the base at the portal, on the 
 telephone line, to communicate with the 
 base atop the borehole, on the fire moni- 
 tor line, even though there was no physi- 
 cal connection between the two. Whenever 
 a vest was within a few feet of mine con- 
 ductors , there was an obvious improvement 
 in clarity and signal strength. 
 
 Although this test was preliminary, 
 it clearly highlights the potential of 
 using MF coimnunications for search and 
 rescue operations. Much more work is 
 necessary to measure range from mine wir- 
 ing whenever the mine is not operating as 
 would be the case during search and res- 
 cue operations. An operational mine pro- 
 duces considerable levels of acoustic and 
 EM noise which reduces MF system range. 
 
 3.7.5 Emergency Warning Systems 
 
 Many types of emergency warning sys- 
 tems are available for alerting under- 
 ground personnel. One example is the 
 stench warning system, which introduces a 
 distinctive odor into the airstream. 
 Visual signals or radio paging could also 
 be used to alert underground personnel. 
 A preferred warning system would operate 
 over existing wiring, such as the twisted 
 pair of a pager phone system, and broad- 
 cast an audio warning that can be heard 
 throughout the active areas of the under- 
 ground complex. Before deciding on an 
 alarm system, factors that affect the 
 range over which an audio alarm can be 
 heard should be considered. The most im- 
 portant factors are the noise background 
 found in mines, the attenuation that the 
 mine environment imposes on the alarm 
 signal, and the attention-getting quality 
 of different alarms. 
 
80 
 
 The intensity of a sound is the en- 
 ergy in the sound wave. It is customary 
 to express intensities or pressure levels 
 in decibels. The term "loudness" refers 
 to the response of the human ear to 
 sound. Experiments have established that 
 the loudness of a tone is a function of 
 both frequency and intensity, with the 
 ear most sensitive to frequencies in the 
 region of 1 to 2 kHz. In other words, 
 for tones with the same intensity, tones 
 in the l,000-to-2,000-Hz region appear 
 louder than those above or below this 
 region. 
 
 Figure 3-58 shows the noise level 
 for a typical continuous miner, with 
 noise samples taken at the operator's po- 
 sition and with the conveyor running. To 
 estimate the masking effects of these 
 samples, we must first transform the 
 curves so that they refer to sound levels 
 on a per cycle basis. This has been done 
 in figure 3-59. The center curve, la- 
 beled "Mask noise source," plots the av- 
 erage of figure 3-58 in terms of the 
 sound level per cycle of bandwidth. The 
 upper curve, labeled "Detection threshold 
 at noise source," shows the estimated 
 threshold level as a function of tone 
 frequency. The curve shows that tones 
 
 200 315 
 FREQUENCIES 
 
 FIGURE 3-58. - Cutting with conveyor on 
 (operator position). 
 
 FIGURE 3-59. = Detection thresholds. 
 
 between 250 and 1,500 Hz require a level 
 in excess of 80 dB to be just detectable. 
 If we allow an additional 10 dB to insure 
 detectability , alarm tones would have to 
 have a sound level of at least 90 dB at 
 the operator's position. 
 
 If we move 15 feet away from the op- 
 erator's position (the bottom curve in 
 figure 3-59) , these sound levels are re- 
 duced considerably. This curve shows 
 that at 800 Hz a level of 60 dB is re- 
 quired, and thereafter the required level 
 decreases until at 6,000 Hz it is about 
 40 dB. 
 
 As mentioned earlier, the ear is 
 most sensitive in the region of 1,000 to 
 2,000 Hz and decreases at higher frequen- 
 cies. Figure 3-59, however, shows that 
 the higher the frequency of a tone (up to 
 8,000 Hz), the more detectable it is. 
 The spectrum of the masking noise is the 
 cause of this apparent contradiction. 
 The background noise is high at the 
 frequencies where the ear is sensitive 
 and decreases with frequency. 
 
 In addition to overcoming background 
 noise, planners must compensate for 
 attenuation of the warning tone. Experi- 
 mental and theoretical investigations 
 are in close agreement on the attenua- 
 tion of sound in room-and-pillar mines. 
 Figure 3-60 shows a plan of one experi- 
 ment on attenuation of sound. For this 
 experiment, a 100-dBA source was mounted 
 at the position shown in the figure, and 
 
81 
 
 DATA GATHERING 
 POSITIONS 
 
 ® 
 
 ^^ B^ ra F: 
 
 / 
 
 ® 
 
 <^ 
 
 100 
 
 _l_ 
 
 150 
 _1_ 
 
 SCALE (FEET) 
 
 FIGURE 3-60. - Planof mine in whichexperiment 
 was conducted. (Coal seam height, 76 inches.) 
 
 the sound levels at the points labeled 1 
 through 4 were recorded. Figure 3-61 is 
 typical of the data obtained. It plots 
 attenuation as a function of frequency at 
 the four points. 
 
 In practice, an audio warning source 
 must be some distance from the personnel 
 it is intended to alert, and it is desir- 
 able that the warning be detectable above 
 the background noise from as far away 
 as possible. The greater the distance 
 the sound must propagate, the louder the 
 source must be; hence, the greater the 
 hazard that the source will damage the 
 hearing of someone who is inadvertently 
 close to it when it is actuated. In an 
 actual emergency, the risk of subject- 
 ing a miner to intense sound may be 
 
 FIGURE 3-61, - Attenuation as a function of 
 frequency. 
 
 considered justified; but to insure reli- 
 ability, warning systems must be routine- 
 ly tested, preferably in an operationally 
 useful way, such as signaling the end of 
 a shift. (Fire stations routinely test 
 their sirens by sounding off at noon or 
 some other prearranged time.) In addi- 
 tion, any system is subject to false 
 alarms and/or pranks. Considering these 
 factors, the presence of a really intense 
 noise source might be regarded as an un- 
 warrantable menace. 
 
 Table 3-4 combines the effects of 
 background noise level and attenuation of 
 the alarm tone to show the sound level 
 required at the source for the warning to 
 be just detectable by the operator of a 
 continuous miner. The significance of 
 these numbers is best explained by taking 
 a particular example. The entry for 
 1,000 Hz under 210 feet is 100. This 
 means that at 1,000 Hz the source level 
 required to just alert the operator of a 
 continuous miner who has "normal hearing" 
 is 100 dB when the source is 210 feet 
 away from him. 
 
 TABLE 3-4. - Sound level required at source for warning 
 to be just detectable at operator's position on a 
 continuous miner, dB 
 
 Frequency (Hz) 
 
 Distance from source 
 
 
 70 ft 
 
 140 ft 
 
 210 ft 
 
 280 ft 
 
 250 
 
 93 
 
 96 
 
 100 
 
 101 
 
 500 
 
 95 
 
 99 
 
 103 
 
 105 
 
 1,000 
 
 94 
 
 96 
 
 100 
 
 103 
 
 2,000 
 
 91 
 
 95 
 
 99 
 
 103 
 
 4,000 
 
 88 
 
 91 
 
 102 
 
 107 
 
82 
 
 There are systems commercially 
 available that can satisfy the require- 
 ments of audio warning systems using ex- 
 isting mine wiring. These systems use 
 the mine paging telephone network as the 
 emergency alarm system. This approach 
 requires the addition of an alarm signal 
 generator compatible with the pager phone 
 operation. The paging telephone and ex- 
 ternal remote speakers act as the alarm 
 sounding units. The alarm signal can be 
 transmitted using a standard mine paging 
 telephone and an acoustically coupled 
 alarm signal generator or by using a ded- 
 icated on-line alarm signal generator, as 
 shown in figure 3-62. Alarm signals are 
 fed onto the pager phone line in one of 
 two ways. 
 
 The first way uses the small porta- 
 ble alarm signal generator shown at the 
 top of figure 3-62. When operated, this 
 unit emits an audio alarm via a small 
 speaker. The speaker is equipped with a 
 suitably sized rubber gasket that enables 
 the sound to be efficiently coupled into 
 the microphone of any standard pager 
 phone. Units of this type are commonly 
 used in conventional telephone applica- 
 tions to remotely control such items as 
 telephone answering machines and WATS 
 line access. It can be seen that sound- 
 ing an alarm in this way is a little 
 awkward since three buttons must be 
 pushed simultaneously, but this provides 
 a safeguard against an accidental alarm. 
 In addition, the portable units need only 
 be entrusted to responsible individuals, 
 which is a safeguard against pranksters. 
 
 ALTERNATIVE 
 ^ METHODS OF 
 SOUNDING 
 THE ALARM 
 
 REMOTE SPEAKER 
 
 ALTERNATIVE ALARMS 
 
 FIGURE 3-62. - Useof theexisting pagertele- 
 phone network as an emergency alarm system. 
 
 The second way of sounding an alarm 
 on the system is to use the on-line alarm 
 signal generator shown in figure 3-62. 
 When the button on this unit is pressed, 
 it places the correct dc signals on 
 the line to actuate the pager phones 
 and electronically transmits the alarm 
 signal. 
 
 BIBLIOGRAPHY 
 
 1. Adams, J. W. , W. D. Bensema, and 
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 Mine. BuMines OFR 37-75, June 1974, 
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 2. Bensema, W. D. , M. Kanda, and 
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 3. Bergeron, 
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83 
 
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 15. Dushac, H. M. Portable Remote 
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 Proc. 4th WVU Conf. on Coal Mine Electro- 
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 9. . Trackless-Trolleyless 
 
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 10. Dobroski, H. , Jr. A Coaxial 
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 11. . Improved Rail Haulage 
 
 Communications. Proc. 4th WVU Conf, on 
 Coal Mine Electrotechnology , Morgantown, 
 W. Va., Aug. 2-4, 1978, pp. 29-1—29-14. 
 
 18. Lagace, R. L. , W. G. Bender, 
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 19. Laubengayer, W. C. , K. Michael 
 Ware, R. D. Gehring, L. R. Wilson, R. P. 
 Decker, and D. T. Anderson. Research 
 and Development Contract for Coal Mine 
 Communication System. Volume 4. En- 
 vironmental Measurements. BuMines OFR 
 69(4)-75, November 1974, 75 pp.; NTIS PB 
 244 900. 
 
 12. . Radio Paging. Paper in 
 
 Underground Mine Communications (in Four 
 
 Parts). 2. Paging Systems. BuMines 
 IC 8743, 1977, pp. 29-33. 
 
 20. Long, R. G. , J. D. Foulkes, P. M. 
 Kay, and S. J. Lipoff. Systems Study of 
 Metal and Nonmetal Mine Communications. 
 BuMines OFR 32-82, August 1979, 231 pp. 
 
 13. 
 
 Remote Control of Circuit 
 
 Breakers. Paper in Underground Mine Com- 
 munications (in Four Parts). 3. Haulage 
 Systems. BuMines IC 8744, 1977, 
 pp. 24-27. 
 
 21. Murphy, J. N. , and H. E. 
 Parkinson. Underground Mine Communica- 
 tions. Proc. IEEE, V. 66, No. 1, January 
 1978; p. 26. 
 
84 
 
 22. Parkinson, H. E. , and J. D. 
 Foulkes. Conventional Telephone Equip- 
 ment. Paper in Underground Mine Communi- 
 cations (in Four Parts). 1. Mine Tele- 
 phone Systems. BuMines IC 8742, 1977, 
 pp. 3-17. 
 
 23. Sacks, H. K. Refuge Shelter Com- 
 munication System. Paper in Underground 
 Mine Communications (in Four Parts). 
 4. Section-to-Place Communications. Bu- 
 Mines IC 8745, 1977, pp. 73-76. 
 
 24. 
 
 Trapped-Miner 
 
 and Communication Systems. 
 
 Location 
 Paper in 
 
 Underground Mine Communications (in Four 
 Parts). 4. Section-to-Place Communica- 
 tions. BuMines IC 8745, 1977, pp. 31-43. 
 
 25. Sacks, H. K. , and R. L. Chufo. 
 Hoist Radio Communications. Paper in 
 Underground Mine Communications (in Four 
 Parts). 3. Haulage Systems. BuMines 
 IC 8744, 1977, pp. 28-41. 
 
 26. Spencer, R. H. , P. O'Brien, and 
 D. Jeffreys. Guidelines for Trolley Car- 
 rier Phone Systems. BuMines OFR 150-77, 
 March 1977, 170 pp.; NTIS PB 273 479. 
 
85 
 
 CHAPTER 4. —COMPUTERIZED MINE MONITORING'' 
 
 4.1 Introduction 
 
 Monitoring systems can have numerous 
 uses in the mine. They can aid in the 
 efficient management of the mine by pro- 
 viding environmental trend data, pro- 
 duction and maintenance control, and 
 communications. In some cases, they can 
 provide justification to petition the 
 Mine Safety and Health Administration 
 (MSHA) for a variance of one of the man- 
 datory safety standards. They may also 
 increase the gross revenues of the mine 
 by increasing the amount of coal produced 
 or increase profits by reducing the cost 
 of producing that coal. 
 
 No single system will satisfy the 
 requirement of all mines. Some may re- 
 quire simple hard-wired status-reporting 
 systems; others, multipurpose computer- 
 based systems that collect, analyze, and 
 store data and perhaps control some mine 
 functions. Even though systems vary in 
 complexity, they are all composed of 
 three functional components. The first 
 component is sensors that measure the en- 
 vironmental or production parameters and 
 produce an electrical signal that is fed 
 into the telemetry. The second is telem- 
 etry devices that receive the signal from 
 the sensors and transmit it in either an- 
 alog or digital format to the third com- 
 ponent, analysis and display equipment. 
 This equipment receives the transmitted 
 signal and either stores it for later 
 analysis or displays it. The analysis- 
 display equipment ranges from simple 
 strip chart recorders with preset alarms 
 to computers, cathode-ray tubes (CRT's), 
 and line printers that can also provide 
 production reports. 
 
 4.2 Uses of a Mine Monitoring System 
 
 A list of potential uses for 
 mine monitoring systems, including both 
 
 production-related functions and those 
 related to health and safety, was used to 
 develop a questionnaire. It was present- 
 ed to representatives of the mining com- 
 munity to determine their current moni- 
 toring priorities. 
 
 The responses indicated that the in- 
 dustry's priorities fall into the follow- 
 ing two categories: 
 
 First priority — 
 
 Production and haulage 
 
 Maintenance 
 Second priority — 
 
 Ventilation 
 
 Communication 
 
 Fire monitoring 
 
 Personnel 
 
 The survey shows that production- 
 oriented systems were the most appealing 
 to the questionnaire respondents. Since 
 even small improvements in production ef- 
 ficiency and maintenance can have a large 
 financial impact, the desirability of 
 monitoring systems that focus on these 
 areas is understandable. 
 
 The results are summarized in table 
 4-1. The function that scored 100 was 
 viewed as the most beneficial monitoring 
 function. 
 
 ^ From Guidelines for Environmental 
 Monitoring in Underground Coal Mines. 
 Phase I Report. BuMines OFR 180-82, 
 1982, 177 pp.; NTIS PB 83-147777. 
 
86 
 
 TABLE 4-1. - Survey 
 
 1. Output by section 
 
 2. Belt monitoring and control.... 
 
 3. Scheduled routine maintenance.. 
 
 4. Equipment repair history 
 
 5 . Spare parts inventory 
 
 6. Assist in diagnosis of failure. 
 
 7. Power: Fault location 
 
 8. Face equipment operating time.. 
 
 9. Personnel: Shift organization.. 
 
 10. Ground fault detection 
 
 11. Trailing cable failure 
 
 12. Power center monitoring 
 
 13. Monitor car haulage system 
 
 14. Personnel: Locate skills; 
 assist in locating people with 
 needed or critical skills 
 
 15. Ventilation: Eliminate inspec- 
 tions; eliminate or reduce 
 frequency of some periodic 
 inspections 
 
 16. Communications: Reliability.... 
 
 17. Communications: Intelligibility 
 
 18. Paging 
 
 19. Plan new ventilation; help in 
 planning ventilation, including 
 new ventilation shafts 
 
 20. Inspection scheduling; alert 
 foremen or others to scheduled 
 or predictable inspection or 
 repair 
 
 results, weighted rank score 
 
 100 
 99 
 99 
 97 
 95 
 94 
 73 
 68 
 66 
 63 
 62 
 62 
 59 
 
 58 
 
 50 
 49 
 
 45 
 45 
 
 45 
 43 
 
 21. Fire: Beltways; a system to 
 detect and warn of incipient 
 fires in the beltway due to hot 
 rollers or other problems 
 
 22. 
 
 Beltway for intake air; the use 
 of improved fire detectors and 
 monitoring system so as to 
 qualify for a variance and ena- 
 ble use of the beltway for in- 
 take air 
 
 23. 
 
 24. 
 
 25. 
 
 26. 
 
 27. 
 28. 
 
 29. 
 
 30. 
 
 31. 
 
 32. 
 33. 
 
 Communication: 
 station 
 
 Station-to- 
 
 Emergency signaling; direct 
 people during any emergency by 
 signaling 
 
 Detect leaky stoppings; venti- 
 lation monitor to detect open 
 doors, blockages in air course, 
 and leaky stoppings 
 
 Personnel: Emergency; assist 
 in locating and aiding people 
 during an emergency 
 
 Fire: Haulage; detect incipi- 
 ent fires in trolleyways 
 
 Ventilation: Control regula- 
 tors; monitor ventilation, and 
 adjust regulators to improve 
 flow distribution 
 
 Roof fall prediction; automat- 
 ically plot falls and/or micro- 
 seismic activity to predict 
 roof fall 
 
 Cage; monitor the operation of 
 the cage to predict failures or 
 minimize delays 
 
 Fire: Gob; monitor gob areas 
 for fire 
 
 42 
 
 Inventory expendables, 
 Rock bursts 
 
 41 
 40 
 
 40 
 
 37 
 
 30 
 29 
 
 25 
 
 20 
 
 18 
 
 18 
 10 
 10 
 
87 
 
 Responders were also asked to indi- 
 cate the relative importance of other 
 cost and technological factors that may 
 affect user acceptance. Results, on a 
 100-point scale, were 
 
 1. Reliablity of monitoring equip- 
 ment in mine environment 100 
 
 2. Maintenance cost 90 
 
 3. Initial cost 88 
 
 4. Skills required to maintain 
 equipment 75 
 
 5. New technology to mining 62 
 
 Reliability of the equipment was the 
 most frequently cited "very important" 
 factor. 
 
 4.3 Petitions for Modification 
 
 Mine monitoring systems can be used 
 to provide a cheaper and safer alterna- 
 tive to satisfying the mandatory safety 
 standards set forth in 30 CFR 75, provid- 
 ed that the alternate method (in this 
 case, the monitoring system) guarantees 
 no less than the same measure of pro- 
 tection afforded by the standard (30 CFR 
 44) . The extent to which the industry 
 currently takes advantage of these us- 
 ages can be determined by reviewing the 
 Petitions for Modification of Manda- 
 tory Safety Standards. 2 Since ventila- 
 tion regulations appear to be the most 
 likely candidates for modification peti- 
 tions, petitions were reviewed under sub- 
 part D, "Ventilation," in the following 
 sections: 
 
 75.305 Weekly examinations for 
 hazardous conditions. 
 
 ^Sources include the Federal Register, 
 the Bureau of National Affairs, Inc., 
 "Mine Safety and Health Reporter," and 
 the McGraw-Hill 1979 "Guide to Modifica- 
 tion of Safety Standards in Coal Mines." 
 
 75.306 Weekly ventilation 
 examinations. 
 
 75.307 Methane examinations. 
 
 75.310 Methane in virgin territory. 
 
 75.326 Aircourses and belt haulage 
 entries. 
 
 This review identified a number of 
 cases where continuous monitoring was 
 used in a petition for a variance and a 
 number of others that could have used 
 continuous monitoring. Included in the 
 review were petitions that were granted 
 and petitions that were filed, but not 
 acted upon as of the writing of this re- 
 port. General comments on the petitions 
 follow. 
 
 75.305 Weekly Examinations for Haz- 
 ardous Conditions. - This section re- 
 quires weekly inspection of at least one 
 entry of each intake and return air- 
 course, in its entirety, for both methane 
 and for compliance with the mandatory 
 health and safety standards. Typical 
 petitions state that because of poor roof 
 conditions it is not possible to travel 
 the aircourses in their entirety, and 
 offer checkpoint measurements as an al- 
 ternative. Continuous methane (75.305) 
 measurements could be made with a moni- 
 toring system at these checkpoints. Re- 
 quired airflow measurements (75.306) 
 could also be made with the same system. 
 
 Only 1 of the 62 petitions that were 
 granted offered continuous monitoring. 
 An additional 20 petitions were filed, 
 but there was no record of any final de- 
 cisions. One of these petitions did pro- 
 pose to install two methane monitors at 
 specified points. 
 
 75.307 Methane Examinations. - This 
 section requires tests for methane at 
 each working place immediately prior 
 to energizing electrically operated 
 equipment. 
 
88 
 
 One petition was noted in which 
 methane monitoring devices were installed 
 on permissible electric water pumps in 
 the face area to eliminate the meth- 
 ane examinations by a qualified per- 
 son required prior to energizing the 
 pumps. 
 
 75.310 Methane in Virgin Terri- 
 tory. - This section requires that all 
 electric power be cut off and men 
 withdrawn when air returning from vir- 
 gin mining areas contains 2% or more 
 methane. 
 
 Three petitions for modification 
 were granted under the stipulation that 
 continuous automatic methane monitors 
 were used in the return as an alternative 
 to measurements made by certified mine 
 personnel. 
 
 75.326 Aircourses and Belt Haulage 
 Entries. - This section requires that en- 
 tries used as intake and return air- 
 courses be separated from belt haulage 
 entries. 
 
 Fourteen petitions that were grant- 
 ed and ten that were filed but not act- 
 ed upon were reviewed. Of these, seven 
 petitions were granted on the basis 
 of continuous monitoring systems , and 
 seven of the filed petitions proposed 
 continuous monitoring of carbon mon- 
 oxide. A review of MSHA tests that 
 demonstrate the "equivalency" of car- 
 bon monoxide sensors and the customary 
 point-type heat sensors is presented in 
 reference 15. 
 
 In summary, at least 11 continuous 
 monitoring systems have been installed in 
 U.S. underground coal mines for purposes 
 of obtaining a variance from the manda- 
 tory health and safety standards. Eight 
 additional petitions for modification 
 mention such systems. 
 
 4.4 Technical Factors 
 
 The key technical issues are whether 
 the sensors can actually provide the 
 needed input information, the ability of 
 the processing system to interpret cor- 
 rectly the telemetered information, and, 
 finally, overall system reliability. 
 
 4.4.1 Sensors 
 
 Sensors are the critical element in 
 mine monitoring systems since they pro- 
 vide the input data. If the input data 
 are not correct or are not representative 
 of the required measurement, the entire 
 monitoring process is meaningless, i.e., 
 "garbage in, garbage out." One problem 
 with sensors is that their output repre- 
 sents the response of the sensor to a 
 number of parameters in addition to the 
 one that is to be measured. Typical ex- 
 amples are the response to changes in 
 temperature and the poisoning of environ- 
 mental sensors by other gases in the 
 mine. 
 
 The critical problem relates to the 
 ability of the sensor actually to measure 
 the parameter of interest. In particu- 
 lar, ventilation monitoring systems use 
 point air velocity measurements to repre- 
 sent the total airflow at a cross section 
 in the mine. The total airflow is deter- 
 mined either from an empirically derived 
 factor and the point measurement or from 
 actual calibration of the cross section. 
 The problem is further complicated be- 
 cause the only safe location for the 
 sensor is on the rib or roof in the low- 
 flow boundary layer. It is possible to 
 have large changes in the overall airflow 
 with little or no change in the veloci- 
 ties in the boundary layer and conse- 
 quently in the sensor output. The reader 
 is referred to reference 13 for guide- 
 lines for avoidance of these problems in 
 airflow measurement. 
 
89 
 
 4.4.2 Telemetry 
 
 4.4.3 Reliability 
 
 The telemetry system obtains the 
 data from the sensor, converts them to a 
 standard format, sends them to another 
 unit that receives them, checks their 
 authenticity, and then refers them to the 
 analysis-display device. The principal 
 problem in this area is data security, 
 i.e., the error rate for information 
 transmission. The problem is not so much 
 that an error is transmitted but that an 
 error in transmission goes undetected be- 
 cause of the noise on the transmission 
 line. The sensitivity to erroneous data 
 transmission depends upon factors such as 
 the cable used, the local noise field, 
 length of cable run, and data formatting. 
 
 Techniques for detecting erroneous 
 data transmissions have been devised 
 principally by computer manufacturers. 
 Notable among these are IBM's synchronous 
 data link control (SDLC) and Digital 
 Equipment Corp.'s digital data communica- 
 tions message protocol (DDCMP). 
 
 Bureau research (iL~A^»^ indicates 
 that the maximum transmission distance 
 for one undetected random error per year 
 varies between 1.3 and 6.8 miles in an 
 average noise field, and between 0.1 and 
 0.6 mile in an estimated maximum noise 
 field. Since cable runs are frequent- 
 ly several miles, occasional undetected 
 transmission errors can be expected. For 
 typical monitoring applications with fre- 
 quent data refresh, this should not be 
 a factor that causes worry; however, in 
 the case of control systems or the least 
 favorable monitoring circumstances, er- 
 ror rates can be unacceptably high, and 
 corrective measures such as more secure 
 transmission systems and improved error 
 detection protocols are necessary. 
 
 ■^Underlined numbers in parentheses re- 
 fer to items in the list of references at 
 the end of this chapter. 
 
 The final area of concern is system 
 reliability. The questionnaire identi- 
 fied reliability as the prime concern. 
 The Bureau is currently sponsoring re- 
 search that provides a methodology for 
 determining the reliability of systems 
 (12, 17 , 21). This methodology has been 
 used to evaluate expected failure rates 
 of current mine monitoring systems. 
 
 Reliability in monitoring systems 
 takes a number of forms. The first is 
 mechanical reliability of the components. 
 The underground mining environment is no- 
 toriously hard on equipment because of 
 water, dust, potential damage due to mov- 
 ing equipment, and rough handling. 
 Therefore, the enclosures for remote sta- 
 tions should be rugged enough to with- 
 stand the day-to-day rigors typically 
 encountered in underground service. The 
 enclosures should have tight and durable 
 seals if the internal components are 
 sensitive to moisture or dust. All ex- 
 terior switches and buttons should also 
 be sealed or be durable enough to with- 
 stand constant use in the presence of 
 dirt and moisture. Cables should be dur- 
 able enough to withstand occasional rough 
 treatment. 
 
 The second aspect of reliability is 
 electrical power reliability. Since pow- 
 er outages are all too common in under- 
 ground mining, some type of backup power 
 or uninterruptable power supply should be 
 provided for this system. Such a power 
 supply is particularly important for mon- 
 itoring systems that provide essential 
 health and safety information such as 
 main fan operation, fire detection, and 
 methane content. Obtaining these data is 
 important during the common day-to-day, 
 short-term power outages, but it is just 
 as important to have such information 
 during emergency situtions such as roof 
 falls, fires, or explosions. It is also 
 required for system approval. 
 
90 
 
 4.5 Commercially Available Mine 
 Monitoring Equipment 
 
 4.5.1 Introduction 
 
 The mine monitoring systems dis- 
 cussed in this report are electromechan- 
 ical systems that remotely sense various 
 environmental and operational parameters 
 and transmit the data to a central loca- 
 tion where the data are analyzed and/or 
 displayed. On the basis of this defini- 
 tion, it is reasonable to discuss the 
 system in terms of three basic functions: 
 sensing, data transmission (or telemetry) 
 and data analysis and display. In the 
 case of monitoring and control systems, 
 such as systems that automatically and 
 remotely deenergize face equipment when 
 the methane content at a specified lo- 
 cation reaches a predetermined level, 
 the control operation presents a fourth 
 function. 
 
 Sensing can be divided into two gen- 
 eral categories: environmental and oper- 
 ational or production sensing. The first 
 category of sensors is designed to mea- 
 sure various aspects of a mine's environ- 
 ment to assist in maintaining a safe en- 
 vironment for underground personnel. The 
 parameters that are ordinarily of concern 
 are gas (i.e., carbon monoxide, methane, 
 oxygen, etc.) content, air velocity, air 
 temperature, differential pressure, and 
 humidity. Typically, the data are used 
 to detect and locate potentially hazard- 
 ous conditions (i.e., fires, gas bursts, 
 etc. ) so that the appropriate measures 
 can be taken. Production sensors are 
 used to monitor the operating status of 
 various pieces of underground equipment 
 to detect production bottlenecks, equip- 
 ment malfunctions, maintenance require- 
 ments, etc. Examples of production pa- 
 rameters that are typically of interest 
 are belt output, face equipment opera- 
 tion, belt slippage, blockages, and bear- 
 ing temperatures or vibration. 
 
 Telemetry is the process of trans- 
 mitting the data output of the sensors to 
 
 the control center that is usually lo- 
 cated aboveground. The output of the 
 sensors can be either a simple status in- 
 dication, sometimes called a binary, con- 
 tact closure, or status output (such as 
 high-low, open-closed) or it can be a 
 continuously variable function of time 
 (such as air velocity, methane concentra- 
 tion, etc.). While the continuously var- 
 iable data can provide significantly more 
 information than the simple status data, 
 how much more depends on the accuracy of 
 the measurement. 
 
 As a practical matter, it is gener- 
 ally not feasible to run a separate con- 
 ductor or conductor pair to each sensor. 
 Therefore, telemetry systems typically 
 incorporate several remote stations or 
 "outstations , " each of which accepts and 
 encodes the output of a number of sensors 
 and transmits the encoded data along a 
 common cable to the control center. The 
 two most common encoding techniques are 
 (1) frequency domain multiplexing and (2) 
 time domain multiplexing. Frequency do- 
 main multiplexing has the advantage that 
 data from all monitoring points are re- 
 ceived at all times, although the number 
 of monitoring points is limited by the 
 overall bandwidth of the system. Time 
 multiplexing can be expanded, at least in 
 principle, to accommodate as many moni- 
 toring points as desired. However, each 
 point is sampled only intermittently 
 (i.e. , the receiver obtains data from 
 only one monitoring point at a time) 
 since the system interrogates the moni- 
 toring points sequentially. The cycle 
 time, or time between successive sam- 
 plings of the same point, is the time the 
 system requires to interrogate all of the 
 monitoring points. 
 
 Time multiplexed systems, the more 
 common of the two, often transmit data in 
 the digital format. That is, a series of 
 high-low state indications is transmitted 
 to indicate the status of the monitor 
 point. A common technique to accomplish 
 this transmission is to use frequency 
 shift key (FSK) encoding. This encoding 
 
91 
 
 process uses two different frequencies 
 (for example, 3,000 and 2,000 Hz) to rep- 
 resent the high and low states, rather 
 than high and low level signals of the 
 same frequency. The FSK encoded data are 
 less affected by noise on the transmis- 
 sion line than data transmitted in simple 
 high-low digital format. In addition, 
 current signal detection techniques make 
 it very easy to detect single frequency 
 signals in the presence of noise. 
 
 The third basic function of a moni- 
 toring system is the analysis and display 
 of the measured data. These operations 
 are normally accomplished in an above- 
 ground control center. Most of the sys- 
 tems have the ability to trigger audio- 
 visual alarms if a sensor detects that 
 its predetermined threshold (such as 1% 
 methane in a return airway) has been ex- 
 ceeded. Most of the systems can also 
 provide hard copy documentation of the 
 alarms and display the actual values de- 
 tected by the sensors, either on meters 
 or CRT's. The computer-based systems 
 have the added capability of data storage 
 for trend analysis, record keeping, and 
 reporting. 
 
 In the following discussion on com- 
 mercially available equipment, a distinc- 
 tion is made between the system suppliers 
 and the sensor suppliers. The distinc- 
 tion is made since in many cases the 
 system supplier expects the mine to 
 select not only the parameters to be 
 monitored but also the sensors to be 
 used. The system supplier then config- 
 ures a monitoring system using both in- 
 house hardware and hardware from outside 
 suppliers to provide the mine with the 
 desired information. Ordinarily, the da- 
 ta telemetry-analysis and display equip- 
 ment is the supplier's own brand, while 
 the sensors are obtained from outside 
 companies. In most cases, the supplier 
 will assume "full system responsibility." 
 That is, it will not only provide the 
 telemetry-analysis and display equipment, 
 but will also ensure proper interfaces 
 for any sensors selected by the mine, 
 provide software to process the data. 
 
 and assist the mine during installation, 
 testing, and operator training. The 
 costs for these services, however, may 
 sometimes be broken out separately from 
 the hardware costs. For mines that pre- 
 fer to use in-house personnel for these 
 tasks, systems suppliers that restrict 
 themselves to providing the hardware 
 alone may be worthwhile. 
 
 4.5.2 Telemetry-Analysis and Display 
 Systems 
 
 Table 4-2 summarizes the major mine 
 monitoring systems currently available in 
 the United States. Three of the systems, 
 Davis, Hawker Siddeley, and Transmltton, 
 were originally based on the MINOS system 
 developed by the National Coal Board in 
 Great Britain. Systems offered for sale 
 in the United States may, however, differ 
 from the original MINOS system. Most of 
 the systems have the capacity for accept- 
 ing input from a wide variety of environ- 
 mental and production sensors. An indi- 
 cation of the extensive use of monitoring 
 systems abroad can be obtained by compar- 
 ing the U.S. -foreign installations for 
 the two British systems. While these 
 systems are used extensively abroad, they 
 are just beginning to be accepted in the 
 United States. 
 
 As discussed previously, most of the 
 systems use an FSK format for data trans- 
 mission. The exceptions are Conspec, 
 Mundix, and Transmitton, which use a 
 direct binary transmission. 
 
 While the range in the number of 
 monitoring points and cable length is 
 substantial, most systems should provide 
 sufficient capacity for typical usage. 
 
 In terms of system costs, one 
 manufacturer uses a "rule-of-thumb" of 
 $50,000 for the central station and 
 $20,000 per mine section. 
 
 The final category of table 4-2 in- 
 dicates which suppliers usually assume 
 overall system responsibility. 
 
92 
 
 TABLE 4-2. - Mine monitoring systems currently available in the United States 
 
 
 Aquatrol 
 
 Conspec 
 
 Davis 
 
 Giangarlo 
 
 Hawker 
 Siddeley 
 
 Kidde 
 
 Current installations: 
 Coal mining. ............ 
 
 1 
 
 1 
 
 3,000 
 
 FSK 
 
 NA 
 
 2 
 
 '1 
 
 110 
 NAp 
 
 40-75 
 5-10 
 
 Yes 
 
 4 
 
 12 
 
 Several 
 
 DB 
 768 
 
 4+S 
 
 10.8 
 
 110 
 12 
 
 25 
 0.25 
 
 Yes 
 
 1 
 
 
 
 FSK 
 '5,080 
 
 2+S 
 
 4+S 
 
 20 
 
 110 
 NAp 
 
 100 
 25 
 
 Yes 
 
 14 
 
 8 
 
 FSK 
 26,400 
 
 2+S 
 
 20 
 
 110 
 12 
 
 15-23 
 33-7 
 
 Yes 
 
 >100 
 NA 
 NA 
 
 NA 
 '6,944 
 
 7+S 
 
 8 
 
 110 
 NAp 
 
 NA 
 NA 
 
 Yes 
 
 4 
 
 Metal-nonmetal mining... 
 Other industries 
 
 Specifications: 
 
 Data transmission 
 
 Maximum number of moni- 
 toring points. 
 
 Cable (number of con- 
 ductors). 
 
 Maximum cable miles., 
 length. 
 
 Power requirements, V: 
 Ac 
 
 
 100 
 
 FSK 
 1,024 
 
 1+S 
 
 Coax. 
 
 10 
 
 110 
 
 Dc 
 
 NAp 
 75-100 
 
 Cost, thousand dollars: 
 Central station. ........ 
 
 Outstation. ............. 
 
 3-4 
 
 Overall system 
 responsibility. 
 
 Yes 
 
 
 MSA 
 
 Mundix 
 
 Outokompu 
 
 R.F.L. 
 
 Sangamo 
 Weston 
 
 Trans- 
 mitton 
 
 Current installations: 
 Coal mining. ............ 
 
 1 
 
 
 
 FSK 
 72,000 
 
 2+S 
 
 8 
 
 110 
 12 
 
 25 
 
 1 
 
 Not 
 normally. 
 
 
 
 1 
 1 
 
 DB 
 24,096 
 
 4+S 
 
 (HF) 
 
 128 
 
 110 
 24 
 
 80 
 3 
 
 Yes 
 
 
 
 1 
 >100 
 
 FSK 
 Unlimited 
 
 2+S 
 
 >6 
 
 110 
 24 
 
 5-100 
 3.3 
 
 Yes 
 
 1 
 
 
 
 100 
 
 FSK 
 4128 
 
 2+S 
 
 >10 
 
 NAp 
 12 
 
 1.3-1.5 
 0.7 
 
 Not 
 normally. 
 
 
 
 1 
 
 >100 
 
 FSK 
 8,192 
 
 2+S 
 
 4+S 
 
 '2 
 
 110 
 12 
 
 15-100 
 5-15 
 
 Not 
 normally. 
 
 200 
 
 Metal-nonmetal mining. . . 
 Other industries 
 
 Specifications: 
 
 Data transmission 
 
 Maximum number of moni- 
 toring points. 
 
 Cable (number of con- 
 ductors) . 
 
 Maximum cable miles., 
 length. 
 
 Ac 
 
 
 
 
 DB 
 7,392 
 
 4+S 
 
 10 
 
 110 
 
 Dc 
 
 NAp 
 
 50-60 
 
 Cost, thousand dollars: 
 Central station 
 
 Outstation. ............. 
 
 4-8 
 
 Overall system 
 responsibility. 
 
 Yes 
 
 DB Direct binary. 
 
 FSK Frequency shift key, 
 
 'Extendable. 
 
 2Digital inputs. 
 
 NA Not available. 
 NAP Not applicable. 
 ^Includes CO monitor. 
 ''# per station. 
 
 S Shield. 
 
93 
 
 4.5.2a Systems Suppliers 
 
 The system is typically purchased 
 from a vendor who will supply the 
 telemetry-analysis and display equipment. 
 Since this vendor will usually ask the 
 mine operator to specify the sensors used 
 in the system, sensor operating princi- 
 ples and available sensors are discussed 
 separately. 
 
 All of the systems suppliers, with 
 the exception of R.F.L. , provide com- 
 puter-based systems with printers, CRT's, 
 software packages and auxiliary power 
 in case of main mine power failure. The 
 listing is of necessity incomplete, it 
 does not represent endorsement by the 
 Bureau of Mines, nor is responsibility 
 assumed for any errors that may have 
 occurred in system performance descrip- 
 tions. Much of the material was ab- 
 stracted from telephone conversations 
 with and brochures received from the 
 designer-manufacturers . 
 
 Aquatrol Corp. 
 2258 Terminal Road 
 St. Paul, MN 55113 
 (612) 636-3950 
 
 Aquatrol Corp. markets an Intel 
 8085-based monitoring and control system 
 that is sold primarily to water treatment 
 facilities. It has full system capabil- 
 ity including color CRT, printer, and 
 262,000 random-access memory (RAM) floppy 
 disk storage. The system will accommo- 
 date up to 98 outstations on the master 
 trunk, with up to 18 data channels per 
 outstation. Each channel can be analog, 
 resolved to 12 bits, or the 12-bit digi- 
 tal word can be treated as individual 
 binary level inputs. 
 
 Similarly, each outstation can out- 
 put up to 6 analog channels or 12 binary 
 drive levels for each analog channel less 
 than 6. 
 
 The cable is two-conductor voice 
 grade, operable to 5,000 ft or, with an 
 extender, further. Power is 110-volt ac 
 or 12-volt dc for both the central or the 
 remotes. 
 
 Telemetry is at 
 RS232C format. 
 
 300 baud using 
 
 A training program about a week long 
 is offered at their plant. They will 
 assume system responsibility and quote a 
 maintenance contract if desired. 
 
 Conspec Controls, Inc. 
 901 Fuhrmann Blvd. 
 Buffalo, NY 14203 
 (716) 854-4769 
 
 The Conspec Senturion Series 200 
 monitoring system consists of a central 
 processor that has a CRT display and key- 
 board, two printers, one or more "data 
 concentrators," which are interface de- 
 vices, and a capability for up to 768 
 sensing (monitoring and/or control) 
 points on any of four trunks, each capa- 
 ble of carrying 128 locations. At the 
 sensing location, the electronics neces- 
 sary to interface a sensor onto the trunk 
 (including entering its address) are 
 housed on a 4- by 6-inch accessor card, 
 which in many cases is incorporated into 
 the sensor package. 
 
 Transmission is over four-conductor 
 cable; two for power and two for signal. 
 The power required is up to 30 volts dc, 
 and the maximum end-to-end length is 
 4,000 ft. If greater distances are re- 
 quired, a trunk extender can be used; 
 powered by 110 volts ac, it permits oper- 
 ation to 40,000 ft. A 600-baud (bit per 
 second) FSK telephone modem is also 
 available. 
 
 Data transmission is in a noise- 
 resistant binary format. There are no 
 remote stations, other than the accessor 
 at each measurement location. Either 
 binary or analog data can be transmitted. 
 
 Display is in a 20-character alpha- 
 numeric format (20 letters or numbers) so 
 that interpretation is simplified. A 
 complete software package is offered that 
 has the capability to alarm on thresholds 
 and set high-low points. Set points can 
 be entered from the keyboard. There is 
 central reset of remote points, a pro- 
 gramed restart or load-shedding feature. 
 
94 
 
 time of day or week programing, and se- 
 quencing initiated by alarm or keyboard 
 entry. 
 
 Alarm states are printed and dis- 
 played separately. Routine information 
 such as end of shift reports may also be 
 generated. 
 
 Standard accessors available include 
 interfaces for thermistors, pressure, 
 RTD, power, current (4-20 mA, 0-5 volts, 
 0-1 mA) , potentiometer, alarm states, 
 two- or three-state load control, and 
 electrical demand. 
 
 John Davis and Sons (Derby) Ltd. 
 Alfeton Road 
 Derby DE2 4AB 
 England 
 
 Service Machine Co., Inc. 
 Box 8177 
 
 6072 Ohio River Road 
 Huntington, WV 25702 
 Attn: Mr. J. H. Nash 
 
 A variety of system configurations 
 is available. Outstations have been 
 designed primarily for haulage, machin- 
 ery monitoring, and communication. For 
 example, the type 25200 equipment ac- 
 cepts eight thermal probes and two other 
 transducers, such as pressure and flow. 
 Thresholds are set locally with a poten- 
 tiometer. Such a unit would be appropri- 
 ate as a compressor, fan, or pump moni- 
 tor. There are local and remote stop 
 modes. 
 
 Similarly, the FMSl type 25000 ac- 
 commodates six transducer inputs, and has 
 six relay outputs for indication and/or 
 control. It can perform level detection, 
 temperature detection, and time delay 
 functions. Applications listed include 
 haulage, bunker, pump, and refrigeration 
 plant monitoring. It is powered with 
 110-volt main power. 
 
 Other communication, conveyor con- 
 trol, and signaling devices are also 
 offered. 
 
 Davis of Derby offers a data trans- 
 mission system consisting of a surface 
 master station with two video displays, a 
 control keyboard, and a communications 
 switchboard. Associated with this cen- 
 tral station are up to 127 outstations, 
 each capable of monitoring up to 40 
 transducers. Interconnection is by ei- 
 ther two- or four-conductor shielded ca- 
 ble. Telemetry is accomplished digitally 
 with FSK coding. 
 
 Outstations can be wired to accommo- 
 date prestart warning, belt slip, and 
 other transducers, including temperature 
 and pressure. 
 
 Virtually all Davis of Derby under- 
 ground equipment is housed in flameproof 
 housings, and most circuitry is designed 
 to be intrinsically safe. Certification 
 and design are to international stan- 
 dards, such as Cenelec Standard 50 020 
 and IS1902 Class I. Circuitry is low- 
 power CMOS. Standby battery power is 
 available with automatic changeover. 
 
 Giangarlo Scientific Company, Inc. 
 2500 Baldwick Road 
 Pittsburgh, PA 15205 
 (412) 922-8850 
 
 The Giangarlo system consists of a 
 central processor with associated outsta- 
 tions. The outstations are microcomputer 
 controlled and are capable of accepting 
 up to five input boards. Each board may 
 consist of either 8 analog inputs, 16 
 digital inputs, or 8 relay inputs, and 
 each board has light-emitting diode (LED) 
 status lamps for troubleshooting. There 
 is battery backup power. Telemetry re- 
 quires a three-conductor shielded cable 
 with data packaged in an ASCI II serial 
 FSK digital format, run at 300 to 1,200 
 baud. Remote stations can alarm on their 
 own, with visual and audible alarms. 
 
 At the central computer, software 
 is available that will use redundancy 
 checks of data to identify faulty trans- 
 mission. It can display data, set remote 
 alarm thresholds, and perform control 
 
95 
 
 functions. Central computer alarms are 
 independent of remote alarm status. 
 
 There is provision for interface 
 from the central computer to a teletype, 
 CRT display, disk memory, or another 
 computer. 
 
 Input parameters that might be 
 measured and transmitted to the central 
 station include carbon monoxide, carbon 
 dioxide, methane, temperature, air veloc- 
 ity, radon, belt slippage, belt speed, 
 weight, pressure, power, etc. 
 
 Hawker Siddeley Dynamics 
 
 Engineering Ltd. 
 Manor Road 
 
 Hatfield, Hertfordshire ALIO 9LP 
 England 
 Hatfield (07072) 68234 
 
 United Technologies Bacharach 
 301 Alpha Drive 
 Pittsburgh, PA 15238 
 Attn: Mr. David M. Nelson 
 (412) 784-2137 
 
 Hawker Siddeley is one of the quali- 
 fied manufacturers of the MINOS system. 
 It offers systems for environmental moni- 
 toring, conveyor and bunker control and 
 monitoring, and mine cage monitoring. 
 The "Dynalink" system for conveyor con- 
 trol and monitoring consists of a surface 
 control center with provision for local 
 control and monitoring or for remote 
 control from the remote station. Out- 
 stations are connected to the central 
 station with six-conductor cable at dis- 
 tances up to several miles. Sixteen 
 outstations can be carried on one cable, 
 and seven cables can be accommodated 
 
 The control station has a dual visu- 
 al display. Monitored data, or change of 
 state, or even mimic diagram graphics can 
 be displayed. The central computer is a 
 DEC PDP U/34 or CAI ALPHA LS1-2/20G. 
 Microprocessors used are Intel 8080 and 
 8085. There is dual control with auto- 
 matic switchover for reliability. 
 
 Each outstation can accommodate 32 
 input channels, expandable in increments 
 
 of eight. Power is 110 volt, 50 Hz, or 
 any standard mine supply. 
 
 The bunker-conveyor monitoring and 
 control system can automate haulage from 
 the vicinity of the face to the sur- 
 face, including bunker monitoring and 
 metering. 
 
 Their mine cage monitor maintains 
 the cage within speed and distance lim- 
 its, comparing cage performance to a de- 
 sired operation profile every 0.1 sec and 
 making the appropriate corrections. 
 
 The environmental monitor can accept 
 either analog or digital inputs from 
 sensors such as anemometers, methanom- 
 eters, pressure sensors, and bearing tem- 
 perature probes. 
 
 Kidde Automated Systems 
 (Formerly S.R. Smith Co., Inc.) 
 7256 County Line Road 
 Deerfield, IL 60015 
 (312) 272-8012 
 
 The S.R. Smith System uses a remote 
 "data collection panel" architecture. 
 Each remote station is capable of receiv- 
 ing up to 16 contact closure inputs. The 
 remote stations also contain relays that 
 are capable of control functions such as 
 starting-stopping belts. A new system 
 has a capability for the transmission of 
 analog levels. 
 
 The central computer facility uses 
 Digital Equipment Corp. display and anal- 
 ysis equipment. A CRT displays informa- 
 tion from any of 1,024 monitor points, 
 connected to any of 64 remote stations 
 per channel. Telecommunication is over a 
 two-wire pair, or coaxial line, or 3002 
 Telco for each channel. 
 
 A complete interactive software 
 package is available that displays alarms 
 in a prioritized list. Data are also 
 printed on a high-speed printer, as an 
 aid to failure diagnosis and as a perma- 
 nent record. Up to 700 characters of 
 description and instruction are possible 
 per alarm point. 
 
96 
 
 Intended applications include belt 
 monitoring (slip, alignment, power, chute 
 plugging, etc.), power center monitoring 
 (breaker position, voltage in range, open 
 ground, etc.). environmental monitoring 
 (methane within limits, air velocity 
 within limits, etc.), fire monitoring, 
 and fan monitoring (operating, tempera- 
 ture in range, etc.). The system can be 
 used with a card reader to control and 
 monitor people underground or in a con- 
 trolled area. 
 
 Mine Safety Appliances Co. 
 600 Penn Center Blvd. 
 Pittsburgh, PA 15235 
 (412) 273-5000 
 
 Catalyst Research Corp. 
 3706 Crandall Lane 
 Owing Mills, MD 21117 
 (301) 356-2400 
 
 Catalyst Research Corp. has designed 
 and tested a computer based supervisory 
 control and data acquisition system 
 (SCADA) which is also available from MSA. 
 The system can accommodate up to 38 field 
 data stations. Each data station is ca- 
 pable of receiving eight analog inputs, 
 plus any one of the following: 16 digi- 
 tal inputs or 4 digital outputs. 
 
 Mundix Control Systems, Inc. 
 5495 Marion St. 
 Denver, CO 80216 
 (303) 296-1790 
 
 Mundix offers a supervisory control 
 and data acquisition system that can ac- 
 commodate up to 128 outstations. Each 
 outstation can be configured with four 
 electronic interface cards (extensible 
 to 16). Each card can accommodate any 
 one of the following: four analog in- 
 puts (12-bit resolution), eight digital 
 inputs, eight digital outputs, or four 
 analog outputs. 
 
 The total system capacity is 4,096 
 digital inputs or 1024 analog. Telemetry 
 to the central station is accomplished 
 using a 500-kHz digital phase modula- 
 tion technique (Manchester coding). As 
 a result, the system uses two-conductor 
 shielded cable. The maximum transmission 
 distance is stated to be 128 miles. 
 
 The central station is computer- 
 based, and BASIC language programable. 
 There are printers and color or black and 
 white CRT monitors available. Power to 
 the central station is 110-volt ac, while 
 remotes can be powered with either 110- 
 volt ac, 24-volt dc, or 120-volt dc. 
 
 Remote stations can be up to 18 
 miles from the central station. Cabling 
 is by two-conductor No. 19 gage twisted 
 pair. The central station is computer 
 based. It uses a Digital Equipment Corp. 
 LSIll microprocessor with 64,000 bytes of 
 memory. Data transfer from remotes to 
 the central station uses RS232 at up to 
 2,400 baud. 
 
 Interactive, user friendly software 
 is available. Alarm states are indicated 
 audibly and automatically printed. Hour- 
 ly and daily summaries can be generated. 
 
 Outokumpu Engineering 
 4680 Packinghouse Road 
 Denver, CO 80216 
 (303) 371-0540 
 
 There are hundreds of Outokumpu mon- 
 itoring systems installed in Europe, pri- 
 marily in power control applications. It 
 is a computer based system (DEC) with an 
 interactive English control software. 
 The system has a capability for up to 
 30,000,000 bytes of storage on a Winches- 
 ter disk and printer, CRT options. 
 
97 
 
 The remote outstations are typical- 
 ly configured to accept 4 analog in- 
 puts, 16 digital inputs, and 16 digi- 
 tal control outputs. There are two types 
 of outstations, the miniremote described 
 above, and larger, higher capacity units. 
 Outstations can be interconnected in 
 any series parallel configuration. 
 Analog data is resolved to a 12-bit 
 precision. 
 
 Telemetry is accomplished using a 
 serial RS232 FSK format at 300 baud (50- 
 600) . Outstations can be up to 6 miles 
 distant. Two-conductor twisted pair ca- 
 ble is required. 
 
 Power to the central and remote sta- 
 tions is 110-volt ac, 220-volt ac, or 24- 
 volt dc. 
 
 R.F.L. Industries, Inc. 
 Boonton, NJ 07005 
 (201) 334-3100 
 
 R.F.L. Industries manufactures te- 
 lemetry equipment consisting of build- 
 ing blocks that can be combined to 
 satisfy progressively more demanding 
 system requirements. The simplest sys- 
 tem consists of frequency-multiplexed 
 transmitter-receiver pairs that operate 
 with center frequencies between 300 Hz 
 and 30 kHz in spacings of 100 and 120 Hz 
 or more. These devices are available in 
 two configurations: an AM system, in 
 which the carrier is keyed on or off with 
 a 12-volt dc input status, or a system 
 with a switch closure. A more secure 
 frequency-shifted version uses the same 
 audio band center frequencies, but 
 frequency-shift codes the data in any of 
 four codes including a two-frequency code 
 (mark-space) and a three-frequency code 
 (mark-center-space) . Both systems are 
 powered by 12-volt dc. If standard CCITT 
 channel spacing is used, there are 46 
 carrier channels between 300 Hz and 10 
 kHz, with more at higher frequencies. 
 
 If there is a need to transmit ana- 
 log data, the model 64B series converts 
 input voltages in the 0.4- to 2-volt 
 range, or input current 4 to 20 mA, 10 to 
 50 mA, etc., into a square wave output, 
 frequency coded in the 5- to 25-Hz range. 
 After voltage- or current-to-frequency 
 conversion, these low-frequency codes are 
 transmitted by using the frequency shift 
 transmitter-receiver described. 
 
 If large numbers of status-control 
 signals are to be telemetered, the mod- 
 el 66A encoder-decoder will accommodate 
 16 input channels. These data are 
 then time-division-multiplexed (TDM) 
 with a redundant transmission (normal 
 and polarity-inverted, doublescan format) 
 to ensure reliable transmission. There 
 are two parity check bits, so that par- 
 ity errors up to the third order can be 
 detected. Once again the data are 
 telemetered with one of the frequency- 
 shift, frequency-multiplexed transmitter- 
 receiver pairs. The scan time with the 
 normal 60-baud system is 1.1 sec with 
 doublescan and 0.6 sec with single scan. 
 Input data can be binary level or switch 
 closure inputs. Power is 12 volts dc. 
 
 Analog channels can also be accommo- 
 dated on the TDM system. Analog inputs 
 are channeled, one at a time, to an A-D 
 converter. The analog quantity is digi- 
 tized and this parallel digital signal, 
 together with a corresponding digital ad- 
 dress, is fed to a 66A encoder. Trans- 
 mission is now similar to that in the 
 foregoing paragraph. At the receiving 
 station, the message is decoded, and the 
 digital output is channeled to the corre- 
 sponding D-A converter to produce an ana- 
 log output. 
 
 Sangamo Weston, Inc. 
 Industrial Products 
 P.O. Box 3041 
 Sarasota, FL 33578 
 (813) 371-0811 
 
98 
 
 Sangamo Weston manufactures three 
 different monitoring and control systems. 
 Their RECON I has a capability for up to 
 127 remote stations, each with 12 analog 
 inputs, resolved to 12-bit precision, and 
 12 output levels. Channels can be ex- 
 tended fourfold as an option. Operation 
 is either manual or by computer control. 
 Cycle time is 0.5 sec per channel. 
 
 The RECON II has eight 
 remote and can accommodate 
 channels each, resolved 
 precision (0.4%). It is 
 an Intel 8080. The output 
 terminal. Cycle time is 
 channel. 
 
 channels per 
 eight analog 
 to eight-bit 
 controlled by 
 is a printer 
 0.5 sec per 
 
 The RECON III is a computer- 
 controlled system that uses a Digital 
 Equipment Corp. PDP-11/24 programed in 
 RSXllM. Peripherals include a color CRT 
 and printer. It is capable of 256 remote 
 stations. Reporting is by exception 
 (report when exceed limits). Each remote 
 is capable of 16 analog inputs and has 
 16 output control relays (KU series 20A 
 relays). Resolution of input signal is 
 12 bits (0.024%). The system has a data 
 base and graphics edition and a capabil- 
 ity to download to remotes over the data 
 line. Remotes can scan subremotes. 
 
 A microprocessor-based system, MIC- 
 RECON, is under development and will be 
 released soon. 
 
 Transmitton Ltd. 
 Smisby Road 
 Ashby-De-la-Zouch 
 England LE6 5UG 
 0530-415941 
 
 Reliability Technology 
 150 Plum Industrial Court 
 Pittsburgh, PA 15239 
 (412) 325-3121 
 
 Transmitton Ltd. is also qualified 
 to manufacture the British MINOS system. 
 
 The Transmitton control consol consists 
 of one or two color video displays, a 
 keyboard, two switch panels, and a com- 
 munications center. The Transmitton sys- 
 tem, represented in the United States by 
 Reliability Technology, is designed to 
 monitor and control the operation of con- 
 veyors, pumps, electrical switches, shaft 
 elevators, ventilation fans, and bunkers, 
 as well as environmental parameters such 
 as methane, carbon monoxide, air ve- 
 locity, air pressure, temperature, and 
 smoke. 
 
 The system consists of a central 
 control station and up to 168 outsta- 
 tions. Data are transmitted in a digital 
 format, but not frequency coded; i.e., 
 high-low states are transmitted. A four- 
 conductor cable is used. 
 
 Conveyor monitoring includes belt 
 speed, misalignment, belt weight, torn 
 belt, motor voltage current and tempera- 
 ture, vibration, and alarm states. Sonic 
 and visual alarms are available locally 
 and can be controlled locally or remote- 
 ly. Belt start sequencing can be done 
 automatically, so that belts can be shut 
 down to conserve power. 
 
 Pump monitoring might include 
 diagnostics, such as electrical cur- 
 rent drain, pressure, flow, etc. Energy 
 shedding can be accomplished. Power 
 center monitoring can be used to reset 
 breakers remotely or to locate a prob- 
 lem. Software for summary analysis is 
 included. 
 
 The Transmitton systems have been 
 installed in hundreds of deep coal mining 
 operations, internationally. 
 
 4.5.2b Summary 
 
 A summary of the monitoring sys- 
 tems offered by suppliers currently mar- 
 keting in the United States is given in 
 table 4-3. 
 
99 
 
 TABLE 4-3. - Mine monitoring system summary 
 
 Aquatrol 
 
 Conspec 
 
 Davis 
 
 Giangarlo 
 
 Hawker 
 Siddeley 
 
 Kidde 
 
 1 
 
 4 
 
 1 
 
 1 
 
 7 
 
 2 
 
 98 
 
 128 
 
 127 
 
 512 
 
 16 
 
 64 
 
 18 
 
 1 analog or 
 16 digital 
 in; 1 dig- 
 ital out. 
 
 40 
 
 5 boards 
 
 32 
 
 24 
 
 <18 
 
 1 (16 bit) 
 
 8 
 
 18 
 
 NA 
 
 8 
 
 218 analog 
 
 16 (1 bit) 
 
 32 
 
 '16 
 
 NA 
 
 16 
 
 <6 
 
 NAp 
 
 NAp 
 
 NAp 
 
 NA 
 
 NAp 
 
 2 6 analog 
 
 1 (1 bit) 
 
 4 
 
 '16 
 
 NA 
 
 8 
 
 ^1 mi 
 
 44,000 ft 
 
 20 mi 
 
 20 mi 
 
 8 mi 
 
 10 mi 
 
 40-75 
 
 25 
 
 100 
 
 15-23 
 
 NA 
 
 75-100 
 
 5-10 
 
 0.25 
 
 25 
 
 53-7 
 
 NA 
 
 3-4 
 
 MSA 
 
 Mundix 
 
 Outokompu 
 
 R.F.L. 
 
 Sangamo 
 Weston 
 
 Trans- 
 mit ton 
 
 3 
 
 1 
 
 Unlimited 
 
 1 
 
 32 
 
 6 
 
 38 
 
 128 
 
 32 
 
 1 
 
 8 
 
 28 
 
 ^24 
 
 '^16 boards 
 
 20 
 
 8 boards 
 
 32 
 
 28 
 
 8 
 
 '4 
 
 4 
 
 '16 
 
 16 
 
 16 
 
 16 
 
 '8 
 
 16 
 
 '16 
 
 16 
 
 28 analog 
 
 NAp 
 
 '4 
 
 NAp 
 
 NAp 
 
 16 
 
 2 
 
 4 
 
 '8 
 
 16 
 
 '16 
 
 8 
 
 28 
 
 8 mi 
 
 128 mi 
 
 >6 mi 
 
 >10 mi 
 
 32 mi 
 
 10 mi 
 
 25 
 
 80 
 
 5-100 
 
 1.3-1.5 
 
 15-100 
 
 50-60 
 
 1 
 
 3 
 
 3.3 
 
 0.7 
 
 5-15 
 
 4-8 
 
 Trunks 
 
 Outstations per trunk. 
 Total input channels 
 per outstation. 
 
 Inputs per outstation: 
 
 Analog 
 
 Digital 
 
 Outputs per outstation: 
 
 Analog 
 
 Digital 
 
 Transmission distance. . 
 Cost, thousand dollars: 
 
 Central station 
 
 Outstation 
 
 Trunks 
 
 Outstations per trunk.. 
 Total input channels 
 
 per outstation 
 
 Inputs per outstation: 
 
 Analog 
 
 Digital 
 
 Outputs per outstation: 
 
 Analog 
 
 Digital 
 
 Transmission distance. . 
 Cost, thousand dollars: 
 
 Central station 
 
 Outstation 
 
 NA Not available. 
 'Per board. 
 2Times 12. 
 ^Extendable. 
 
 4.5.3 Sensors 
 
 NAp Not applicable. 
 "^Typical, 40,000 ft maximum. 
 5 Includes CO monitor. 
 ^575 maximum data points. 
 
 74,096 digital 
 maximum. 
 
 and 1,024 analog. 
 
 While mine monitoring systems can 
 include both environmental and production 
 sensors, this report focuses on environ- 
 mental sensors. Of particular interest 
 in environmental monitoring are air ve- 
 locity, methane and carbon monoxide con- 
 centration, and respirable dust. 
 
 Air velocity can be measured by us- 
 ing either vane anemometers or acoustic 
 vortex-shedding anemometers. Vane ane- 
 mometers are basically mechanical devices 
 
 in which the airflow causes the vanes or 
 impellers to rotate at a speed propor- 
 tional to the airflow. Although they are 
 most commonly used underground as direct 
 reading, portable instruments, they can 
 also be adapted to a mine monitoring sys- 
 tem. However, despite the advantage of 
 mechanical simplicity, they are suscepti- 
 ble to dirt and moisture concentration. 
 Vortex-shedding anemometers, on the other 
 hand, have no moving parts and use acous- 
 tic signals to measure turbulence caused 
 by the airflow. Both anemometers have 
 the disadvantage that they are fixed 
 
100 
 
 point measurements normally made in the 
 boundary layer near the roof or rib and 
 therefore do not necessarily represent a 
 true measure of the average airflow. 
 
 Methane and carbon monoxide concen- 
 tration can be measured by either heat of 
 combustion (i.e., catalytic combustion) 
 or infrared absorption techniques. For 
 carbon monoxide, electrochemical analysis 
 is also commonly used. The heat-of- 
 combustion sensor is based on the princi- 
 ple that catalytic oxidation (i.e., burn- 
 ing) of a combustible gas such as methane 
 will result in a temperature rise in the 
 sensor in proportion to the gas concen- 
 trations. This technique is widely used 
 in the methane monitors required for face 
 equipment in this country. The infrared 
 sensors are based on the fact that dif- 
 ferent gases have different infrared en- 
 ergy absorption characteristics. These 
 sensors have been in use in South African 
 and German mines for a number of years. 
 Electrochemical analyzers measure the 
 carbon monoxide concentration by chemical 
 reaction with electrodes that are im- 
 mersed in an electrolyte. These sensors 
 have been used recently as part of an 
 early warning belt fire detection system 
 (that in some cases allowed the use of 
 the beltway for intake air). 
 
 Remote monitoring of respirable dust 
 presents a number of technical difficul- 
 ties. Of the various measurement tech- 
 niques currently in use, the beta- 
 attenuation and optical devices appear to 
 be the most suitable for integration into 
 mine monitoring systems. The former uses 
 beta radiation to detect dust concentra- 
 tion, i.e. , the amount of beta absorption 
 due to the dust deposited on a sample 
 plate is proportional to the dust concen- 
 tration. The optical sensors are based 
 on the principle that the dust concentra- 
 tion is proportional to the amount of 
 light reflected by the dust-laden air 
 sample. 
 
 The following sections describe the 
 sensing or measurement techniques for the 
 four parameters of interest that are most 
 applicable to underground mine monitor- 
 ing. That is, only sensors that are 
 
 suitable for remote, fixed point opera- 
 tion underground and that provide an 
 electrical output that can be interfaced 
 with standard telemetry equipment will be 
 discussed. 
 
 4.5.3a Air Velocity Sensors 
 
 There are two basic types of air 
 velocity sensors that are applicable to 
 underground mining: rotating vane ane- 
 mometers and acoustic vortex-shedding 
 anemometers. 
 
 The rotating vane anemometers are 
 mechanical devices with vanes or im- 
 pellers that are rotated or turned by the 
 air flowing through the anemometer. The 
 better instruments use ball bearings that 
 reduce the turning friction of the main 
 shaft on which the vanes are mounted to 
 improve the accuracy at low air veloc- 
 ities. Portable (typically hand held) 
 vane anemometers have been a standard air 
 velocity measuring instrument in under- 
 ground mines for a number of years. The 
 Davis vane anemometer is probably the 
 most common example of these direct read- 
 ing instruments. Recently there has been 
 some interest in adopting these in- 
 struments to remote monitoring systems; 
 for example, American Mine Chemical Co. 
 is currently distributing the British 
 Abbriko anemometer that provides an elec- 
 trical pulse count output proportional to 
 the air velocity. However, while the 
 device does have certain advantages be- 
 cause of simplicity of operation, its 
 susceptibility to excessive dirt and 
 moisture represents a significant dis- 
 advantage. The National Coal Board, ex- 
 perimenting with such anemometers (such 
 as its BA. 1 and BA.2), found that in- 
 creasing the vane diameter increased the 
 torque on the center shaft and thereby 
 reduced the potential of the shaft seiz- 
 ing because of dirt accumulation (9^) . 
 
 The second category of anemometers, 
 acoustic vortex-shedding, measure air ve- 
 locity by sensing the frequency at which 
 vortices are shed from a rod placed in 
 the airstream. The vortices, or eddies 
 in the airstream, are sensed by the ef- 
 fect they have on an acoustic (actually 
 
101 
 
 ultrasonic) pulse transmitted through 
 them. A typical configuration would con- 
 sist of a relatively compact package con- 
 taining transmitting and receiving trans- 
 ducers mounted on opposite sides of a 
 small rod and the electronics required to 
 transmit the data to the appropriate out- 
 station or control panel. Since vortex 
 shedding anemometers have no moving 
 parts, they are particularly well suited 
 for underground mines. However, while 
 they are less susceptible to contamina- 
 tion than the vane anemometers, they are 
 also typically more expensive than the 
 mechanical anemometers. 
 
 It should be pointed out that both 
 types of anemometers are fixed point 
 units and as such have the disadvantage 
 of being able to measure the airflow at 
 only one point in the airway. This re- 
 striction is usually compounded by mount- 
 ing the unit close to the roof or rib of 
 the airway, i.e., in the boundary layer 
 where changes in the average airflow can- 
 not always be accurately sensed. Al- 
 though there are empirical methods of 
 compensating for this measurement defi- 
 ciency, they do not always provide the 
 most satisfactory solution. 
 
 The two major suppliers of acoustic 
 anemometry equipment in this country are 
 J-Tec Associates and Mine Safety Appli- 
 ance (MSA). The J-Tec model VA-216B is 
 approximately 12 by 7 by 4 inches in size 
 and can measure air velocity in two 
 ranges; 50 to 3,000 fpm and 150 to 10,000 
 fpm (±2% of full scale). The unit op- 
 erates on a 12 to 21 volts dc at a max- 
 imum of 35 mA. While the standard output 
 is to 5 volts dc, the unit can also 
 provide 1 to 5 mA or 4 to 20 mA as an 
 option. Calibration, performed at the 
 sensor, is typically recommended at 6- 
 week intervals. The price of the unit 
 runs between $1,000 and $1,500, including 
 output electronics. 
 
 The MSA sonic anemometer can measure 
 air velocities up to 25,000 fpm. The 
 sensor requires 110 volts ac power (30 w) 
 and produces either 0-1 mA or 4-20 mA 
 outputs. The list price of the unit is 
 between $500 and $600. 
 
 4.5.3b Methane Sensors 
 
 There are two primary techniques of 
 detecting and measuring methane concen- 
 tration that are suitable for use in mine 
 monitoring systems: heat of combustion 
 and infrared absorption. Of the two, the 
 heat of combustion, or catalytic combus- 
 tion, sensors are the most common in this 
 country. These sensors detect the pres- 
 ence and concentration of methane by mea- 
 suring the temperature rise of a cata- 
 lytic element that oxidizes (i.e., burns) 
 the methane at very low temperatures 
 without a flame. The temperature rise in 
 the catalyst is proportional to the meth- 
 ane content of the air surrounding the 
 sensor. 
 
 There is some difference in the 
 technique by which the sensors expose the 
 catalyst to the gas mixture to be mea- 
 sured. Some devices rely on diffusion of 
 the gas mixture through a porous metal 
 flame arrestor screen. These are often 
 referred to as "diffusion-head" type sen- 
 sors. Others use mechanical pumps to 
 drain air samples across the catalyst. A 
 third method, referred to as "sniff and 
 sneeze," alternately draws the sample in 
 and then exhales prior to the next sam- 
 ple. While the diffusion devices have a 
 slower response time they are simpler and 
 do not rely on mechanical pumps that may 
 be affected by dust and moisture. 
 Diffusion-type methane sensors are typi- 
 cally used in the monitoring and auto- 
 matic deenergizing devices required on 
 face equipment in U.S. coal mines. Al- 
 though catalytic combustion sensors are 
 relatively rugged and simple in opera- 
 tion, they do have (at least in princi- 
 ple) a disadvantage in terms of specific- 
 ity. That is, the catalyst temperature 
 will rise in the presence of any combus- 
 tion gas, not just methane. However, 
 this disadvantage is not always a major 
 problem and can be reduced somewhat by 
 operating at a specified temperature or 
 selecting a catalyst that favors a 
 methane-oriented chemical reaction. A 
 second, and possibly more important, dis- 
 advantage is that catalytic sensors are 
 not generally suitable for measuring 
 methane concentrations above 5%. 
 
102 
 
 The second methane sensing technique 
 is based on the absorption, by different 
 gases, of different amounts of infrared 
 radiation. In a typical configuration, 
 infrared energy is passed through a sam- 
 ple cell that has windows that do not ab- 
 sorb in the infrared band. Either the 
 sensor is equipped with a reference cell 
 or the sensor is calibrated by purging 
 the sample cell with nitrogen prior to 
 making any measurements. 
 
 An infrared detector, located on 
 the opposite side of the cell, produces 
 an electrical signal proportional to 
 the difference between the reference and 
 the sample. This signal is, in turn. 
 
 proportional to the methane concentra- 
 tion. Infrared sensors can be used to 
 measure methane concentrations in the en- 
 tire range between 0% and 100%. While 
 these devices are relatively sensitive 
 and specific, they are typically more 
 complex and expensive than the catalytic 
 sensors. 
 
 Table 4-4 summarizes the methane 
 sensors currently available in this coun- 
 try. Although only one infrared sensor 
 is mentioned, it should be noted that 
 several infrared sensors have 
 veloped in other countries, 
 the South African SPANAIR and 
 UNOR. 
 
 been de- 
 among them 
 the German 
 
 TABLE 4-4. - Methane sensors 
 
 Cost' 
 
 Company 
 
 Model 
 
 Measuring 
 principle 
 
 Range, 
 
 % 
 
 Electrical 
 output 
 
 Power 
 requirements 
 
 Appalachian 
 
 Electronics. 
 Bacharach 
 
 CEA 
 
 CSE 
 
 Dynamation 
 
 ERDCO 
 
 GasTech 
 
 General Monitors... 
 
 J-Tec 
 
 MSA 
 
 NMS 
 
 Scott Aviation 
 
 Texas Analytical... 
 NA Not available. 
 
 102A 
 CD800 
 
 RI550A 
 140 
 
 1210EX 
 
 250 
 
 1620 
 
 NA. 
 
 480 
 
 VMIOIB 
 
 1810-0073 
 
 40008561 
 40008015 
 
 1930B 
 
 Catalyst 
 
 Infrared 
 Catalyst 
 
 . . .do. . . 
 
 . . .do. . . 
 
 . . .do. . . 
 
 . . .do. . . 
 
 . . .do. . . 
 
 . . .do. . . 
 
 . . .do. . . 
 
 . .do. . . 
 . .do. . . 
 
 0-99 
 
 0- 5 
 
 0- 2 
 0- 5 
 
 0- 5 
 
 0-50 
 
 0- 5 
 
 0- 5 
 
 0- 5 
 
 0- 5 
 
 0- 2 
 
 0- 5 
 
 0- 5 
 0- 5 
 
 Digital 
 0-100 mV 
 
 0-10 mV 
 0-100 mV 
 
 0-1 mA 
 
 0-50 mV 
 0-100 mV 
 
 0-100 mV 
 
 4-20 mA 
 
 0-5 V dc 
 
 0-1 V dc 
 4-20 mA 
 
 4-20 mA 
 
 4-20 mA 
 
 0-1 mA 
 4-20 mA 
 
 110 V ac 
 110 V ac 
 
 110 V ac 
 
 110 V ac 
 270 V dc 
 
 110 V ac 
 12 V dc 
 
 110 V ac 
 24 V dc 
 
 110 V ac 
 12 V dc 
 
 110 V ac 
 24 V dc 
 
 12-21 V dc 
 
 110 V ac 
 
 12 V dc 
 
 24 V dc 
 
 110 V ac 
 12-24 V dc 
 
 $1,800 
 1,100 
 
 2,200 
 2,200 
 
 NA 
 
 1,200- 
 3,000 
 
 1,100 
 
 1,200 
 
 500- 
 1,000 
 
 500 
 
 300 
 
 400 
 1,400 
 
 'Rounded to nearest $100. 
 
103 
 
 4.5.3c Carbon Monoxide Sensors 
 
 There are three techniques of car- 
 bon monoxide sensing that may be con- 
 sidered suitable for use in mine moni- 
 toring systems : electrochemical reac- 
 tion, catalytic-combustion, and infrared 
 absorption. 
 
 Electrochemical sensors contain a 
 sensing electrode, a counter electrode, 
 and sometimes a reference electrode in an 
 electrolyte (such as sulfuric acid solu- 
 tion for the Energetic Sciences Ecolyzer 
 2000) . The air to be sampled either is 
 allowed to diffuse into the sensor or is 
 drawn in by a mechanical pump. The car- 
 bon monoxide in the air reacts with the 
 electrodes, generating an electric signal 
 proportional to the carbon monoxide con- 
 centration in the air sample. General 
 Electric Co. has developed, in conjunc- 
 tion with the Bureau of Mines, a fuel 
 cell carbon monoxide sensor that uses a 
 solid polymer electrolyte. This design, 
 of course, has the advantage of not hav- 
 ing a liquid electrolyte that can spill. 
 
 IS 
 
 The catalytic combustion technique 
 quite similar to that discussed 
 
 earlier for methane sensing. That is, 
 the air sample is oxidized in the pres- 
 ence of a catalyst, with the resulting 
 temperature rise in the catalyst being 
 proportional to the gas concentration. 
 As mentioned earlier, the basic technique 
 is nonspecific, and carbon-monoxide -spe- 
 cific catalysts and appropriate filament 
 temperatures are required to reduce the 
 interference of other combustible gases. 
 
 The infrared technique is also simi- 
 lar to that described for methane detec- 
 tion. The sensor is made selective for 
 carbon monoxide by modifying the receiver 
 transducer to detect changes in the in- 
 frared wavelengths that are absorbed by 
 carbon monoxide molecules. For a nondis- 
 persive system, the absorption filters 
 must be changed, and for the dispersive 
 systems, the refraction grating may have 
 to be changed. 
 
 Table 4-5 lists several representa- 
 tive carbon monoxide sensor suppliers. 
 While no infrared carbon monoxide sensors 
 are listed, at least one is manufactured 
 in South Africa. It is called the 
 SPANAIR, and it can also be modified for 
 use as a methane detector. 
 
 TABLE 4-5. - Carbon monoxide sensors 
 
 Company 
 
 Model 
 
 Measuring 
 principle 
 
 Range , 
 PPm 
 
 Electrical 
 output 
 
 Power re- 
 quirements 
 
 Cost 
 
 Dynamation. 
 
 Energetic Sciencies 
 
 General Electric. 
 
 MSA. 
 
 Neutronics. 
 
 CO-2300 
 CO-300 
 
 4125 
 15ECS6 
 571 
 910 
 
 Catalyst 
 
 . . .do 
 
 Electrochemical 
 
 Fuel cell. 
 
 Electrochemical 
 
 .do. 
 
 0- 300 
 0- 300 
 
 0- 50 
 
 0-1,000 
 
 0- 100 
 0- 500 
 
 0-4,000 
 
 0-1 mA 
 0-1 mA 
 
 0-1 V dc 
 0-20 mA 
 dc 
 4-20 mA 
 
 } 0-1 V 
 
 117 V ac 
 
 110 V ac 
 
 12 V dc 
 
 110 V ac 
 
 14-28 V dc 
 
 7-38 V dc 
 
 115 V ac 
 
 19-60 V dc 
 
 110 V ac 
 
 $950 
 925 
 
 1,700 
 
 NA 
 
 1,830 
 
 1,200 
 
 NA Not available. 
 
104 
 
 4.5.3d Dust Sensors 
 
 Although there are many types of 
 techniques to measure dust concentrations 
 in mine air, a number of technical diffi- 
 culties limit the availability of dust 
 sensors that would be suitable for use in 
 automatic, remote, mine monitoring sys- 
 tems. For example, while the Bendix-type 
 personal dust sampler has been used suc- 
 cessfully for a number of years, it is 
 not amenable to remote monitoring because 
 the filters must be manually removed, 
 weighed, and replaced. Three measurement 
 techniques that might be adopted to re- 
 mote monitoring are described below; they 
 are optical sensing, piezoelectric sens- 
 ing, and beta attenuation. 
 
 In optical sensors, a beam of light 
 (from either an incandescent or laser 
 source) is directed into a chamber that 
 contains a sample of the dust-laden air. 
 The intensity or brightness of the light 
 that is scattered by the dust cloud in 
 the chamber is governed by the surface 
 area of the dust particles. The inten- 
 sity of the reflected light is typically 
 determined by comparison with a portion 
 of the direct light. While the device is 
 normally manual in operation (the di- 
 rect light is passed through a variable 
 filter that is adjusted until the inten- 
 sity of the direct light equals the re- 
 flected light), at least one company man- 
 ufactures an optical dust monitor with 
 an electrical output. The Japanese-made 
 Horiba monitor provides an output of to 
 20 mA, but since it requires 110 volts 
 ac (10 W) , MSHA approval is necessary if 
 it is going to be used in a return air- 
 way. Another unit, GCA model RAM-1 dust 
 monitor, measures concentrations in the 
 range of 1 to 200 mg/m^ , has a 0- to 10- 
 volt dc output and runs on 110 volts ac. 
 The cost of this unit is approximately 
 $6,000. 
 
 The main disadvantage of this tech- 
 nique is that a direct comparison between 
 dust concentrations by this technique and 
 other methods is possible only when the 
 particle size distribution of the dust in 
 the air sample is the same as that used 
 for instrument calibration. 
 
 The second method is piezoelectric 
 sensing. In this type of dust sensor, 
 particles are drawn through an orifice 
 and deposited on the face of a quartz 
 crystal. This crystal is part of an 
 oscillator whose resonant frequency 
 changes linearly with small changes 
 in crystal thickness (or mass). As 
 particulate mass collects on the crys- 
 tal face, the frequency decreases. 
 Therefore, the rate of frequency change 
 is proportional to the airborne mass 
 concentration. 
 
 The third technique is beta attenua- 
 tion. In beta attenuation instruments, 
 the aerosol is drawn through an orifice, 
 and particles impact on a suitable sur- 
 face. The impact surface is positioned 
 between a beta radiation source and a 
 counter. The amount of beta absorption 
 recorded by the counter is proportional 
 to the dust concentration. The major ex- 
 ample of this technique is the GCA high- 
 concentration dust monitor that GCA de- 
 veloped in conjunction with the Bureau of 
 Mines. 
 
 The major advantage of the GCA dust 
 monitor over the light-scattering moni- 
 tors is that (within certain limits) the 
 GCA unit measures the mass concentration 
 independent of the type of dust and par- 
 ticle size distribution. 
 
 4.6 Existing Mine Monitoring Systems 
 
 Monitoring systems have been in- 
 stalled in a limited number of U.S. 
 coal and metal and nonmetal mines. In 
 addition, numerous systems are installed 
 in foreign countries. In this section, 
 several installations are described; 
 the reasons for their development and 
 comments that are relevant to the cen- 
 tral issue of monitoring and/or control 
 in underground coal mining are given. 
 In cases where the data have been pub- 
 lished and are available, the name of 
 the mine is listed. In cases where 
 data were obtained from mine person- 
 nel and are not generally available, 
 only a letter designation for the mine is 
 provided. 
 
105 
 
 4.6.1 U.S. Undergound Coal Mines 
 
 Mine A 
 
 Coal producer A operates a coal mine 
 in support of its steelmaking operations. 
 A very extensive monitoring and data ac- 
 quisition system was installed in 1978. 
 The system has production, management, 
 maintenance, and safety components. 
 
 In 1978, production was lagging and 
 morale was poor. A human relations audit 
 indicated that section foremen and others 
 felt isolated and left to fend for them- 
 selves. Communications were poor. Sup- 
 port to section foremen was perceived as 
 slow and often ineffectual. 
 
 The system that was installed fo- 
 cused on improved communication and back- 
 up support. It monitors production, pro- 
 vides maintenance support from a data 
 base system, supports underground manage- 
 ment with improved reporting and communi- 
 cation, and enhances safety with tracking 
 and followup of unsafe conditions. 
 
 At present, phone reports are re- 
 quired from each section foreman every 2 
 hours (soon to become every hour when a 
 leaky coaxial radio link system becomes 
 operational). The status reports are re- 
 ceived by an operator who codes the 
 status, including mechanical problems and 
 repair effort underway, onto a data base 
 reporting system. These reports include 
 
 Conditions on section. 
 
 When loading of coal started. 
 
 Mechanical delays. 
 
 Nature of delay. 
 
 Start-stop time. 
 
 Mechanical problems are keyed to ma- 
 chine and location. Information is coded 
 by a communications coordinator, then 
 telemetered over phone lines to a central 
 
 office computer. Since a communications 
 coordinator is the only one to enter the 
 data, it is coded in a uniform fashion. 
 
 In turn, section foremen are able to 
 interrogate the system at any of the 
 three underground communication centers 
 (one at base of portal, and one each at 
 East and West mains). 
 
 Spare parts inventory software was 
 generated from an existing program used 
 in the mill activity. A detailed, pic- 
 torial blowup with parts number callout 
 is available near each section. As a re- 
 sult, the inventory status, location, and 
 ordering (reorder from vendor, if neces- 
 sary) can be accomplished quickly under- 
 ground. Similarly, management can track 
 problems by interrogating the system. 
 
 Because the bihourly reports are 
 relatively complete, formal end-of-shift 
 reports are no longer necessary, and on- 
 going foremen have a current status sheet 
 on which to base their plans. 
 
 This mine installed the cable for 
 the system, purchased modems to enable 
 communication of the data over phone 
 lines , and then leased the remainder of 
 the system, including dedicated phone 
 lines to the computer facility, line 
 printers, CRT displays, etc. 
 
 Software was developed in-house, al- 
 though some of it already existed, since 
 it was in use in the associated steel 
 mill. Software development times were 
 estimated to be 
 
 Production system.... 
 General underground.. 
 
 Maintenance 
 
 Safety 
 
 14 man-months. 
 
 18 man-months, 
 
 Existed. 
 
 12 man-months. 
 
 Surprisingly, one of the most trou- 
 blesome links in the system has been the 
 leased commercial phone lines (2,400 
 baud) used to telemeter data to the cen- 
 tral computer facility. There have been 
 instances of outages for days. 
 
106 
 
 Mine B 
 
 Coal producer B has a monitoring 
 system that uses equipment built by Larse 
 Corp. of Palo Alto, CA. It is a hard- 
 wired, time-domain multiplexed system 
 (channels interrogated sequentially in 
 time), designed originally for building, 
 process control, or energy monitoring. 
 The producer uses the system for belt 
 monitoring and for carbon monoxide and 
 temperature monitoring on the slope (the 
 mine has a propane preheat system) . 
 
 The history of the system is that in 
 1975, local management personnel had de- 
 cided to upgrade the telephone system to 
 a dial-phone system. Their chief elec- 
 trical engineer urged simultaneous in- 
 stallation of a monitoring capability, 
 perhaps integrated with the new phone 
 system. He obtained mine managers' ap- 
 proval, and invested approximately one- 
 third of his time during the next 3 
 years selecting, purchasing, installing, 
 and testing the system. He was assisted 
 underground with cable stringing and 
 other installation tasks about one-sixth 
 of the time. 
 
 The Larse system that was purchased 
 is capable of transmitting only binary 
 data. The sequence time is 3 sec per 
 station. Transmission is over a spare 
 twisted pair of conductors in the mine 
 phone cable. An independent electronics 
 and manufacturing firm developed and fab- 
 ricted solid-state interface circuitry 
 designed to convert voltages on control 
 switch terminals to logic levels compati- 
 ble with the Larse system. The converter 
 was optically coupled so that belt se- 
 quencer circuitry is uncoupled from the 
 monitor, and the converter has no moving 
 parts, enhancing reliability. A printer 
 is used to generate hard copy of alarm or 
 status change. A light panel is also 
 used to display status. 
 
 Mine personnel are evaluting the de- 
 sirability of installing a J-Tec velocity 
 and methane system. Their most likely 
 application would be a continuous monitor 
 
 of worked out panels, eliminating the 
 need for a 4-hour inspection mandated by 
 the State. 
 
 Extensions of this system might in- 
 clude ventilation control. In that case, 
 a variable-speed synchronous motor drive 
 on their fan might be used to reduce 
 energy demand when the full output is not 
 needed. At present, 1,600 hp is drawn 
 for ventilation. Original plans called 
 for some airflow and methane monitoring, 
 but this step was deferred as "too expen- 
 sive with little payback." 
 
 The monitoring system is maintained 
 by one man, who also maintains the tele- 
 phone system (which requires much more 
 time than maintaining the monitoring 
 system) . 
 
 Mine C 
 
 Operator C has several mines that 
 have very gassy seams and heavy over- 
 burden with difficult roof control prob- 
 lems. As a result, personnel at this 
 mine have received a variance from MSHA 
 allowing the use of beltways as an inlet 
 aircourse. 
 
 They have 5 years of experience with 
 carbon monoxide monitors, using Energetic 
 Sciences sensors. The current Energetic 
 Sciences monitors that are purchased have 
 two level alarms and a built-in battery 
 pack for uninterruptable power supply. 
 
 The telemetry system is an S.R. 
 Smith system that transmits digital in- 
 formation only and necessitates a dual 
 threshold modification to the Ecolyzer. 
 False alarms are generated with the 
 Ecolyzer as the result of power intermit- 
 tencies, a prime factor in the insistence 
 on an uninterruptible battery supply. To 
 enhance reliability, redundant PDP-11/4 
 computers have been installed to analyze 
 and display data. Maintenance by DEC has 
 sometimes been slow, although the system 
 has been completely down only once in the 
 last 2 years , thanks primarily to redun- 
 dant equipment. 
 
107 
 
 Plans include staffing up to one 
 full-time engineer per mine for the moni- 
 toring system. Although the system ini- 
 tially gave frequency "nuisance" alarms, 
 those problems have largely been solved, 
 with the occasional exception of false 
 alarms associated with the monthly cali- 
 bration procedure. They also conduct a 
 weekly inspection. Where alarms are dis- 
 played at the surface, the appropriate 
 site is notified by telephone. 
 
 Future plans include monitoring of 
 methane and airflow in returns. They 
 have a methane drainage research project 
 for which they have currently purchased 
 methane and airflow monitors. During a 
 6-week, period in the winter of 1981, they 
 lost 10 shifts of longwall production as 
 a result of gas outs , shutdowns that are 
 sometimes mine wide and are the motiva- 
 tion for their ventilation monitoring and 
 control plans. 
 
 Adjustment and deployment of the 
 monitoring system hardware, including 
 sensors, is a nuisance factor. There 
 have also been incidents of either will- 
 ful or careless destruction of equipment, 
 particularly sensors. 
 
 two mines are equipped with belt fire 
 monitoring equipment and two more are be- 
 ing outfitted. The two that are present- 
 ly on-line have been operating for about 
 6 months. The system consists of Ecolyz- 
 er carbon monoxide sensors, with up to 40 
 monitors per mine, a Giangarlo transmis- 
 sion system, and Niagra Scientific compu- 
 tational and display equipment. There is 
 no maintenance contract. The software 
 was provided by Giangarlo. The system 
 scans stations at a rate of about 1 per 
 second. The cabling for the system is 
 military surplus wire that is four- 
 conductor twisted No. 19 wire with steel 
 armor, which mine personnel purchased in- 
 expensively. It has been found that the 
 digital (frequency shift) signals can be 
 transmitted up to about 3 miles with vir- 
 tually no problems. The alarm algorithm 
 at present is simply a level exceedance 
 alarm. There have been four cable fail- 
 ures in the last 6 months, two failed 
 open and two failed shorted, owing to ac- 
 cidents with large boulders on the belt. 
 In the case of cables failing open, the 
 system continued to operate up to the ca- 
 ble break, and in the case of cabling 
 failing shorted, the entire system was 
 faulted until repaired. 
 
 They also monitor bearing tempera- 
 ture and water gage on their main ven- 
 tilation fans, using a FSK audio band 
 t e lame try s cheme . 
 
 The software for the S.R. Smith sys- 
 tem was written by a software specialist. 
 The costs for software were viewed as 
 substantial. 
 
 Mine D 
 
 Coal producer D has made a very sub- 
 stantial commitment to its mine-wide 
 monitoring, data acquisition, and con- 
 trol. Its mines are deep, with typically 
 more than 2,000-foot overburden, are very 
 gassy, and have difficult roof control 
 problems. As a result, the producer has 
 received permission to use the belt pas- 
 sageway for inlet air, provided that 
 carbon monoxide is monitored. Currently 
 
 It is estimated that maintenance 
 of the system requires about two man- 
 shifts per week, including daily, week- 
 ly, and monthly inspections. The daily 
 inspection is a physical inspection by 
 the fire boss, the weekly inspection in- 
 volves electrical tests, and the monthly 
 inspection involves span calibration. 
 The maintenance effort was estimated to 
 be approximately one-half inspections 
 and calibrations and one-half repair 
 functions. 
 
 Ac power tends to be unreliable and 
 intermittent. As a result, a gel-cell 
 power has been installed for each sensor 
 with a 48-hour capacity and continuous 
 trickle charge. In the event of inadver- 
 tent disconnect and resultant battery 
 discharge, the Ecolyzer indicates a false 
 alarm. 
 
108 
 
 All four systems were expected to be 
 operating by 1982. Following completion 
 of this carbon monoxide fire monitoring 
 system, the system will be extended to 
 monitor belt operation. In the No. 3 
 mine, there are 25 belts, each typically 
 4,000 feet long. The intent of the belt 
 monitoring is to display remotely infor- 
 mation about which belts are down and 
 possibly, diagnostics as to failure mode. 
 
 Because the seam being mined is ex- 
 tremely gassy, the required amount of air 
 is very high. This demand, combined with 
 the extreme depth, results in very high 
 water gage and very high ventilation 
 costs. As a result, variable pitch ven- 
 tilation fans have been installed, and 
 methane and airflow will be monitored in 
 the returns and the fan pitch modified 
 accordingly to reduce ventilation costs. 
 When a 2,000-hp fan was increased to a 
 larger 7,000-hp variable-pitch fan, the 
 power costs increased $1,900 per day at 
 that mine. Plans also exist to monitor 
 the temperature and vibration of fan 
 bearings. 
 
 In addition, the producer is also 
 automating its billing, inventory con- 
 trol, and maintenance functions, such as 
 maintenance history, equipment inventory, 
 lubrication histories, preventive mainte- 
 nance, etc. , as well as automating some 
 mapping and plotter routines. These com- 
 putational facilities will use their own 
 computers and programs and not piggyback 
 on the fire monitoring system. 
 
 Other monitoring plans include 
 production tonnage monitoring, by sec- 
 tion and shift, as well as power cen- 
 ter monitoring, particularly on longwall 
 sections. 
 
 Mine E 
 
 Mine monitoring at operation E has 
 the following three distinct functions: 
 
 Trapped miner location-roof fall 
 monitoring. 
 
 Ventilation monitoring at 11 
 stations. 
 
 Fan monitoring. 
 
 Trapped Miner-Roof Fall Detection 
 
 The trapped miner system was origi- 
 nally installed several years ago as a 
 Bureau-funded research project. Six com- 
 mercial, 40-Hz resonance, moving coil 
 geophones are implanted about 2,000 feet 
 apart in 50-ft-deep holes backfilled 
 with sand. Their seismic output is fed 
 to a Texas Instruments 980A computer that 
 computes the epicenter of any event ob- 
 served by three or more geophones. The 
 software package includes graphics that 
 plot the trapped miner-roof fall location 
 with a triangle if three geophones 
 received the signal, a square if four 
 were activated, and an asterisk if five 
 or more sensed the disturbance. The co- 
 ordinates and computed confidence margin 
 (error) are printed. Triangulation in 
 the southern mine sections where the 
 equipment is located is to within 50 to 
 100 feet. A disturbance count is also 
 recorded. The trapped miner equipment is 
 all located on the surface. 
 
 Ventilation Monitoring 
 
 Mine personnel at operation E were 
 interested in upgrading mine technology 
 and therefore were receptive to a Bureau 
 of Mines request for a cooperative devel- 
 opment. An electrical engineer from re- 
 search was assigned to design and super- 
 vise the fabrication and installation of 
 a ventilation monitoring (methane, carbon 
 monoxide, air velocity, temperature) sys- 
 tem and to assist in the development of a 
 trapped miner location system. The 
 telemetry system he designed is an audio 
 frequency, FSK serial transmission sys- 
 tem. It has a capacity for 16 stations 
 and a capability for up to eight analog 
 inputs at each station (0-5 volts dc). 
 Data are telemetered in an audio band. 
 The system is powered with a converter 
 that uses power from the trolley wire. 
 
109 
 
 converting -345 volts dc trolley power to 
 13 volts dc. At each station this 13- 
 volt power is again converted to ±15 and 
 ±5 for logic and control circuits. Meth- 
 ane is detected by mounting a Bacharach 
 cell directly on top of the explosion- 
 proof container that houses the elec- 
 tronics. Access to the box is via multi- 
 pin Amphenol environmental connectors. 
 
 Carbon monoxide is detected with an 
 Ecolyzer 4000 device, from which the 110- 
 volt converter has been removed. Air 
 velocity is measured with a J-Tec ane- 
 mometer, mounted on a 3-ft-long pole near 
 the center of the passageway. Flow cali- 
 bration is accomplished with a Davis 
 anemometer. 
 
 The processing display package 
 prints hourly summaries of the low read- 
 ing, high reading, and hourly average at 
 each station. 
 
 Manual cross-checks of the system by 
 the fire boss on his round are used to 
 verify system accuracy and reliability. 
 Each station is interrogated electroni- 
 cally every 5 min. The system has a 
 capacity for 16 stations and a maximum 
 cycle rate of 2.5 seconds per station. 
 
 Telemetry cables (unshielded twisted 
 pairs) are strung adjacent to the trolley 
 wires. The operation is gradually relo- 
 cating these cables, since the most fre- 
 quent failure mode for the system is a 
 loss of telemetry because of a cable cut 
 by a derailed trolley pantograph. Be- 
 cause of line losses, the sensors must be 
 within 500 feet of the power supply and 
 telemetry system. 
 
 Data are processed at the surface 
 (thresholding, time averaging, high and 
 low peak) and stored on a Phillips cas- 
 sette. The cassettes can be processed 
 with a print routine to generate hard 
 copy. 
 
 A problem encountered during devel- 
 opment of the wire power-to-12-volt dc 
 
 converter was that there are massive 
 transients on the trolley wire that the 
 converter could not handle. The solu- 
 tion was to use a high wattage voltage 
 divider with a zener limit, then dc-to- 
 dc convert the voltage-divided, filtered 
 dc. 
 
 The operation has asked for exten- 
 sion of the ventilation monitoring system 
 to the north to cover the other half of 
 the mine. However, the research staff is 
 now committed to other projects. 
 
 All maintenance and operation is 
 performed in-house. Spare boards are 
 stocked for the TI 980 and the computer 
 is maintained by swapping out defective 
 boards. Normal maintenance and record 
 keeping takes about 45 min per day. The 
 system costs a little over $100,000. In- 
 dividual stations cost $2,000 to $2,500. 
 The biggest maintenance cost, by far, is 
 line repair. 
 
 Fan Monitoring 
 
 The primary ventilation fans are 
 continuously monitored. Local system dc 
 power and fan head, in inches of water, 
 are reported hourly with a low, average, 
 and high reading. Alarm status is, of 
 course, immediately printed. 
 
 Mine F 
 
 Operator F has three mines that 
 share a power distribution system and 
 that have very substantial monitoring in- 
 strumentation. Mine personnel have also 
 begun development of a data base acquisi- 
 tion system. The system consists of sev- 
 eral layers, developed sequentially. 
 
 The initial installation was a sur- 
 face system for fans and circuit break- 
 ers. Later, haulage monitoring and un- 
 derground fire detection were added. A 
 large data base supervisory control 
 system is now being installed that will 
 be used to upgrade the fan monitoring 
 to include vibration and temperature 
 
110 
 
 monitoring so as to (1) anticipate fail- 
 ure, (2) detect smoldering fires by us- 
 ing carbon monoxide monitors, (3) con- 
 trol utility costs by power shedding when 
 desirable, (4) extend haulage monitor- 
 ing and control, and/or (5) acquire and 
 store maintenance data on machines for 
 preventive maintenance and failure 
 analysis. 
 
 Quotations for this system are being 
 solicited from mine monitoring system 
 vendors and particularly from process 
 control and energy management equipment 
 vendors. A brief summary of the equip- 
 ment to date follows. 
 
 Surface System 
 
 ing given to power center control and 
 load shedding. At present, overall power 
 drain data are provided to dispatchers, 
 so that they can shed "unnecessary" or 
 lower priority loads; however, shedding 
 rarely happens because of production 
 pressures and the overwhelming complexity 
 of the problem. Effective control re- 
 quires mine-wide information, analysis, 
 and synthesis of the data. 
 
 The operation buys power from the 
 local utility at 69 kV, then distributes 
 and transforms the power for the opera- 
 tions. Power billing is in proportion 
 to the highest 30-min average, using 
 sequential time windows (not a floating 
 window) . 
 
 The surface system monitors ventila- 
 tion fans and the status of circuit 
 breakers. At present, there are 13 fans 
 in the complex and 28 monitoring substa- 
 tions. The system was developed in-house 
 in 1973 and uses a hard-wired FSK tech- 
 nique, with a 40-channel capacity. 
 
 Underground Haulage and Fire Detection 
 
 At present there are about six sec- 
 tions of instrumented haulage. A CRT 
 display at the dispatcher indicates the 
 number of available empty cars at each 
 station, an estimated time of depletion 
 of cars (a function of whether the mine 
 is cutting), the number of loaded cars, a 
 count of loaded cars for the shift, and 
 whether the belt is running. 
 
 Heat sensor fire detection outputs 
 are also displayed. Data acquisition 
 is accomplished with a Westinghouse 
 neumalogic system. A deluge system is 
 controlled by the output. 
 
 Power Monitoring 
 
 The three mines have a monthly util- 
 ity bill of several hundred thousand dol- 
 lars. As a result, attention is being 
 
 Methane Monitoring 
 
 Methane is continuously monitored in 
 one abandoned area. 
 
 Mine G 
 
 Operator G has a Motorola Intract 
 2000, UHF high-band transmitter-receiver 
 system that is used at three mines. Per- 
 sonnel at this mine monitor ventilation 
 fan operation, sensing water gage, and 
 measuring bearing temperature and vibra- 
 tion. In each instance, the alarm level 
 exceedance is the only data transmitted. 
 In the case of circuit breakers, they 
 have the capability to reset, trip, and 
 monitor each of these. 
 
 Each mine typically has several fans 
 and perhaps 20 circuit breakers physical- 
 ly distributed over the countryside, per- 
 haps up to 4 or 5 miles distant from the 
 portal. Electrical power is either 7,200 
 volts ac three-phase or 300-volt dc trol- 
 ley power. In the case of circuit break- 
 ers, the system is used primarily for 
 control and for resetting after interrup- 
 tions. They reset as a test, and, if 
 there is a second interruption, they dis- 
 patch a man to the site. Since fan 
 
Ill 
 
 monitoring is required by law, the system 
 saves personnel assignment to monitor the 
 remote fans. 
 
 At the central station, located in 
 the maintenance shop, data are displayed 
 on a console with eight status lights, 
 designating the presence or absence of 
 threshold exceedance of any one of eight 
 sensed variables at a site. There is al- 
 so a hard-copy printout on a Tl printer 
 of twice-a-day status, plus alarms. One- 
 third spares are maintained, and Motorola 
 gives a 3-to-8-week turnaround time for 
 repairs. The system has been on-line for 
 15 months, with many initial problems 
 that have evidently been worked out. 
 
 Two fully qualified technicians 
 with FCC licenses are on the staff and 
 a third is being sought. Testing and 
 qualification certification is contracted 
 out. 
 
 Monitoring of this sort dates to 
 1969 when it began with a FEMCO hardware 
 system. In 1975, an intermediate radio 
 system was added and, later, the Infract 
 2000. 
 
 The parent research organization has 
 also experimented with the feasibility of 
 monitoring cars, as well as production 
 activity, on each section, to improve the 
 efficiency of the dispatch of cars to 
 operating sections. This was a test of 
 instrumentation and technique, using one 
 section. They measured 
 
 1. Is the section operating? 
 
 a. Continuous miner off. 
 
 b. Continuous miner tramming. 
 
 c. Continuous miner cutting. 
 
 2. At the section, how many — 
 
 a. Cars at ramp. 
 
 b. Empty cars waiting. 
 
 c. Full cars. 
 
 d. Empty — full conversions. 
 
 3. How many cars available at the 
 dump site? 
 
 The goal was to reduce the "no empty" de- 
 lays at the section. 
 
 The dispatcher has voice communica- 
 tion with the locomotive engineer and the 
 loading supervisor at each section. De- 
 velopment of the design began with the 
 following constraints: 
 
 1. Commercial sensors to be used, 
 perhaps repackaged. 
 
 2. Existing phone lines to be used 
 (Gaitronics) [but not same wires, i.e., 
 use phone-quality wiring. Cable is fig- 
 ure 8, wire-supported, with 40 conductors 
 in pairs. ] . 
 
 The sensors tested were 
 
 1. Infrared photocell (car count). 
 
 2. In-track magnetic proximity 
 (count) . 
 
 3. Sonic level (full-empty). 
 
 4. Current sensor at load center. 
 
 a. Off 
 
 b. Tramming 
 
 c. Cutting 
 
 5. Traffic switch positions. 
 
 All data were telemetered to the 
 dispatcher. There are local displays for 
 debugging purposes. Intel microproces- 
 sors were used in remote stations. 
 
 A test of the sensing system was 
 conducted by stationing an observer to 
 note visually car passage, operating 
 
112 
 
 time, empty-full status, and switch posi- 
 tion. The test results were 
 
 Visual Automatic Error 
 
 582 
 
 599 
 
 3% 
 
 230 
 
 233 
 
 1% 
 
 820 
 
 813 
 
 
 49 
 
 49 
 
 
 
 Direction 
 
 Empty-full 
 
 Operating 
 
 time min. . 
 
 Switch position 
 
 It was concluded that sensing reliability 
 was good and quite adequate. 
 
 The most difficult problems were (1) 
 the deployment-redeployment of sensors, 
 and (2) cost-complexity of the system. 
 
 It was concluded that this type of 
 rail haulage monitoring is technically 
 feasible; however, the economic viabil- 
 ity is probabilistic. There is econom- 
 ic payback only if all sections are 
 instrumented, since intersection co- 
 ordination is the goal. The operator 
 opted for a computer simulation at this 
 point to evaluate the economics, using 
 a modified version of the Penn State 
 RailSim program. The results indicated 
 substantial incremental improvement in 
 productivity, but since the operation 
 is market limited, the economics for 
 such a system were not quite attractive; 
 i.e., it cannot easily sell more coal per 
 year and cannot capitalize on the poten- 
 tial to run less days per year because 
 of large capital costs and institutional 
 factors. 
 
 A simpler less costly system might 
 be viable. For example, a system that 
 reported 
 
 1. Whether the section is operating 
 (power center). 
 
 2. What the car count is at the 
 dump site. 
 
 3. Other data manually input by the 
 dispatcher. 
 
 Since use of longwall sections is 
 expected soon, problems with haulage 
 
 coordination will be intensified. "In- 
 tangibles" (safety, morale, health, labor 
 resistance, sabotage, etc.) are hard to 
 evaluate. 
 
 A "smart sensor" standalone is being 
 considered that — 
 
 1. Obtains power off rail line. 
 
 2. Senses a parameter, converts 
 data to a secure format. 
 
 3. Transmits wireless. 
 
 Passive (unpowered) sensors are used 
 wherever possible, and sensors are mini- 
 mized (more manual input). 
 
 Power Monitoring 
 
 Billing of power used by energy- 
 intensive industrial users is based on a 
 base rate plus a peak load factor. Mine 
 G's local utility expects to go to a peak 
 factor based on the highest 5-min aver- 
 age, perhaps highest floating 5-min aver- 
 age. Although the potential savings are 
 very substantial, an automatic power 
 management system is clearly required to 
 react within 5-min windows. 
 
 A very ambitious computerized main- 
 tenance management program has also been 
 begun. There seems to be a likelihood 
 that haulage and ventilation monitor- 
 ing and/or control will eventually be 
 attempted. 
 
 Mine H, Federal No. 2 
 
 Sensors for carbon monoxide and 
 methane were placed so as to compare 
 methane concentrations entering and leav- 
 ing each section. Differential pressures 
 were also measured to highlight ventila- 
 tion problems owing to partial blockages, 
 etc. Temperature, humidity, and air 
 velocity were also measured at dozens of 
 sensor stations. Computer control was 
 performed at West Virginia University. 
 These early tests served to prove that 
 the task of underground monitoring was 
 technologically feasible (2). 
 
113 
 
 More recently, a similar system has 
 been used to test the concept of ventila- 
 tion control (1). Ventilation param- 
 eters, including methane, carbon monox- 
 ide, temperature, humidity, airflow, and 
 differential pressure, were measured. 
 Ventilation regulators (louver door de- 
 sign) were operated electronically, and 
 the resultant ventilation redistribution 
 was monitored. 
 
 Mine I 
 
 Drainage of methane in advance of 
 mining is an effective way of minimiz- 
 ing methane content at the working face. 
 Since most coal mining countries have 
 enacted safety regulations that require 
 that face equipment be shut down if the 
 methane content in the air reaches a 
 predetermined level, methane drainage 
 has production as well as safety bene- 
 fits. Because methane liberation in- 
 creases with rate of coal production, 
 methane drainage is more desirable in 
 highly mechanized longwall operations 
 where production rates are relatively 
 high. As a result, methane drainage is 
 used extensively in Europe where longwall 
 mining has become very common. In 1975, 
 approximately 7.5 billion cubic feet of 
 methane was drained in Polish mines , 
 and over 21 billion cubic feet were 
 drained from mines in the Federal Repub- 
 lic of Germany (7). Recently, methane 
 drainage has also become more important 
 as the production rates in U.S. mines 
 have increased. 
 
 Because of the inherent design that 
 roof falls, bottom heaving, etc., may 
 rupture the methane drainage piping, 
 the Bureau of Mines has specified 
 that drainage systems be equipped with 
 'a "fail-safe" monitoring and control 
 system (22) . Basically, the system 
 should be able to shut off the flow of 
 methane into the drainage pipe in the 
 event of unsafe conditions such as a 
 power failure or break in the drainage 
 piping. 
 
 One such system has been success- 
 fully employed in the Bethlehem Mines, 
 Marianna No. 58, located in Marianna, Pa. 
 This system consists of MSA methane sen- 
 sors located at 500-foot intervals in the 
 airway containing the methane drainage 
 pipe. The output from these intrinsical- 
 ly safe sensors is transmitted to a con- 
 trol panel located in a nearby fresh air 
 entry. Also located in the fresh air 
 entry is a small (1.05-cfm) air compres- 
 sor that supplies air through PVC tubing 
 strapped to the drainage pipe, to the 
 pneumatically actuated shutoff valves lo- 
 cated at each borehole. The valves are 
 spring loaded and held open by the air 
 from the compressor. 
 
 The system can also incorporate 
 a "receiving" station (located above 
 ground) that contains meters that in- 
 dicate the methane content measured by 
 the sensors and recorders that provide a 
 continuous log of the sensor output. 
 
 In the event of a power failure, the 
 compressor would cease to function, the 
 air pressure in the PVC tubing (normally 
 55 psig) would drop, and the valves would 
 close. If the methane content in the 
 airway containing the drainage pipe ex- 
 ceeds 1% (due to a leak in the pipe) , the 
 PVC tubing is vented by solenoids located 
 on the control panel and the valves again 
 shut down the methane flow. In addition, 
 if a roof fall ruptures the drainage 
 pipe, it will also rupture the other PVC 
 tubing that is strapped to the top of the 
 pipe, automatically dropping the pressure 
 and closing the valves. Finally, if a 
 sensor fails to operate or a cable is 
 severed, the solenoids will vent the PVC 
 tubing and again close the borehole 
 valves. 
 
 References 10 and 18 discuss this 
 system in more detail. 
 
 A summary of the monitoring systems 
 reviewed is presented in table 4-6. 
 
L14 
 
 TABLE 4-6. - Summary of monitoring systems surveyed in U.S. underground coal mines 
 
 
 A 
 
 B 
 
 C 
 
 Function. ............ 
 
 Production, maintenance, 
 
 communication. 
 Input from section 
 
 foremen. 
 
 6 
 
 Belt monitoring. .......... 
 
 Belt fire monitor 
 
 Parameters monitored. 
 Size: 
 
 Belt operation, belt 
 sequencing, motor tem- 
 perature and power, CO. 
 
 40 
 
 10 
 
 2- and 4-conductor 
 
 Digital 
 
 equipment status. 
 CO, fan temperature and 
 pressure, degasifica- 
 tion pumps, fire sup- 
 pression operation. 
 
 15 to 20. 
 
 Input per station.. 
 Telemetry: 
 
 Cables 
 
 NA 
 
 1 6 maximum . 
 
 
 
 Format 
 
 Voice and digital 
 
 conductor, shielded. 
 Digital. 
 Line printer, CRT. 
 
 2-level. 
 
 Data display 
 
 Line printer, CRT's above 
 
 and below ground. 
 No 
 
 Line printer, light panel. 
 Yes 
 
 Year installed 
 
 1978 
 
 1978 
 
 1978. 
 
 Comments ....•.•.■...• 
 
 Production reports, man- 
 power planning. 
 
 Planned expansion — air 
 velocity, CH4 monitoring. 
 
 Allowed 75,326 
 
 
 variance. 
 
 
 D 
 
 E 
 
 F 
 
 Function. ............ 
 
 Belt fire monitor. •••■■•■• 
 
 Environmental, fan opera- 
 tion, roof falls. 
 
 CO, CH4 , air velocity, fan 
 power and pressure, seis- 
 mic activity, battery 
 voltage. 
 
 11 
 
 4 
 
 Fan operation, haulage, 
 fire, environmental. 
 
 Fan operation, haulage, 
 heat-CO, CH4 . 
 
 26. 
 
 16 maximum. 
 
 Parameters monitored. 
 
 Size: 
 
 Remote stations.... 
 
 Input per station.. 
 Telemetry: 
 
 Cables 
 
 CO 
 
 13 
 
 1 
 
 4-conductor, shielded 
 
 Digital 
 
 
 Digital 
 
 shielded. 
 Digital. 
 Line printer, CRT, 
 
 light panel. 
 Yes. 
 
 Data display 
 
 
 Line printer, plotter, 
 CRT, light panel. 
 
 Yes 
 
 1974 
 
 At station and aboveground 
 1981 
 
 
 1978. 
 
 Comments. 
 
 Allowed 75.326 variance, 
 planned expansion — belt 
 monitor, ventilation 
 monitor. 
 
 Result of cooperative 
 agreement with Bureau of 
 Mines. 
 
 Planned expansion — fan 
 temperature, vibra- 
 tion, maintenance 
 data. 
 
 
 G 
 
 H 
 
 I 
 
 Function. ■■...■...... 
 
 Fan operation. ............ 
 
 Environmental monitoring.. 
 
 CH4 , CO, temperature, rel- 
 ative humidity, airflow, 
 differential pressure. 
 
 12 
 
 10 to 20 
 
 Methane drainage. 
 CH4. 
 
 >4. 
 
 Parameters monitored. 
 
 Size: 
 
 Remote stations. ... 
 
 Fan operation, tempera- 
 ture, pressure, 
 vibration. 
 
 10 
 
 Input per station.. 
 Telemetry: 
 
 Cables 
 
 8 
 
 1. 
 
 Radio link.. •..■•..••■...•• 
 
 2— conductor. .............. 
 
 4— conductor . 
 
 Format ..•.....•..•• 
 
 FM 
 
 Digital. • 
 
 Analog. 
 
 Strip chart, meter. 
 
 2-level. 
 
 Data display 
 
 Alarm. .•.......••.■.. 
 
 Line printer, light panel. 
 Yes 
 
 Mine maps, CRT, teletype.. 
 2-level 
 
 Year installed. ...... 
 
 1979 
 
 1972 
 
 1977. 
 
 
 Had prototype haulage mon- 
 itor, planned expansion — 
 maintenance data. 
 
 WVU experiment , Bureau of 
 Mines sponsored. 
 
 Controls drainage 
 valves, result of 
 Bureau of Mines 
 Research. 
 
 
 NA Not available. 
 
 NOTE. — Alphabetic designators are those used for mine designations in text. 
 
115 
 
 4.6.2 U.S. Underground Metal-Nonmetal 
 Mines 
 
 Mine J 
 
 Although mine J is not a coal mine, 
 its physical layout and mining technique 
 are very coal-like. It is a very exten- 
 sive, single-level, room and pillar mine 
 that extracts about 5 million tons per 
 year, working about 7 feet of a 9-foot 
 seam that is 1,200 feet deep. The mine 
 is classified gassy because of the pres- 
 ence of oil shale above and below the 
 seam. However, it is allowed use of the 
 beltway for fresh air, since metal- 
 nonmetal mines are governed by 30 CFR, 
 Part 57, which is less restrictive than 
 the sections governing coal. 
 
 Up to 10 continuous miner sections 
 and a longwall are operated. Underground 
 equipment is electric, with a combination 
 of rail and belt haulage. 
 
 A very satisfactory 50-station haul- 
 age monitoring system has been installed, 
 and an additional system is on or- 
 der. There is about 20 miles of belt 
 conveyors. 
 
 The system, which was built by Aqua- 
 trol Corp., monitors belt functions such 
 as belt slip, plug up, sequencing, align- 
 ment, etc. 
 
 Mine K 
 
 Operator K operates a multilevel 
 copper mine, and has an ambitious load- 
 shedding program underway. The haulage 
 is electric-powered track and belt. Face 
 operation is with diesel-powered track- 
 less vehicles. Sump pumps run up to 
 5,000 hp. The utility bill is based on a 
 15-min peak load factor, which is hoped 
 to be reduced greatly by load shedding. 
 The hardware and telemetry will be pur- 
 chased from Harris Co. Enlargement to a 
 mine-wide supervisory control and data 
 acquisition system, including airflow, 
 perhaps production monitoring data, etc. , 
 
 is anticipated. An example with rapid 
 payback is automatic measurement of air 
 quality following a large explosive shot. 
 At present, a rescue team is dispatched 
 to certify the work areas. The result is 
 a 3- or 4-hour mine-wide shutdown, which 
 is, of course, expensive. 
 
 4.6.3 Foreign Mines 
 
 Canada 
 
 As in many countries, mine monitor- 
 ing in Canada has become more important 
 in recent years. The Canadian Mining Re- 
 search Laboratories (CANMET) has been 
 working on carbon monoxide and methane 
 monitoring in western Canada since 1976. 
 One such monitoring system, which uses an 
 electrochemical carbon monoxide analyzer, 
 was installed in a Kaiser Resources mine 
 to detect mine heating. Air samples were 
 drawn through 1/2-inch OD polyethelene 
 tubing from distances up to 7,000 feet 
 (8^). In 1978, this system detected a 
 significant rise in the carbon monoxide 
 levels in one return, indicating the po- 
 tential onset of heating. This early in- 
 dication of heating was credited with 
 allowing the mine to alleviate the situ- 
 ation without interrupting the mining 
 schedule. 
 
 CANMET began working on methane mon- 
 itoring in the coal mines in western Can- 
 ada in 1977. This work centered around 
 the use of remote methane analyzers, 
 again by drawing air samples through 
 polyethelene tubing. In one mine with a 
 history of sudden gas bursts, the system 
 successfully documented the sudden meth- 
 ane liberations (8) . 
 
 More recently, the Cape Breton De- 
 velopment Corp. has installed a computer- 
 based mine monitoring system in its coal 
 mine in Nova Scotia (6^). The system, 
 which monitors methane concentrations, 
 air velocities, air pressures, fan vibra- 
 tion, machine temperatures, and methane 
 pump pressure, was supplied by Transmit- 
 ton Ltd. 
 
116 
 
 Poland 
 
 As is true for much of Europe, the 
 Polish coal mining regulations consider 
 continuous methane monitoring in the 
 return airways with automatic deener- 
 gizing as an adequate alternative to 
 individual monitoring on each piece 
 of face equipment (as is done in the 
 United States). Furthermore, the methane 
 threshold values permitted by the regu- 
 lations are also increased in mines where 
 continuous monitoring and automatic de- 
 energizing are used (16). As discussed 
 earlier, such regulations tend to encour- 
 age the use of remote mine monitoring 
 systems . 
 
 The Polish Research and Development 
 Center for Mining Mechanization, Electro- 
 technics and Automation Systems (EMAG) 
 has been working on ventilation monitor- 
 ing and control systems for Polish coal 
 mines for a number of years. The methane 
 monitoring systems currently consist of 
 remote sensors, telemetering equipment, 
 and a miniprocessor , located in a central 
 station that receives and analyzes the 
 data. The system has the capability of 
 automatically switching off electric pow- 
 er if the methane concentrations exceed 
 the specified level. The system also 
 maintains permanent records of warnings 
 and alarms and provides summary reports 
 on methane liberation cycles, etc. 
 
 interest in methane monitoring and con- 
 trol is not great. The Swedish mines do 
 use diesels , however, and an effort is 
 underway to monitor carbon monoxide, 
 nitrogen dioxide, etc. In addition, some 
 work has been done on the development of 
 automatic air regulation. Because meth- 
 ane liberation is not a problem in Swed- 
 ish mines, mine air regulation, which 
 would be based on the number of diesels 
 operating and on their location, is cur- 
 rently being studied (19) . 
 
 U.S.S.R. 
 
 Automatic monitoring and control of 
 mine ventilation systems has been studied 
 in the U.S.S.R. for a number of years. 
 This work has resulted in the development 
 of an automatic ventilation monitoring 
 and control system called ATMOS. This 
 computer-based system is reportedly (14) 
 able to monitor ventilation parameters 
 (such as methane concentration, airflow, 
 etc.), calculate the required airflows, 
 and provide the system operator with in- 
 formation on the appropriate fan and reg- 
 ulator settings. Ventilation corrections 
 are made on a weekly basis. 
 
 The system has been operationally 
 tested in two mines, and the Ministry of 
 Coal Mining is currently in the process 
 of commercial development of the ATMOS 
 system. 
 
 For fire monitoring, EMAG has also 
 developed a carbon monoxide monitoring 
 system based on a catalytic sensor and an 
 ionization fume sensor. The system has 
 been tested, experimentally, in Polish 
 coal mines. 
 
 EMAG has also worked on developing 
 automatically controlled air regulators 
 for coal mines. The regulator, which 
 has pneumatically or hydraulically actu- 
 ated vanes , is intended to become part of 
 an overall mine monitoring and control 
 system. 
 
 Sweden 
 
 Since most of the mining in Swe- 
 den is metal mining in nongassy mines. 
 
 United Kingdom 
 
 The development and use of remote 
 mine monitoring systems is probably more 
 advanced in the United Kingdom than in 
 any other country, apparently for two 
 reasons. 
 
 The first is that British mining 
 regulations tend to encourage the use of 
 such systems. For example, in contrast 
 with U.S. regulations requiring that 
 methane monitoring devices be installed 
 on each piece of face equipment that can 
 deenergize the equipment once the estab- 
 lished methane threshold is exceeded, 
 British regulations permit monitoring of 
 the return airways for methane if the 
 data are transmitted to a central control 
 
117 
 
 station that can remotely deenergize the 
 affected equipment. 
 
 The second, and probably the more 
 important reason, is that the British 
 coal industry is nationalized and is sup- 
 ported by a centralized research organi- 
 zation (the National Coal Board's Mining 
 Research and Development Establishment — 
 MRDE). This arrangement has greatly fa- 
 cilitated the development, testing, and 
 implementation of mine monitoring systems 
 in the United Kingdom. 
 
 In the 1970' s, the MRDE focused its 
 attention on developing a universal moni- 
 toring and control system that could be 
 used throughout the coal mining industry. 
 The system, called MINOS (for Mine Opera- 
 ting System) , is based on a common core 
 of equipment that consists of a control 
 console, central computer(s), and periph- 
 erals. The application software is also 
 the result of MRDE development. The mon- 
 itoring systems are supplied to the mine 
 by several independent companies that 
 are free to market a variety of trans- 
 ducers, data transmission equipment, and 
 accessories. The concept of a universal 
 computer-based operating system has per- 
 mitted the MRDE to achieve certain econo- 
 mies in the development of the system and 
 tends to reduce interface problems. 
 
 The applications of the MINOS moni- 
 toring/control systems can be divided in- 
 to the following six basic categories: 
 
 1. Ventilation monitoring. 
 
 2. Coal face monitoring. 
 
 3. Coal clearance monitoring. 
 
 4. Coal conveying monitoring. 
 
 5. Fired plant monitoring. 
 
 6. Preparation plant monitoring. 
 
 Ventilation monitoring in the United 
 Kingdom is currently being accomplished 
 via tube bundle air sampling as well as 
 telemetering of data from electromechan- 
 ical transducers. In the former, air 
 
 samples are drawn through tubes and ana- 
 lyzed by using a gas analyzer located in 
 a surface lab facility. In the latter, 
 the output of the transducers is fed to 
 outstations that encode and transmit the 
 data to the surface via a communications 
 cable. In either case, the environmental 
 data (methane, oxygen, carbon monoxide 
 levels, airflow, differential pressure, 
 etc.) are analyzed, stored, and displayed 
 on video monitors located on the central 
 control panel. The displays and hard- 
 copy reports can consist of warnings, 
 alarms , actual values , or graphs of long- 
 term trends (9). As of 1980, there were 
 approximately two such systems either in 
 operation or scheduled for installation 
 (3). 
 
 Development of a system for moni- 
 toring face equipment performance began 
 in 1977. One version of the system, 
 called FIDO (Face Information Digested 
 On-Line) was installed and tested in four 
 collieries by 1980. The National Coal 
 Board plans to install the system in an 
 additional 24 collieries that have ap- 
 proximately 100 active coal faces (20). 
 Although the system originally monitored 
 only face equipment operations, the NCB 
 plans to expand the system to provide 
 data on such parameters as roof height, 
 pick force, and equipment orienta- 
 tion, and eventually to permit automatic 
 control of such equipment as longwall 
 shearers. 
 
 Monitoring and control of under- 
 ground coal conveying systems is rela- 
 tively advanced in the United Kingdom, 
 with the first system in operation in 
 1972. The systems provide stop-start 
 logic sequencing in addition to sensing 
 of such parameters as bearing tempera- 
 tures, blocked chutes, motor operation, 
 etc. As of 1980, there were approximate- 
 ly 30 such systems in operation in the 
 United Kingdom ( 20 ) . 
 
 Monitoring and control of coal 
 preparation plant operations (such as 
 conveying, reagent mixing, etc.) is a 
 relatively new application for the MINOS 
 system. The system began development 
 testing in the Lea Hall colliery in 1978. 
 
118 
 
 A decision on expanding the use of the 
 system will depend on the results of this 
 inplant demonstration. 
 
 It should be pointed out that mine 
 monitoring systems based on the MINOS 
 concept are currently being manufactured 
 in the United States. 
 
 Germany 
 
 In recent years, remote monitoring 
 of methane and carbon monoxide has re- 
 ceived increased attention in German coal 
 mines. One reason is that the German 
 Federal Regulations on mine health and 
 safety make such systems desirable and, 
 in some cases, necessary. For example, 
 the German regulations permit higher 
 methane threshold values if constant mon- 
 itoring is carried out by permanently in- 
 stalled recording instruments that can 
 telemeter the data to a remote control 
 center that can automatically deenergize 
 the electrical face equipment (16) . An- 
 other example is the German requirement 
 for automatic recording of carbon monox- 
 ide levels along all belt entries. 
 
 It has been estimated that as many 
 as 1,400 methane and 1,200 carbon mon- 
 oxide measuring devices were in use in 
 the Ruhr district in 1978. Most of these 
 provided remote transmission of the mea- 
 surement data to a central control sta- 
 tion. In addition to the methane and 
 carbon monoxide sensors, some 500 fixed- 
 point air velocity sensors were also es- 
 timated to be operating in Ruhr district 
 mines as of 1978. 
 
 In addition, investigations have 
 been conducted in Germany into the use of 
 minicomputers as well as microcomputers 
 to receive and process the data from the 
 remote sensors. The primary purposes are 
 (1) the reduction of false alarms through 
 trend identification and signature 
 
 matching and (2) the manipulation, pres- 
 entation, and storage of large amounts of 
 monitoring data. 
 
 South Africa 
 
 Remote automatic detection of un- 
 derground mine fires is a major concern 
 for South African mining companies. This 
 is particularly true for the deep level 
 gold mines because of the large amount 
 of timber required for roof support in 
 these mines. Instances of spontaneous 
 combustion in South African coal mines 
 have also been reported in recent years. 
 For example, between 1968 and 1973 more 
 than 23 mine fires occurred in one South 
 African mining district. Approximately 
 65% of these were attributed to spontane- 
 ous combustion (11). 
 
 Two basic approaches have been used 
 in remote monitoring for underground mine 
 fires. In coal mines, an infrared gas 
 analyzer drawing air samples through 
 polyethelene tubes has been used to sense 
 carbon monoxide levels ( 11 ) , and a com- 
 bination of infrared gas analyzers and 
 ionization chamber detectors has been 
 used in the underground gold mines (23). 
 In the latter case, the electrical sig- 
 nals from the transducers were tele- 
 metered to a control room located above 
 ground. In the control room, data (car- 
 bon monoxide level for the gas analyzer 
 and ionization level for the combustion 
 and particle detector) are recorded in 
 analog form on continuous logs. In addi- 
 tion, the system has the capacity to 
 initiate alarms if specified levels are 
 exceeded. 
 
 Although some difficulties were en- 
 countered with the ancillary equipment 
 (such as recorders) in the tube system 
 and dirt and condensation in the telem- 
 etry system, both have proven effective 
 in detecting mine fires. 
 
REFERENCES 
 
 119 
 
 1. Aldridge, M. D. , and R. S. Nutter, 
 An Experimental Ventilation Control Sys- 
 tem. Proc. 2d Internat. Mine Ventila- 
 tion Cong., Nov. 4-8, 1979, Reno, Nev. , 
 pp. 230-238. 
 
 2. Aldridge, M. D. , N. S. Smith, 
 R. E. Swartwout, and D. T. Worrell. Con- 
 clusions From the WVU Monitoring Experi- 
 ments. Proc. 2d WVU Conf. on Coal Mine 
 Electrotechnology , June 12-14, 1974, Mor- 
 gantown, W. Va. , pp. 18-1 — 18-4. 
 
 3. Barham, D. K. Progress in Coal- 
 face Monitoring and Control in the 
 United Kingdom. Proc. 5th WVU Conf. 
 on Coal Mine Electrotechnology, July 30- 
 31 — August 1, 1980, Morgantown, W. Va. , 
 pp. 16-1 — 16-18. 
 
 4. Bredeson, J. G, H. Hashemi, and 
 K. Snedhar. Data Security for In-Mine 
 Transmission. Final Report — Part 11. 
 BuMines contract J0308024; for informa- 
 tion contact R. W. Watson, Pittsburgh 
 Research Center, Pittsburgh, Pa. 
 
 5. Bredeson, J. G. , J. L. Kohler, and 
 H. Singh. Data Security for In-Mine 
 Transmission. Final Report — Part 1. Bu- 
 Mines OFR 76-81, February 1981, HI pp.; 
 NTIS PB 81-221998. 
 
 6. Brezovec, 
 Mine Conditions. 
 August 1981, pp. 
 
 D. Computer Monitors 
 Coal Age, v. 86, No. 8, 
 56-60. 
 
 7. Cervik, 
 ane Drainage 
 
 J. Experience With Meth- 
 From Horizontal Bore- 
 holes. Proc. 2d Internat. Mine Ventila- 
 tion Cong., Nov, 4-8, 1979, Reno, Nev., 
 pp. 257-264. 
 
 8. Chakravorty, R. N. , and R. L. 
 Woolf. Environmental Monitoring for 
 Safety in Underground Coal Mining. CIM 
 Bull., V. 72, No. 801, January 1979, 
 pp. 100-107. 
 
 9. Corbett, A. W. The Use of Compu- 
 ters for the Continuous Monitoring of the 
 Environment at Brodsworth Colliery. Min. 
 Eng. (London), v. 138, No. 210, March 
 1979, pp. 641-650. 
 
 10. Irani, M. C. , F. F. Kapsch, P. W. 
 Jeran, and S. J. Pepperney. A Fail-Safe 
 Control System for a Mine Methane Pipe- 
 line. BuMines RI 8424, 1980, 11 pp. 
 
 11. Joubert, F. E. , I. F. Buchan, and 
 J. D. R, Beukes. Report on Carbon Monox- 
 ide Monitoring System for the Early De- 
 tection of Incipient Fires in Mines. J. 
 Mine Ventilation Soc. South Africa, v. 
 29, No. 4, April 1976, pp. 74-80. 
 
 12. Kacmar, R. M. Reliability of 
 Computerized Mine-Monitoring Systems. 
 BuMines IC 8882, 1982, 12 pp. 
 
 13. Kohler, J. L. An Evaluation of 
 the Air Velocity Sensing Unit in the Bu- 
 reau of Mines Remote Monitoring Sys- 
 tem. Ongoing BuMines contract J0308027; 
 for information contact E. D. Thimons , 
 Pittsburgh Research Center, Pittsburgh, 
 Pa. 
 
 14. Kot, V. I., and E. F. Karpov. 
 Coal Mine Ventilation - Monitoring and 
 Control. Proc. 5th WVU Conf. on 
 Coal Mine Electrotechnology, July 30- 
 31~Aug. 1, 1980, Morgantown, W. Va. , 
 pp. 15-1 — 15-14. 
 
 15. Miller, E. J., P. M. Turcic, and 
 J. L. Banfield. Equivalency Tests for 
 Fire Detection Systems for Underground 
 Coal Mines Using Low Level Carbon Monox- 
 ide Monitors. Proc. 2d Internat. Mine 
 Ventilation Cong., Nov. 4-8, 1979, Reno, 
 Nev. , pp. 236-246. 
 
 16. North American Mining Consultants 
 Inc. Single Entry Longwall Study. U.S. 
 Dept. Energy contract ET-77-C-01-9052, 
 October 1979; available from U.S. Depart- 
 ment of Energy, Germantown, Md. 
 
 17. Nutter, R. S. Hazard Evalua- 
 tion Methodology for Computer Controlled 
 Mine Monitoring/Control System. Pres. 
 at Western Electro Technology Conf., 
 Reno, Nev., September 1981; available 
 from R. S. Nutter, West Virginia Univer- 
 sity, Morgantown, W. Va. 
 
120 
 
 18. Prosser, L. J. , Jr. , G. L. Fin- 
 finger, and J. Cervik. Methane Drain- 
 age Study Using an Underground Pipeline, 
 Marianna Mine 58. BuMines Rl 8577, 1981, 
 21 pp. 
 
 19. Rustan, A. Review of Develop- 
 ments in Monitoring and Control of Mine 
 Ventilation Systems. Proc. 2d Internat. 
 Mine Ventilation Cong., Nov. 4-8, 1979, 
 Reno, Nev. , pp. 223-229. 
 
 20. Tregelles, P. G. Automation in 
 UK Mines: Achievements, Goals & Prob- 
 lems. Coal Min. and Processing, v. 17, 
 No. 9, September 1980, pp. 110-122. 
 
 Instrumentation. BuMines OFR 1-82, June 
 1981, 137 pp.; NTIS PB 82-146325. 
 
 22. Tongue, D. W. , D. D. Schuster, R. 
 Neidbala, and D. M. Bondurant. Design 
 and Recommended Specifications for a Safe 
 Methane Gas Piping System. BuMines OFR 
 109-76, July 1976, 97 pp.; NTIS PB 259 
 340. 
 
 23. Van der Walt, N. T. , B. J. Bout, 
 Q. S. Anderson, and T. J. Newington. 
 All-Analogue Fire Detection System for 
 South African Gold Mines. J. Mine Venti- 
 lation Soc. South Africa, v. 33, No. 1, 
 January 1980, pp. 2-13. 
 
 21. Trelewicz, K. Environmental Test 
 Criteria for the Acceptability of Mine 
 
CHAPTER 5.— COMMUNICATION SYSTEM DESIGN AND IMPROVEMENT 
 
 121 
 
 5.1 Introduction 
 
 This chapter analyses the parameters 
 influencing initial design of communica- 
 tion systems, for new mines and upgrading 
 existing systems. 
 
 Paragraph 5.2 outlines those varia- 
 bles that must be taken into account dur- 
 ing the design stages of a new wired 
 phone system. Recommended features, gen- 
 eral requirements, and how they can be 
 implemented are treated in this section. 
 
 Paragraph 5.3 describes ways of 
 improving or extending the range of trol- 
 ley carrier phone systems and pager phone 
 systems already installed in the mine. 
 
 5.2 New Phone System Design 
 
 The task of designing an adequate 
 communication, control, and monitoring 
 system for an underground mine must be 
 addressed on a system basis. In addition 
 to insuring that effective voice communi- 
 cation is established, any new system 
 should take into account present and 
 future requirements of remote control and 
 monitoring functions. Chapter 4 illus- 
 trated the drastic savings in response 
 time that can be realized when remote 
 control and monitoring are integrated 
 into the overall communication system. 
 The importance of including control and 
 monitoring in the overall design plan for 
 any system cannot be overemphasized. 
 
 Because each mine is unique, and 
 thus usually has its own special operat- 
 ing characteristics and communication 
 requirements, there is no such thing as 
 "the one best system" to meet the 
 requirements of all mines. The optimum 
 communication, control, and monitoring 
 system for a mine must be one that has 
 been tailored to meet the special 
 requirements of that particular mine. 
 Factors that must be considered during 
 system design include — 
 
 a. Type of mine and mining methods 
 (low- or high-seam coal, deep hardrock 
 
 mine, stope caving, longwall, room and 
 pillar, etc.). 
 
 b. Maximum number of working 
 sections. 
 
 c. Expected mine growth rate and 
 eventual maximum size. 
 
 d. Haulage methods (tracked trol- 
 ley, diesel, belt, etc.). 
 
 e. Underground power distribution 
 system (dc, ac, or both). 
 
 f. Features desired (two-way radio 
 paging, private line capability for emer- 
 gency use, etc.). 
 
 g. Redundant or backup systems 
 for use during outages of the normal 
 system. 
 
 Although no two mines are alike, the 
 following items have been established as 
 the main characteristics desired for any 
 underground communication system: 
 
 1. Multiple Communication Paths to 
 Outside — the objective here is to give 
 all telephones a second method of commu- 
 nicating with the surface. 
 
 2. Audible Emergency Signaling — the 
 communication system provides the main 
 means of alerting miners during emergen- 
 cies. The system should include means to 
 broadcast distinct audible signals for 
 emergency signaling. Initiation of these 
 signals should probably be controlled 
 from a central outside point, such as a 
 surface control room. 
 
 3. Emergency Override — provisions 
 should be included to permit any con- 
 versation to be overridden with emergency 
 communication. 
 
 4. Selective Area Page — as mines 
 grow larger it is apparent that the 
 entire telephone system paging mode need 
 not be activated each time a call is 
 initiated. When the general area of a 
 
122 
 
 person to be paged is known, only the 
 pagers in that area would be activated. 
 
 5. Simultaneous Conversation Capa- 
 bility — although the ultimate for this 
 characteristic would be a private line 
 for each telephone, this channel capacity 
 may not be necessary in some mines. In 
 general, each working section does not 
 produce much communication activity. 
 Haulage and maintenance activities domi- 
 nate telephone use. Since these activi- 
 ties tend to originate on the basis of 
 mine "areas," it appears that providing 
 different areas of the mine with a sepa- 
 rate communication circuit could meet the 
 simultaneous conversation need and main- 
 tain circuit simplicity. 
 
 6. Manual or Automatic Connection 
 Between Subsystems — provisions must be 
 made for connecting telephones within the 
 telephone system, and provisions should 
 be made for connecting the telephone sys- 
 tem into the other communication systems 
 used at the mine. 
 
 7. Remote Signaling — the design of 
 the telephone equipment and circuits 
 should be compatible with frequency divi- 
 sion multiplexed equipment so frequencies 
 above 3,000 Hz can be used for control 
 and monitoring applications. 
 
 5.2.1 Wired Phone Systems 
 
 The options open to a designer dur- 
 ing planning stages for a hard-wired 
 phone system include 
 
 Single-pair phone system 
 
 Multipair phone system 
 
 Multiplexed phone system 
 
 5.2.1a Single-Pair Systems 
 
 Many different types of wire can be 
 used for single-pair (party-line) commun- 
 ication systems (table 5-1). Smaller 
 gage wire may be satisfactory if the num- 
 ber of telephones in the system is small 
 and the distance between them is short. 
 However, for most applications, a larger 
 
 gage wire is chosen to improve the ten- 
 sile strength of the wire, as well as to 
 reduce the overall resistance of the run. 
 
 TABLE 5-1. - Single-pair cable 
 
 Description 
 
 Wire 
 gage, 
 AWG 
 
 Loop resist- 
 ance , ohms 
 per mile 
 
 Plastic-insulated 
 nonjacketed build- 
 ing wire. .......... 
 
 18 
 
 18 
 16 
 14 
 
 19 
 
 18 
 
 67 
 
 Type SO, neoprene- 
 jacketed portable 
 cable 
 
 Buried distribution 
 wire. .............. 
 
 67 
 42 
 27 
 
 83 
 
 Plastic drop wire 
 (copper-clad steel) 
 
 223 
 
 An inexpensive wire used for inter- 
 connecting mine phones is vinyl-plastic- 
 coated, 18-gage, two-wire, twisted-pair 
 building wire. Unjacketed wire of this 
 type provides little environmental pro- 
 tection for the copper conductors ; there- 
 fore it must be located out of the way of 
 the mining equipment and carefully sus- 
 pended to avoid moisture penetration. 
 
 The 14-gage neoprene-jacketed type 
 (see fig. 5-1) is recommended for most 
 underground applications. The greater 
 mechanical strength, reduced loop resist- 
 ance, and superior moisture resistance of 
 this cable makes it ideal for communica- 
 tion applications. 
 
 The best method of getting a feel 
 for the design considerations of a 
 single-pair system is to design a system 
 for a representative moderate-sized mine. 
 An example of such a mine is shown in 
 figure 5-2. This mine has the following 
 characteristics: 
 
 FIGURE 5-1, • Single-pair type SO neoprene cable. 
 
123 
 
 ^—SINGLE PAfR UNDERGROUND 
 
 SINGLE PAIR FOR LOOP BACK ABOVE GROUND 
 
 @ PERMANENT TELEPHONES IN MAIN HAULAGEWA 
 O SEMI-PERMANENT TELEPHONES AT BUTT ENTRM 
 ® FREQUENTLY MOVED TELEPHONES AT WORKING 
 
 FIGURE 5-2, - Single-pair installation in typical mine. 
 
 Less than 2 years old 
 
 6 square miles in total area 
 
 3.5 miles of main haulageway 
 
 0.8-mile-long average submain 
 
 Average panel size of 800 feet by 
 2,100 feet 
 
 Average working section size of 
 300 feet by 400 feet 
 
 5 working sections per shift 
 
 A maximum of 6 active working 
 sections 
 
 17 fixed mine pager phones presently 
 installed 
 
 The fixed-telephone, single-pair 
 communication system shown in figure 5-2 
 complies with the Federal Coal Mine 
 
 Health and Safety Act of 1969, in that it 
 provides two-way communication between 
 the surface and each working section. 
 Additional phones were installed at the 
 intersections of the main haulageway and 
 the submains , and at the intersections of 
 the submains and the butt entries to all 
 active sections. 
 
 Based on the physical characteris- 
 tics of the mine, the total length of 
 single-pair cable required can be calcu- 
 lated for this stage of development as 
 follows: 
 
 Miles 
 
 1 main haulageway 3.5 
 
 3 submains (0.8 mile 
 
 each) 2.4 
 
 6 active sections 
 
 (3,000 feet per section). 3.4 
 
 Total 9.3 
 
 The 3.4 miles of section cable as- 
 sumes the reuse of the cable as the work- 
 ing sections move from one panel to 
 another. At this stage in the mine's 
 development, 15 panels have been driven 
 or are being driven which would have 
 required 8.5 miles of section cable if 
 reusing it had not been assumed. There- 
 fore, the total cable miles needed are 
 
 9.3 if section cable reused 
 
 14.4 if section cable not reused 
 
 The least expensive wire for the 
 above application is plastic-insulated, 
 nonjacketed 18 AWG building wire. How- 
 ever, the high loop resistance (67 ohms 
 per mile) of the 18-gage wire will make 
 future expansion impractical; therefore 
 we should consider a larger gage wire. 
 
 A more suitable cable due to its low 
 loop resistance is 14 AWG, type SO, neo- 
 prene wire. The 14 AWG neoprene cable 
 uses annealed copper conductors so that 
 it can withstand severe mechanical abuse. 
 (The cable is designed for use as power 
 supply cable on portable equipment.) If 
 the 3,000 feet of 14 AWG neoprene wire 
 used for each active section is mounted 
 
124 
 
 on a reel and travels with the working 
 section phone into the panel, then we can 
 plan on reusing this wire when developing 
 future panels. The cost of expanding to 
 6 submains and 60 panels would involve 
 only the additional wire for 3 submains, 
 assuming we can reuse the section wire. 
 
 replacing the single-pair cable in the 
 main haulageway and the submains with 
 multipair cable. In a new mine, it would 
 mean calculating the maximum channel 
 requirements expected during the life of 
 the mine and specifying the proper multi- 
 pair distribution system. 
 
 The economic importance of reusing 
 section wire can be elaborated on by the 
 following calculations for 54 lengths of 
 additional section wire needed to reach 
 the 6-submain development stage if the 
 section wire is not reused. Each length 
 is 3,000 feet, or 0.57 mile. 
 
 54 lengths x 0.57 mile per length 
 = 30 miles of additional cable 
 
 The cost of this additional cable 
 can be a significant part of the total 
 cost of the entire single-pair system. 
 Although material costs are greatly 
 reduced if section wire is reused, some 
 additional labor costs are involved in 
 the removal of cable once a panel has 
 been completed. 
 
 Another alternative that can be 
 employed is to use high-quality 14 AWG 
 wire for the main and submains , and then 
 use a less expensive lighter gage wire 
 for the panels and not reuse this wire. 
 A low-cost 18-gage building wire may be 
 acceptable as section wire, because its 
 high resistance is not a problem for the 
 short length involved. 
 
 5.2.1b Multipair Systems 
 
 A single-pair cable system restricts 
 the mine communication system to a 
 single-channel multiparty configuration. 
 Introducing multipair cable into the mine 
 communication system allows one to expand 
 the number of channels to whatever is 
 necessary for efficient voice traffic. 
 In an existing mine, this would mean 
 
 The hardware for a multipair system 
 is of proven reliability and has stood 
 the test of time. All of these materials 
 have been used for aerial distribution 
 systems in the telephone industry and 
 were refined over the years to survive in 
 any part of the world with a minimum of 
 preventive maintenance. Because it was 
 designed to be installed and maintained 
 by linemen working in all kinds of 
 weather while standing on ladders, on 
 aerial platforms, or in manholes, multi- 
 pair equipment can be handled by elec- 
 tricians in the underground environment. 
 The only new skill that mine personnel 
 may have to learn is the splicing of 
 small-diameter wires. However, crimp- 
 type splice connectors are available to 
 simplify the splicing of multipair 
 cables. 
 
 Table 5-2 shows the major character- 
 istics of multipair cable available from 
 telecommunication cable manufacturers. 
 Figure-8 cable is recommended for the 
 mine application because the messenger 
 wire adds considerable tensile strength 
 to the cable, and the installation is 
 similar to that of trolley wire. 
 
 The previous section described a 
 single-pair cable system using a repre- 
 sentative moderate-size fictitious mine. 
 The same mine will be used to analyze a 
 multipair cable system (fig. 5-3). 
 
 Using a cable distribution and load- 
 ing plan that will allow the servicing of 
 no more than two sections per twisted 
 pair, a minimum of three pairs is 
 
 TABLE 5-2. - Range of multipair cables commercially available 
 
 Number of pairs 3-400 
 
 Messenger size (diameter) inch.. 0.134-0.250 
 
 Conductor size AWG. . 26-19 
 
 Conductor dc resistance at 68° F...ohms per mile.. 43-220 
 
125 
 
 FIGURE 5»3, - Multipair installation in typical mine, 
 
 required to handle the six working sec- 
 tions. The main haulageway phones con- 
 nected across a single party line require 
 an additional pair for a total of four 
 pairs, each of which extends back to a 
 centralized location such as the dis- 
 patcher's office. A six-pair cable 
 placed in the main haulageway will accom- 
 modate the above required pairs while 
 leaving an extra two pairs for future 
 expansion. Three-pair cable may be ap- 
 propriate for the submalns because no 
 more than four sections will be active 
 per submain at any one time. A single- 
 pair cable can be used between the panel 
 entry phone, located in the submain, and 
 the section phone, which must move with 
 the section crew. 
 
 Due to the 3.5-mile length of the 
 main haulageway and assuming that a maxi- 
 mum of seven phones will be connected 
 in parallel across one pair, a 19-gage 
 
 six-pair cable has been selected for the 
 haulageway. The submains with only two 
 phones per pair and run lengths of less 
 than 1 mile can use 22-gage wire. A 
 splice case at every third section entry 
 should be sufficient in this application 
 and will reduce labor costs. 
 
 The section cable can be a single 
 pair but must be strong enough to with- 
 stand the wear caused by the almost con- 
 stant phone relocating required in the 
 working section. A 3,000-foot reel of 
 wire that travels with the section phone 
 would reach any location in an 800- by 
 2,100-foot panel. Plastic drop wire has 
 been chosen for the section cable. This 
 wire is made up of two 18 AWG copper- 
 covered steel wires laid in parallel and 
 coated with a black flame-resistant poly- 
 vinyl chloride insulation. The high 
 strength of this cable allows for long 
 spans which make for quick temporary in- 
 stallations and also reuse of the cable. 
 A stainless steel drop wire clamp can be 
 hooked to roof bolts or nailed to timbers 
 for support. 
 
 In cost comparisons between single- 
 pair and multipair systems (2^),^ the wir- 
 ing costs for multipair installations 
 were less expensive because the smaller 
 gage wire allowed in the multipair cable, 
 due to fewer phones placed in parallel 
 per pairs, kept the per-mile cost of 
 multipair cable competitive with that of 
 the larger gage single-pair cable. 
 
 Two questions worthy of considera- 
 tion at this point are. How well does a 
 multipair communication system meet the 
 needs of the mine user? and What 
 improvements can be incorporated into a 
 multipair system that are not possible 
 with the present day single-pair mine 
 telephone system? 
 
 Advantages 
 
 More Channels . — Using multipair ca- 
 ble, a system can be designed with as 
 
 ^Underlined numbers in parentheses re- 
 fer to items in the bibliography at the 
 end of this chapter. 
 
126 
 
 many channels as are deemed necessary 
 for the particular application, the only 
 limits being cost and complexity. 
 
 Private Channels. — Individual pairs 
 can be assigned to each working section, 
 thereby producing a private channel 
 between the section and the mine commu- 
 nications center. 
 
 Zone Paging. — The communication cen- 
 ter can page over an individual pair 
 so that only the section of the mine 
 concerned with the transmission need be 
 disturbed. This would eliminate the 
 present situation of requiring miners in 
 all sections to listen to all pages. 
 
 Direct Dialing. — Pairs can be dedi- 
 cated to connect underground dial phones 
 directly to the company's private auto- 
 matic branch exchange (PABX) or directly 
 to a central office through an approved 
 interface. This would allow key loca- 
 tions in the mine to dial each other, 
 place outgoing calls, or receive incoming 
 calls via the local exchange without 
 relaying messages through the communica- 
 tion center. Provisions for preventing 
 abuse of the latter two features could 
 also be included. 
 
 Remote Monitoring. — Extra pairs in 
 the cable may be used for monitoring the 
 mine environment and/or equipment. 
 
 Disadvantages 
 
 Increased Operating Costs. — A multi- 
 pair system incorporating all of the 
 above advantages will cost more than a 
 single-pair system, even though the 
 multipair cable may cost less than the 
 single-pair cable. This is due to the 
 additional cost of a central switching 
 equipment required for multipair systems. 
 For a particular application, the 
 increased efficiency and other benefits 
 must be weighed against the added in- 
 stallation and maintenance costs in order 
 to establish its true worth. 
 
 Training Costs. — The maintenance per- 
 sonnel assigned to install and main- 
 tain this equipment will have to be 
 
 trained to use the different splicing 
 techniques required and to troubleshoot 
 this somewhat more complex system. 
 
 5.2.1c Multiplexed Phone Systems 
 
 Multiplex telephone systems achieve 
 their private channel capability via 
 electronic means on a single cable. 
 Multiplexing can be via time division 
 multiplexing (TDM) or frequency division 
 multiplexing (FDM). Although TDM systems 
 have been developed and provide certain 
 advantages, a multitude of disadvantages 
 tend to make this type of multiplexing 
 unattractive for mine telephone systems. 
 
 FDM systems have been developed and 
 tested in underground mines with consid- 
 erable success. These systems can be 
 divided into ones that require a central 
 switching station for system control and 
 those that do not. In a central switch- 
 ing system, most if not all of the system 
 intelligence resides in the central unit 
 which assigns frequencies, provides power 
 for the phones, and generates ringing and 
 busy signals. These systems are gener- 
 ally permitted only in nongassy mines. A 
 serious disadvantage of such a system is 
 that a failure in the central unit can 
 render the entire system inoperative. 
 
 A system that does not rely on a 
 central switching unit has been developed 
 by the Bureau of Mines. The system is 
 based upon microprocessor control, where 
 intelligence is resident in each tele- 
 phone. Eight-channel voice or data com- 
 munications is possible. The system uti- 
 lizes FDM at medium frequencies (340-650 
 kHz) and is designed for a 10-mile cable 
 plant. A failure of any one phone nor- 
 mally affects the multiplex feature of 
 that phone only. Each phone also in- 
 cludes a resident pager phone capability 
 such that even a total failure of the 
 microprocessor intelligence will not 
 normally inhibit a user from making a 
 call. This feature is essential in any 
 modern telephone system for underground 
 mines. Supervisory feedback and a vis- 
 ual message-leaving capability (as is 
 required in several States) are also 
 included. 
 
127 
 
 5.2.2 Cable Selection 
 
 Telephone transmission is made over 
 wires which represent a considerable 
 fraction of the cost of any telephone 
 system. As an example, figure 5-4 shows 
 three broad categories of equipment in 
 which telephone companies invest. The 
 "transmission" category not only repre- 
 sents wires, but also includes multiplex 
 systems, microwave systems, and other 
 wire substitutes. Since transmission 
 equipment accounts for about half of the 
 total investment, telephone companies put 
 considerable effort into planning the 
 layout and the growth of their transmis- 
 sion facilities. Cable costs account for 
 even a greater percent of the expense in- 
 volved in an underground communication 
 system. Therefore, mine planners should 
 also carefully plan the network and re- 
 vise the plan on a scheduled basis. 
 
 The general environment in an under- 
 ground mine imposes severe physical re- 
 quirements on communication cable. Insu- 
 lation is required to withstand exposure 
 to moisture, abrasion, and rough han- 
 dling; to afford protection against some 
 level of accidental contact with higher 
 
 FIGURE 5-4, - Telephone company investments. 
 
 voltages; and to not support combustion 
 in case of fire. 
 
 Twisted-pair construction is advised 
 to reduce the effects of induced noise or 
 interference. The 14 AWG solid-conductor 
 twisted pair, with suitable insulation 
 dielectric and outer protective jacket is 
 very rugged, and will withstand the rough 
 handling and stress imposed by abrasion 
 against timbers or falling debris. For 
 the smaller diameter wires, such as 
 19 AWG, a figure-8 cable is recommended. 
 In this construction, a steel "messenger" 
 or support wire is added to the twisted- 
 pair bundle, so that the overall cross 
 section resembles a figure 8. The -steel 
 messenger cable provides additional 
 strength and support so that minimum 
 strain is applied to the signal-carrying 
 twisted pair. 
 
 Solid conductor is advised, rather 
 than the more easily handled multistrand 
 wire. The multistrand cable is subject 
 to corrosion buildup on the surface of 
 the individual conductor strands, which 
 in time could reduce the conductivity of 
 a splice or connection and become the 
 source of added noise and reduced signal 
 level. Conditions within an underground 
 mine dictate the use of press-on or 
 twist-on connectors as common practice to 
 complete a splice. Such practices are 
 not compatible with the use of multi- 
 strand wire. 
 
 The choice of wire size is deter- 
 mined by the configuration of the tele- 
 phone system and the type of phone in 
 use. Major factors to consider in the 
 choice of wire size are the total length 
 of cable run, the number of phones in the 
 circuit, the average distance between the 
 phones, and the characteristics of the 
 ringing or calling circuit in each phone. 
 
 In pager phone systems, the paging 
 relay circuit is one of the more critical 
 parameters to consider in the choice of 
 pager phone wire size. The normal audio 
 signal imposed on the cable is about 1 to 
 2 MW; this signal level is sufficient to 
 operate a phone receiver at satisfactory 
 
128 
 
 volume over several miles of cable as 
 small as 19 AWG. The limiting condition 
 is the ability to reliably operate the 
 paging relays. In this regard, the cable 
 impedance, or resistance per unit length, 
 as it affects the available dc voltage at 
 the paging relay, is more influential 
 than audio loss. Calculation of the min- 
 imum wire size that will insure reliable 
 operation of all paging relays must take 
 into account three major parameters: 
 paging circuit impedance, battery volt- 
 age, and wire losses. 
 
 Some pager phones use electromechan- 
 ical relays that have an impedance of 
 about 2,500 ohms while other systems use 
 electronic or semiconductor switching 
 circuits that have an impedance of from 
 8,000 to 50,000 ohms. The minimum dc 
 voltage required to operate any of these 
 relays is about 1.5 to 4 volts. To 
 insure a safety margin, it is recommended 
 that at least 5 volts dc be available at 
 all telephone paging relays. It is eas- 
 ier to obtain this minimum voltage with 
 the higher impedance circuits. 
 
 Available battery voltage is a func- 
 tion of the condition of the battery and 
 the load it must operate. In a 12-volt 
 system, the battery is at the end of its 
 useful life when the dc voltage under 
 load condition approaches 8 volts. For a 
 24-volt system, a battery is at the end 
 of its useful life when the available dc 
 
 voltage under load approaches 16 volts. 
 There is no specific time at which the 
 battery can be identified as not usable. 
 However, it is generally agreed that the 
 levels just stated are typical of the end 
 of a battery's useful life and indicate 
 that it should be replaced. 
 
 In many pager phones, the internal 
 circuit has been designed so that the 
 total battery voltage is not available on 
 the line for operation of paging relays. 
 Circuitry in such phones can add a series 
 dc resistance of from 10 to 100 ohms to 
 limit the short-circuit drain to levels 
 of operation that are intrinsically safe. 
 A pager phone system can draw significant 
 current from the battery in the "paging" 
 phone. This causes an internal voltage 
 drop which significantly reduces the 
 effective voltage presented to the line. 
 Estimates of this effect, for a variety 
 of conditions, are shown in table 5-3. 
 
 Wire loss per unit length is a func- 
 tion of wire diameter and system configu- 
 ration. These factors include total wire 
 used, telephone spacing, number of 
 phones, and input impedance. All of 
 these factors must be considered together 
 in view of the expected battery voltage 
 at end of useful life (8 or 16 volts), 
 the relay impedance (2,500 ohms or 
 greater than 8,000 ohms), and the inter- 
 nal voltage drop because of circuit 
 losses. 
 
 TABLE 5-3. - Effect of paging circuit impedance 
 (Electromechanical relays, 2,500 ohms) 
 
 Battery voltage. 
 
 Limiting 
 
 Available battery voltage 
 
 dc volts 
 
 resistance, 
 ohms 
 
 on the line, dc volts 
 
 
 10-phone system 
 
 20-phone system 
 
 24-volt battery: 
 
 
 
 
 24 (new) 
 
 10 
 100 
 
 23.75 
 19 
 
 23.5 
 18 
 
 16 (near end of life). 
 
 10 
 100 
 
 15.5 
 13 
 
 15.5 
 12 
 
 12-volt battery: 
 
 12 (new) 
 
 10 
 100 
 
 11.8 
 10 
 
 11.6 
 8 
 
 8 (near end of life).. 
 
 10 
 100 
 
 7.8 
 6.5 
 
 7.6 
 6 
 
129 
 
 The simplest calculation is to as- 
 sume a basic ladder configuration, where 
 all phones are in parallel on the same 
 single two-wire cable, strung the length 
 of the installation (fig. 5-5). This ba- 
 sic installation is the one most normally 
 considered when calculations are made to 
 determine minimum wire size. Tables 5-4 
 and 5-5 indicate the minimum wire size 
 for both electromechanical and electronic 
 relays, with average phone spacing of 1/4 
 and i/2 mile. 
 
 In a 12-volt system, with electro- 
 mechanical 2,500-ohm relays, only 12 
 hones spaced 1/4 mile apart over 3 miles 
 can be used with 19 AWG wire. However, 
 20 phones can be used over a 5-mile run 
 if 14 AWG wire is used. If electronic 
 8,000-ohm relays were used, the 24-volt 
 system could support 33 phones over 8 
 miles of cable using 19 AWG wire. 
 
 Tables 5-4 and 5-5 do not take into 
 consideration line losses caused by poor 
 splices, dampness, or defective phones. 
 However, they do illustrate comparative 
 conditions as a guide for system design 
 and component selection. 
 
 Consideration of a topography that 
 involves a multiple-branch system may 
 result in a design that can use a smaller 
 diameter wire. Conditions in mines nor- 
 mally degrade even the best of systems — 
 moisture causes signal leakage; erratic 
 
 m 
 
 o 
 
 m 
 
 o 
 
 fqf 
 
 Rw = LIME IMPEDANCE 
 
 Rp - PHONE INPUT IMPEDANCE 
 
 FIGURE 5-5. - Basic ladder configuration. 
 
 or incorrect branch connections and 
 splices tend to reduce performance — so 
 that using detailed calculations to de- 
 termine marginally usable minimum wire 
 size is not a recommended practice. It 
 makes more sense to determine a minimum 
 wire size for safe operating level and 
 then use that size as a guide to select 
 or recommend a wire that meets all the 
 specifications. For multiple-branch con- 
 figurations, the following rules of thumb 
 can be used to estimate minimum wire size 
 without extensive calculation: 
 
 1. Determine 
 configuration. 
 
 present telephone 
 
 TABLE 5-4. - 1/4-mile pager phone spacing 
 
 System 
 
 19 AWG 
 
 14 AWG 
 
 12-volt, 2,500-ohm relay 
 
 24-volt, 2,500-ohm relay 
 
 12-volt, 8,000-ohm relay 
 
 24-volt, 8,000-ohm relay 
 
 3 miles, 12 phones 
 
 5 miles, 20 phones 
 
 5 miles, 20 phones 
 
 8 miles, 33 phones 
 
 5 miles, 
 9 miles. 
 
 20 phones 
 36 phones 
 
 9 miles, 36 phones 
 >9 miles, >36 phones 
 
 TABLE 5-5. - 1/2-mile pager phone spacing 
 
 System 
 
 19 AWG 
 
 14 AWG 
 
 12-volt, 2,500-ohm relay 
 
 24-volt, 2,500-ohm relay 
 
 12-volt, 8,000-ohm relay 
 
 24-volt, 8,000-ohm relay 
 
 4.5 miles, 9 phones 
 7 miles, 14 phones 
 
 7.5 miles, 15 phones 
 13 miles, 26 phones 
 
 7.5 miles, 
 13 miles. 
 
 15 phones 
 26 phones 
 
 13 miles, 26 phones 
 >18 miles, >36 phones 
 
130 
 
 2. Estimate probable growth of the 
 telephone configuration. 
 
 3. Sketch the future telephone 
 configuration. 
 
 4. Examine the sketch to determine 
 the longest combined path that takes into 
 account a majority of the telephones. 
 
 5. From table 5-2 or 5-3 determine 
 the minimum wire size for the longest 
 path needs. 
 
 6. The added loads of the other 
 branches will not greatly affect the 
 determination of minimum wire size and 
 can be ignored for such an estimate. 
 
 The 21 pager phones shown in the top 
 panel of figure 5-6, spaced an average of 
 1/4 mile apart, are connected in a 
 branching system, which can be repre- 
 sented by the impedance diagram shown in 
 the bottom panel. The longest path is E 
 
 (A) y,-MlLE PAGER PHONE SPACING 
 
 (Bl EQUIVALENT CIRCUIT 
 
 FIGURE 5-6. - Branching ladder network. 
 
 to D to J, which includes 10 phones over 
 about 2.5 miles of cable. If we examine 
 table 5-2, we find that with 2,500-ohm 
 mechanical relays in a 12-volt system, 
 19 AWG wire is adequate for the 
 configuration. 
 
 This type of rule-of -thumb estimate 
 is adequate to identify approximate 
 requirements for wire size, but it does 
 not replace necessary detailed calcula- 
 tions for a major installation with many 
 branches. It must also be emphasized 
 that calculation of minimum wire size 
 identifies the bottom limit of a marginal 
 condition and good engineering practice 
 dictates some margin of reliability. The 
 general manufacturers' recommendation of 
 14 to 16 AWG twisted pair for systems us- 
 ing 2,500-ohm electromechanical relays is 
 sound, particularly for a 12-volt system. 
 
 For systems using semiconductor pag- 
 ing circuits (with impedances of 8,000 
 ohms or greater), 19 AWG is usually ade- 
 quate. This is particularly true for 24- 
 volt systems, but also applies to most 
 12-volt systems that have high-impedance 
 switching circuits. 
 
 In summary, the cable wire size de- 
 pends on a series of factors that include 
 the total number of telephones in an in- 
 stallation, the total length of cable run 
 (distance between the farthest phones), 
 the configuration of branch lines, the 
 available battery voltage, and the type 
 of paging relay used. The preferred ca- 
 ble, regardless of wire size, is a 
 twisted pair of solid conductor wires, 
 with individual insulation around each 
 wire in the pair and an outer abrasion- 
 resistant covering of waterproof, flame- 
 retardant material. 
 
 5.2.3 Summary 
 
 The basic system choices that may be 
 selected when choosing an underground 
 wired phone system consist of — 
 
 Single pair . — This is a party line 
 system in which all phones are on the 
 same channel. 
 
131 
 
 Multipair. — A private line system 
 with each phone or group of phones con- 
 nected to the system center by its own 
 individual wire pair. 
 
 Multiplex. — A private line system 
 using a single cable, with the audio to 
 and from each phone multiplexed onto the 
 common cable. 
 
 In all of these systems, telephone 
 transmission is made over wires which 
 represents a considerable fraction of the 
 cost of the entire system. Since trans- 
 mission equipment accounts for about half 
 of total investment, companies should put 
 considerable effort into planning the 
 layout and growth of their transmission 
 facilities. 
 
 In planning mine communication 
 systems, the pairs or voice channels 
 that will be needed in the future and 
 the mobility of the telephones involved 
 should be kept in mind. In addition, 
 pairs that will be needed for purposes 
 other than for telephones (telemetry, 
 remote monitoring, etc.), which inci- 
 dentally may exceed voice communica- 
 tion needs, should also be taken into 
 
 performance of a trolley carrier system, 
 and the second treats telephone systems. 
 
 5.3.1 Trolley Carrier Phone Systems 
 
 account. 
 
 5.3 Improving Existing (In-Place) 
 Phone Systems 
 
 The two types of communication sys- 
 tems commonly used to date in underground 
 mines are as follows: 
 
 Carrier current radio system using 
 the trolley line. 
 
 Various types of telephone system. 
 
 Because these systems have gained 
 such widespread usage, methods for up- 
 grading and improving presently installed 
 systems are presented in the following 
 sections. The first deals with improving 
 
 ' Approved and nonapproved equipment may 
 not share the same cables; check with 
 MSHA for details. 
 
 WARNING 
 
 Some of these procedures are un- 
 dertaken with the trolley wire ener- 
 gized; therefore, they are extremely 
 hazardous. Extreme caution must be 
 exercised to avoid potentially lethal 
 shock. The fuses used in the test 
 leads serve only to protect equipment 
 and do not in any way reduce the 
 shock hazard to personnel. Only per- 
 sonnel thoroughly familiar with elec- 
 trical work on trolley wires should 
 conduct these procedures. The perma- 
 nent connection of components should 
 be done with power removed. Care 
 should also be taken to insure that 
 components and equipment are suitable 
 for use in the desired application. 
 
 The trolley carrier phones used for 
 dispatch purposes in electrical rail 
 haulage mines often show problems in pro- 
 viding coverage over the entire haulage 
 system. Direct communication between the 
 dispatcher and vehicles in certain areas 
 of the mine is often difficult or impos- 
 sible. The major reason for these diffi- 
 culties is the effects that loads placed 
 across the trolley wire or rail have on 
 transmission. 
 
 Both theory and experiment show that 
 the trolley wire-rail by itself is a rel- 
 atively good transmission line for car- 
 rier phone frequencies. In fact, on an 
 unloaded trolley wire-rail transmission 
 line, a distance of 35 miles could be ex- 
 pected for communication range. Communi- 
 cations over a real trolley wire-rail can 
 never achieve this range because the many 
 loads across the trolley wire-rail absorb 
 and reflect carrier signal power. The 
 list of these loads is long and includes 
 rectifiers, personnel heaters, signal 
 lights, vehicle motors, vehicle lights, 
 and the carrier phone itself. It is 
 probable that the net signal attenuation 
 
132 
 
 rate for a trolley wire-rail with typical 
 loads placed across it yields a useful 
 range as low as 3.5 miles. The problem 
 of obtaining good signal propagation is 
 further aggravated by branches of the 
 trolley wire where the signal splits in a 
 totally uncontrollable way. Lack of 
 proper signal termination at the ends of 
 the trolley wire-rail further degrades 
 signal propagation. The vehicles repre- 
 sent moving loads on the transmission 
 line and add a further complication to 
 obtaining or predicting good signal pro- 
 pagation. Also, advancing the mine face 
 means that the transmission network 
 changes with time, yielding more uncer- 
 tainty to the quality of transmission. 
 
 The seriousness of the bridging 
 loads can be seen by reference to fig- 
 ure 5-7 where the losses for typical 
 loads are tabulated. Using this chart, 
 one can make an estimate of the total 
 signal loss by adding the individual 
 losses (in decibels). 
 
 In the past, whenever poor trolley 
 carrier communications existed, attempts 
 were made to remedy the problem using 
 "Z-boxes," or signal couplers to the 
 phone line. Z-boxes are not permissible, 
 are usually not the best solution, and 
 may actually introduce more problems then 
 they solve. Mines are full of Z-boxes 
 that have been disconnected and abandoned 
 
 fin 
 
 
 LOAD 
 
 OHMS 
 
 
 \ 
 
 HEATEHS 
 
 JO -100 
 
 
 RECTIFIERS 
 
 2-10 
 
 
 VEHICLE MOTORS 
 
 60-500 
 
 50 
 
 VEHICLE LIGHTS 
 
 60-120 
 
 
 CARRIER PHONES 
 
 20-200 
 
 
 MINE LIGHTS 
 
 500-2000 
 
 40 
 
 30 
 20 
 10 
 
 1 1 
 
 
 because of poor performance. It is 
 recommended that solutions other than 
 Z-boxes be used to improve the perform- 
 ance of trolley carrier phone systems. 
 
 The most straightforward way of 
 treating the trolley wire-rail to make it 
 into a functional carrier signal trans- 
 mission line is to physically remove from 
 the trolley wire-rail all of the bridging 
 loads that impede carrier signal propaga- 
 tion. The steps in this process follow: 
 
 1. Identify the bridging loads. 
 List all the bridging loads across the 
 trolley wire-rail. Consult figure 5-7 to 
 estimate the seriousness of the impedi- 
 ment to carrier signal propagation that 
 each load represents. 
 
 2. Determine which loads can be 
 removed from the trolley wire-rail and be 
 operated from mine ac power. 
 
 For practical reasons, physical 
 removal of bridging loads has severe lim- 
 itations. Certain critical loads, in- 
 cluding rectifiers, vehicles, lights, 
 motors, and carrier phones themselves, 
 cannot be removed from the trolley wire- 
 rail. In some instances, none of the 
 loads can be removed from the trolley 
 wire-rail, and efforts to improve signal 
 propagation must involve other methods. 
 
 Studies conducted have revealed 
 alternative ways of increasing the range 
 and quality of existing trolley carrier 
 phone systems. These methods include — 
 
 Isolated 
 frequency 
 
 loads at 
 
 the 
 
 carrier 
 
 BRIDGING LOAO lOHMSI 
 
 FIGURE 5-7. " Signal loss versus bridging load. 
 
 Using a dedicated line 
 
 Using a remote transceiver 
 
 5.3.1a Isolating Loads at the Carrier 
 Frequency 
 
 Figure 5-7 shows that as the bridg- 
 ing resistance is increased, the signal 
 loss decreases. The "isolating loads" 
 method involves adding passive circuit 
 elements (inductors and capacitors) in 
 
133 
 
 series with the particular load to reduce 
 the effects of the bridging load. The 
 circuit elements do not affect dc equip- 
 ment (motors, lights, etc.) being powered 
 from the trolley wire, but they do, if 
 properly chosen, add high impedance at 
 the carrier frequency. Rectifiers, heat- 
 ers, and vehicle lights are the bridging 
 loads that most seriously degrade re- 
 ceived signal levels and should be 
 treated first to improve received signal 
 levels. 
 
 5.3. la. i Rectifiers 
 
 There are three means of raising the 
 effective carrier frequency impedance of 
 a rectifier. The most practical method 
 depends on where the rectifier is in- 
 stalled. If it is located relatively far 
 from the rail (beyond 40 feet), the feed 
 wires represent sufficient inductance 
 that can be resonated, thereby raising 
 the effective impedance as seen by the 
 trolley wire-rail (fig. 5-84). If the 
 rectifier setback is short (less than 40 
 feet) , two techniques can be used to 
 raise the effective impedance: (1) A 
 fixed high-current inductor can be added 
 in series with the rectifier and that 
 inductor can then be tuned to raise the 
 effective impedance (fig. 5-8S); or 
 (2) the inductance of the trolley wire- 
 rail can be used to resonate short sec- 
 tions of the trolley wire-rail near the 
 bridging load to raise the effective 
 bridging impedance (fig. 5-8C). The ways 
 of applying each of these means are de- 
 scribed below. 
 
 a. Resonating the Feed Wire Inductance 
 
 The following steps are required to 
 tune the rectifier feed wires: 
 
 1. Attach a 1,000-volt (some sys- 
 tems may require even higher voltage com- 
 ponents) , l-pF or larger, oil-filled 
 capacitor directly across the plus and 
 minus terminals inside the rectifier. 
 (This capacitor serves to reject 
 rectifier-generated interference in the 
 carrier frequency band. ) 
 
 2. At the far end of the feeder 
 wires, as near to the trolley wire-rail 
 
 FEED WIRE INDUCTANCE 
 
 ADDED TUNING CAPACITOR 
 
 A RESONATING THE FEEDWIRE INDUCTANCE 
 
 ADDED TUNING CAPACITOR 
 
 ADDED FIXED INDUCTANCE 
 
 B. RESONATING AN ADDED FIXED INDUCTANCE 
 
 ADDED 
 TUNING 
 CAPACITOR 
 
 C RESONATING THE TROLLEY WIRE/RAIL INDUCTANCE 
 
 FIGURE 5-8. - Ways of raising the impedance 
 of a rectifier. 
 
 as practical, install the temporary test 
 set shown in figure 5-9. This test set 
 comprises a decade capacitor, isolating 
 and protection devices, and a tuned 
 voltmeter. Usually two feed wires are 
 run from the rectifier to this point. 
 Only one need be treated. 
 
 3. The dispatcher is called from a 
 jeep parked nearby and asked to key on 
 his transmitter for 20 seconds or so. 
 The decade capacitor box is switched 
 through its range of operation and left 
 at the position of maximum signal, as 
 indicated by the tuned voltmeter. (The 
 decade box should have enough range 
 to peak the voltmeter.) This value of 
 signal should be larger than when the 
 decade capacitor is at its off position. 
 The two values — the voltage when the dec- 
 ade capacitor is off and the maximum 
 
134 
 
 TO VOLTMETER 
 
 WARNING I 
 
 THIS CAPACITOR 
 
 S ESSENTIAL WITHOUT 
 
 EDO. TROLLEY 
 VOLTAGE IS APPLIED TO 
 THE VOLTMETER 
 
 OR MORE 
 
 
 FIGURE 5=9. => Test configurations for tuning 
 feeder wire. 
 
 FIGURE 5-10. - Permanent installation of tun= 
 ing elements for feeder wire. 
 
 WARNING 
 
 Some of these procedures are undertaken with the trolley wire energized; there- 
 fore, they are extremely hazardous. Extreme caution must be exercised to avoid 
 potentially lethal shock. The fuses used in the test leads serve only to protect 
 equipment and do not in any way reduce the shock hazard to personnel. Only per- 
 sonnel thoroughly familiar with electrical work on trolley wires should conduct 
 these procedures. The permanent connection of components should be done with 
 power removed. Care should also be taken to insure that components and equipment 
 are suitable for use in the desired application. 
 
 value — should be logged, preferably on a 
 mine map. There should be an appreciable 
 increase in voltage for this condition, 
 at least 1 1/2 to 1, and in some in- 
 stances up to 10 to 1. The value of the 
 capacitance that produces the maximum 
 voltage should be noted from the value 
 indicated on the decade capacitor, and a 
 suitable capacitor of that value should 
 then be installed in a permanent fashion, 
 as shown in figure 5-10. When this in- 
 stallation has been made, a final check, 
 using the tuned voltmeter, should be made 
 to ascertain that the originally indi- 
 cated increased voltage is obtained. 
 
 For this procedure, it is important 
 that the tuned voltmeter be tuned to the 
 precise transmission frequency of the 
 
 dispatcher. A preliminary test can 
 easily ascertain that this condition has 
 been met by sweeping the tuning dial of 
 the tuned voltmeter through the region 
 near the transmitted frequency and leav- 
 ing it at the position where maximum 
 response is indicated. 
 
 b. Resonating an Added Fixed Inductance 
 
 When the setback is short, an added 
 inductor made of a coil of feeder wire 
 may be used to provide a series induct- 
 ance that can be tuned. Because feeder 
 wire is expensive, a coil in the so- 
 called Brooks form, which yields the max- 
 imum inductance per length of wire, 
 should be used. See Appendix A (Mine E) 
 for an actual installation example. 
 
135 
 
 The approximate form is shown in 
 figure 5-11. A reasonable bending radius 
 for the typical thousand-circular-mils 
 cable used for such feeder wires is 2 
 feet; therefore this dimension is approx- 
 imately fixed. Four turns at this diam- 
 eter yield an inductance of approximately 
 25 )H, which is adequate for tuning most 
 rectifiers. The coil should be installed 
 in the room in which the rectifier is lo- 
 cated and should be kept a few feet away 
 from the coal to prevent added losses at 
 the carrier frequency. The exact value 
 of inductance is unknown, so the coil 
 will have to be tuned in much the same 
 manner as discussed previously for reso- 
 nating the feeder wires. 
 
 Figure 5-12 illustrates the test 
 setup. The dispatcher is called and 
 asked for a 20-second transmission. The 
 decade capacitor is switched through its 
 positions and left at the position that 
 yields the maximum voltage. (The decade 
 box should have enough range to peak the 
 voltmeter.) The received voltage with 
 the decade capacitor in the "off" posi- 
 tion and the maximum voltage should be 
 
 NOTES 
 
 MATERIAL 2Xd LUMBER 
 DIAGONAL BRACKING. OMITTED FOR 
 EASE OF DRAWING, IS REQUIRED 
 
 5S 
 
 noted, preferably on a mine map. When 
 the best capacitor value has been found 
 in this manner, the test set is re- 
 moved and a suitable capacitor of the 
 value found during the test is perma- 
 nently attached to the coil, as shown in 
 figure 5-13. When completed, a last test 
 is made to verify that the improved sig- 
 nal reception is obtained. 
 
 c. Resonating the Trolley Wire-Rail 
 Inductance 
 
 A method that can be applied if the 
 rectifier setback is short, and it would 
 be impractical to install a fixed induc- 
 tor in series with the rectifier feed 
 wires, is to tune the trolley wire-rail 
 
 1000 V 
 
 DECADE 
 
 CAPACITOR 
 
 WARNING 
 
 
 CAPACITOR 
 
 5 mtd, 1000 VOLTS, 
 
 IS ESSENTIAL WITHOUT 
 
 IT TROLLEY VOLTAGE IS 
 
 APPLIED TO THE 
 
 VOLTMETER 
 
 TROLLEY WIRE 
 
 OR GROUND 
 
 
 
 r 
 
 r 
 
 c 
 
 \ 
 
 f-: 
 
 '^g'^ ' 
 
 B 
 
 "3 
 3^" 
 
 1 
 
 
 
 E 1 
 
 e PIECES A, THRU Ag 
 4 PIECESe@3' -0" 
 4 PIECESC@3' - II" 
 2 PIECES D®4' -2" 
 4 PIECES E®4'-5" 
 
 J ' ' V '. . t 
 
 WARNING 
 
 Some oflliese procedures arc uiidcrtaketi with ihe trolley 
 wire energized; therefore, they are extremely hazardous, 
 fc-xtreme caution must be exercised to avoid ])otential!y 
 lethal shock. The fuses used in the test leads serve only 
 to jirotect eqiiipinent and do not in any way reduce the 
 shock, hazard to personnel. Only personnel tliorouj^hly 
 familiar witli electrical work on IroUev wires should con- 
 duct these |)rocediires. The permanent connection ot 
 components should be <lone with power removed. Care 
 should also be taken to insme tlial components and equi])- 
 meiit are suitable for use in the desired application. 
 
 FIGURE 5-11.- Coil form. 
 
 FIGURE 5-12. - Test setup for tuning fixed 
 inductance. 
 
136 
 
 CAPACITOR 
 MOUNTED IN 
 FUSE HOLDER 
 IN UTILITY BOX 
 
 a mine map. Remove the test fixture, and 
 install permanently a suitable capacitor 
 of the indicated value. Request another 
 transmission to verify that the signal 
 improvement observed during the test was 
 maintained after permanent installation. 
 
 2. Repeat the same procedure as in 
 step 1, but at a place in the opposite 
 direction along the trolley wire-rail; 
 that is, 80 feet or so on the other side 
 of the feed point. Keep records as 
 before. 
 
 3. Return to the first point and 
 measure and record the signal level for a 
 dispatcher's transmission. 
 
 5.3.1a.ii Heaters 
 
 WARNING 
 
 Some oflliesL' procedures are undertaken with ihe trolley 
 wire energized; therefore, they are extremely hazardous, 
 txtreme caution must l)e exercised to avoid potentially 
 lethal shock. The fuses used in the test leads serve only 
 to ])rotect equipment and do not in any way reduce the 
 shock hazard lo personnel. Only personnel thorouslily 
 familiar with electrical work on troUev wires should con- 
 duct these jMocedures. The permanent coinieclion of 
 components should be done with power removed. Care 
 should also be taken to insure that components and equip- 
 ment are suitable for use in the desired application. 
 
 FIGURE 5=13o - Permanent attachment of tun= 
 ing capacitor to fixed inductor. 
 
 as shown in figure 5-8C 
 described below: 
 
 The steps are 
 
 1. Locate a position about 80 feet 
 along the trolley wire-rail from the 
 place where the rectifier feed wires 
 attach to the trolley wire-rail. Install 
 the test set as illustrated in figure 5-9 
 between the trolley wire and rail. 
 Request the dispatcher to transmit for 
 about 20 seconds. Switch the capacitor 
 box through its values to find a maximum 
 signal level, as indicated on the tuned 
 voltmeter. (The decade box should have 
 enough range to peak, the voltage level. ) 
 Note the value with the decade capacitor 
 in the "off" position and the value to 
 maximum signal, and list these values on 
 
 Personnel heaters with a wide range 
 of wattage ratings are used; however, 
 1,000 watts is likely to be the lowest, 
 and each heater of this rating or higher 
 poses a significant signal loss to the 
 carrier system. Such heaters will range 
 in resistance from 360 ohms for a 1-kW 
 unit on a 600-volt line to 18 ohms for a 
 5-kW unit on a 300-volt line. Current 
 will range from 1.5 to 17 amperes for 
 corresponding conditions. Unlike recti- 
 fier currents, these heater currents are 
 sufficiently low that commercial induc- 
 tors can be used untuned to provide 
 isolation of heaters, thereby avoiding 
 the step of individually tuning each 
 isolator. 
 
 To raise the impedance level to 
 300 ohms at 100 kHz using an untuned 
 inductance requires an inductor of 
 500 pH. While it would be convenient to 
 find a single inductor usable for all 
 such loads, the wide range of direct 
 currents that must be handled (1.5 to 
 17 amperes) makes it necessary to select 
 each inductor on an individual basis. 
 
 The procedure for treating personnel 
 heaters follows: Locate the heater ele- 
 ment. Measure the carrier frequency 
 voltage at this load, using the tuned 
 voltmeter and a dispatcher's transmis- 
 sion. Note this value on a mine map. 
 Disconnect the heater and permanently 
 
137 
 
 attach a 500-yH inductor in series with 
 the element. Reconnect the heater and 
 measure the voltage produced across the 
 heater and inductor in series, using the 
 tuned voltmeter and a dispatcher's trans- 
 mission. An improvement in voltage of up 
 to 10 to 1 can be expected. Note the new 
 received voltage on the mine map. Repeat 
 this procedure for each personnel 
 heater. 
 
 5.3.1a.iii Vehicle Lights 
 
 Mine vehicles, including locomo- 
 tives, jeeps, and portal buses, all draw 
 substantial power from the trolley wire. 
 Much of this power is used for motive 
 purposes. Motors represent a relatively 
 high impedance at trolley carrier fre- 
 quencies, particularly for jeeps and por- 
 tal buses and to a lesser extent for 
 locomotives. However, a part of the 
 power is used for headlights on the 
 vehicles. Most conventional vehicles use 
 150-watt, 32-volt, PAR-type lights for 
 this purpose. The difference between 
 32 volts and the trolley voltage is taken 
 up with a ballast resistor. A single 
 light circuit of this type presents a 
 resistance of about 50 ohms on a 300-volt 
 circuit and about 110 ohms on a 600-volt 
 circuit. Because some vehicles use two 
 lights at a time, and some only one, the 
 bridging loads represented by the vehicle 
 lights range from 110 to 25 ohms per 
 vehicle. These values are sufficiently 
 low that treatment is desirable. 
 
 The procedure for treating vehicle lights 
 follows: Insert a 10-ampere, SOO-yH 
 inductor in series with the light circuit 
 of each vehicle. Make sure that the 
 inductor is only in series with the light 
 circuit and is not in series with the 
 motor or trolley phone circuits. Because 
 of the variable conditions faced by the 
 vehicles, it is not of much utility to 
 check the before and after carrier fre- 
 quency voltages found on vehicles, but 
 the tuned voltmeter could be used for 
 this purpose if so desired. 
 
 5.3.1a.iv Other Loads 
 
 Other loads can also adversely 
 affect propagation on the trolley wire- 
 rail; for example, signal and illumina- 
 tion lights. As noted earlier, an indi- 
 vidual light bulb, or a string of such 
 lights, does not impose much insertion 
 loss. However, if there are many lights, 
 the total effect could be substantial. A 
 way to estimate whether such lights 
 affect propagation significantly is to 
 count the number of lights on the trolley 
 wire-rail between the dispatcher and the 
 farthest place in the mine, and calculate 
 the total bridging resistance. Approxi- 
 mate value of loss versus bridging load 
 can be estimated from figure 5-7. If the 
 toal loss is less than 6 dB, only margin- 
 al improvements will result from treating 
 these lights. If the loss is more than 
 6 dB, consideration should be given to 
 treating the lights. It would be a rath- 
 er unusual situation to find lights that 
 really represented a significant impedi- 
 ment to propagation of a trolley wire- 
 rail. However, when marginal signal lev- 
 els exist, the lights could well make the 
 difference between marginal and fully 
 usable signal levels. 
 
 The most effective way to treat such 
 lights would be to take them off the line 
 and operate them from the ac system. 
 This practice is being used in some of 
 the newer mines. In old mines, where ac 
 power is not available, little can be 
 done. In some instances, the power rat- 
 ing of the lights could be reduced, 
 thereby raising the value of the bridging 
 impedance. Fixed inductors could also be 
 used but would only have small effects 
 because the light strings (typically 
 three 100-watt, 115-volt lights in series 
 in a 300-volt system) already have a 
 fairly high resistance (approximately 300 
 ohms for the example above). 
 
 Other loads are comprised of such 
 equipment as pumps and other motor- 
 driven devices. However, these devices 
 
138 
 
 generally have high enough Impedances and 
 are placed so infrequently that they re- 
 sult in minimal loading effects. 
 
 5.3.1b Using a Dedicated Wire 
 
 As previously mentioned, the trolley 
 wire-rail is an inefficient transmission 
 path because of the many loads that exist 
 on the line. In the dedicated-wire tech- 
 nique, an independent wire (called the 
 "dedicated wire") is run down the entry- 
 way with the trolley line on the wide 
 side, but not connected to the trolley 
 line in any manner. 
 
 Such a wire, since it is unloaded, 
 has a very low attenuation rate. There- 
 fore, if a signal is transmitted on the 
 dedicated wire, the signal strength re- 
 mains high. Since the trolley line and 
 dedicated wire are located in the same 
 entry, there is a mutual electromagnetic 
 coupling between them. (The effects of 
 loads on the trolley line are transferred 
 to the dedicated wire, and the high sig- 
 nal on the dedicated wire is transferred 
 to the trolley line.) Fortunately, if 
 the separation between the two is large 
 enough (9 feet or more), the loading ef- 
 fects of the trolley line are only weakly 
 transferred to the dedicated wire, so 
 that the attenuation rate stays low. But 
 at the same time, the high signal levels 
 on the dedicated wire are strongly cou- 
 pled to the trolley line. The net result 
 is that communication is now possible in 
 areas where it was not possible before. 
 The procedure for developing a system 
 based on a dedicated wire is divided into 
 the following three steps: 
 
 1. Routing . — Ascertain from a mine 
 map the area of coverage desired, con- 
 sidering that the dispatcher position is 
 the key position. Mark out on this map a 
 route for a single line that runs in the 
 same entryway as the trolley wire-rail to 
 which communication is desired. Avoid 
 branches on this route. If necessary, 
 use a second or third such route to cover 
 all regions of the mine. Short side- 
 tracks need not be covered initially. If 
 a branch on the route will cover the 
 desired region with less length of wire. 
 
 use a branch, but minimize the number of 
 branches. 
 
 2. Installation . — Install the wire; 
 No. 10 or No. 12 wire is well suited to 
 the task. Copper-weld construction is 
 recommended for strength and integrity. 
 This wire must be insulated and also held 
 away from the rib or roof for at least 
 3 inches. Installation must be on the 
 wide side of the entry, and the wire 
 should be located for least exposure to 
 damage. At the far ends of each line, 
 the wire is terminated by a 200-ohm, 
 10-watt resistor to the rail, as illus- 
 trated in figure 5-14. If branches are 
 used, signal-splitting resistors must be 
 included (fig. 5-15) to reduce signal 
 attenuation. 
 
 3. Connection of Transmitter. — Upon 
 completion of the installation of the 
 
 
 INSULATORS 
 
 FIGURE 5-14, - Termination of the dedicated wire. 
 
139 
 
 -UTILITV BOX 
 
 FUSE HOLDERS (31 
 
 DEDICATED WIRE 
 
 GATED WIRE 
 
 FIGURE 5-15. - Signal splitter. 
 
 special-purpose wire, the dispatcher's 
 transmitter should be directly connected 
 to the end or ends of the wire that con- 
 verge on the dispatcher's station. The 
 return wire of the transmitter should go 
 to earth or to the rail. 
 
 As noted before, the use of branches 
 should be minimized. When the routes are 
 short, (considerably less than 10 miles), 
 resort can be made to branches on a dedi- 
 cated wire. When more than one wire is 
 used, they should be run in separate en- 
 tryways. The reason that branches are 
 undesirable is that a branch reduces the 
 signal level by 2 to 1 (6 dB). On short 
 runs, such a loss can be tolerated, but 
 on runs approaching 10 miles, such a loss 
 may be too high. 
 
 5.3.1c Using a Remote Transceiver 
 
 Frequently the dispatcher is located 
 at one edge of the mine complex. If this 
 is the case, the communication range 
 required must be extensive in order that 
 the dispatcher be able to reach motormen 
 on the opposite side of the mine. In 
 some Instances, a convenient way of solv- 
 ing the dispatcher's problem is to use a 
 remote transceiver located at the most 
 favorable place for reaching all parts of 
 a mine complex. The location of such a 
 remote transceiver is likely to be near 
 the center of the rail haulage system of 
 
 the mine, although in certain circum- 
 stances moving it somewhat away from 
 such a center might produce more favor- 
 able results. 
 
 As an example of what might be 
 achieved by this means, consider a dis- 
 patcher's position for which the signal 
 attenuation is 80 dB from his position to 
 the farthest reach of the mine. This at- 
 tenuation means that an initial 25-volt 
 rms signal provided by the dispatcher's 
 transmitter would be reduced to 2.5 mV at 
 the farthest reach of the mine. This 
 level of signal is marginal, and thus the 
 dispatcher would have poor communication 
 to those motors on the far side of the 
 mine. If the dispatcher's transceiver 
 were moved to the center of such a mine, 
 the signal attenuation should drop to 
 one-half, or 40 dB, from this central po- 
 sition to the extremities of the mine. 
 The 40 dB of attenuation would provide 
 signal levels of 250 mV at the extremes 
 of the mine, 100 times bigger than would 
 result if the dispatcher's transmitter 
 were located at one edge of the mine. 
 
 Such a substantial improvement in 
 signal levels throughout the mine would 
 change an otherwise marginal operation 
 into a completely adequate communication 
 system. Insofar as the ispatcher is 
 concerned, his operation would remain the 
 same. He would still have the carrier 
 phone speaker and microphone located at 
 his dispatching position; however, the 
 control and audio signals would be trans- 
 mitted from his position through a 
 twisted shielded pair to the remote 
 transceiver (fig. 5-16). Thus, it would 
 be necessary to run an audio cable for 
 whatever distance was necessary to reach 
 the center of the mine. In mines where 
 multipair telephone cable is used, a pair 
 may be available for this purpose. If 
 not, the expense and inconvenience of in- 
 stalling such a cable would be justified 
 to assure adequate coverage for the dis- 
 patcher's communication system. 
 
140 
 
 IF THE TROLLEY IS SECTI0NALI2ED, COUPLING 
 CAPACITORS (5 uf. 1000 VOLTS) MUST BE USED 
 TO PROVIDE A PATH FOR THE CARRIER 
 COMMUNICATIONS. 
 
 di 
 
 DISPATCHER ' 
 
 TRANSCEIVER 
 
 WARNING 
 
 Some ofLliese procedures arc underiakeii with llie trolley 
 wire crieri;izcd; therefore, they are extremely hazardous, 
 t'.xtreme caution must be exercised to avoid ]>olentially 
 lethal shock. ITie fuses used in the lest leads serve only 
 to |>rotect equipment and do not in any wav reduce ihc 
 shock hazard lo personnel. Only fievsonnel thovouj;lily 
 familiar widi elccirical work on irollev wires sliould con- 
 duct these inoccdures. The permaneni coinieclion ol 
 components shoidd be done with power renu)ved. Care 
 should also be laken to instuc thai com] ion en Is and equip- 
 ment aie suiiable for use in ihe desired application. 
 
 FIGURE 5-16. - Dispatcher's remote transceivero 
 
 5. 3. Id Summary 
 
 A substantial number of the problems 
 associated with maintaining good trolley 
 communication systems can be avoided by 
 advanced planning. For those planning a 
 communication system for a new mine, the 
 following suggestions are offered to as- 
 sure optimum operation of the trolley 
 carrier phone system when installed: 
 
 1. If the trolley wire is section- 
 alized, make sure capacitors (5 \i¥ , 
 1,000 volts) (some systems may require 
 even higher voltage components) tie the 
 sections together. 
 
 2. Plan to operate as many auxil- 
 iary loads as is practicable on mine ac 
 power rather than from the trolley wire 
 power. 
 
 5. Insist that vehicle manufactur- 
 ers indicate the 88- to 100-k.Hz operating 
 impedance of their vehicles, and select 
 vehicles that show high operating imped- 
 ance at the carrier frequency. 
 
 6. If possible, use at least a 50- 
 foot setback for rectifiers that are to 
 be installed in the mine; this setback 
 will permit tuning of the rectifier leads 
 to raise the impedance of the rectifier. 
 
 7. Ask the rectifier manufacturers 
 to supply internal filters in series with 
 the voltage to raise the carrier fre- 
 quency impedance to a high level. 
 
 8. Plan and design isolators for 
 all other appreciable bridging loads 
 across the trolley wire rail. 
 
 5.3.2 Improving Telephone Systems 
 
 As mentioned earlier, hardwired 
 phone systems fall into three major cate- 
 gories: single pair (party-line), multi- 
 pair, and multiplex phone systems. A ma- 
 jor disadvantage of single-pair systems 
 is that each telephone must be used in a 
 party-line arrangement. This prevents 
 simultaneous conversations in the system 
 and reduces its usefulness for discussing 
 maintenance problems or other uses that 
 can tie up the system for long periods of 
 time. Multipair and multiplex systems 
 provide for many simultaneous conversa- 
 tions but until recently did not possess 
 the paging ability. 
 
 All three of these systems can usu- 
 ally be improved if the basic reasons for 
 poor performance or high noise levels are 
 understood. For instance: 
 
 Heavier gage wire presents less at- 
 tenuation to the signal and results in 
 better coverage over greater distance. 
 
 3. Consider the use of a dedicated 
 wire to aid signal propagation. 
 
 Splicing technique has a large ef- 
 fect on signal strength. 
 
 4. Select carrier 
 ers that show a high 
 impedance. 
 
 phone transceiv- 
 value of standby 
 
 Twisted pair cable can reduce noise 
 pickup. 
 
141 
 
 Even when proper precautions have 
 been taken, all hardwired systems are in- 
 herently unreliable. For example, if a 
 telephone line is broken or shorted by a 
 roof fall, all telephones beyond that 
 point are severed from communication to 
 the outside. If the line is shorted, 
 communications in the entire system may 
 be severely affected or lost completely. 
 These deficiencies can be corrected by 
 the following methods: 
 
 Adding loopback to the phone line. 
 
 Sectionalizing the phone system. 
 
 5.3.2a Loopback Methods 
 
 A major disadvantage of any wired 
 phone system is its dependence upon a 
 continuous phone line running throughout 
 the mine. If this phone line is broken, 
 communication with all phones inby the 
 break is lost. Alternate communication 
 paths, or loopbacks, can be established 
 as shown in figure 5-17 to overcome this 
 deficiency. If a line break should oc- 
 cur, the loopback switch can be closed, 
 allowing each and every phone to still 
 communicate with all other phones in the 
 system. 
 
 Another way to implement loopback is 
 to return the phone line to the main 
 shaft using a different underground path. 
 No matter which method of loopback is 
 used, the operation of the systems is 
 similar. During normal operation the 
 
 DISPATCHERS LOCATION 
 
 77777^ 
 
 FIGURE 5.17. - Phoneline loopback. 
 
 loopback switch is left in the open posi- 
 tion. If a line break should occur any- 
 where in the underground phone line, the 
 loopback switch can be closed and each 
 phone will still be able to communicate 
 with other phones. Depending upon the 
 physical layout of the mine, forming an 
 underground loop may actually require 
 less wire than if a single line is strung 
 with many branches running to the indi- 
 vidual phones. It is imperative that the 
 loopback switch be always left open under 
 normal conditions to avoid "masking" line 
 breaks. 
 
 Another method of establishing loop- 
 back is by using an overland radio link. 
 In this type of system the mine telephone 
 signals are returned from the end of the 
 line to the surface through a ventilation 
 shaft or borehole. At the surface a two- 
 way radio base station establishes an 
 overland radio link to a second station 
 near the dispatcher or general mine fore- 
 man's office. Note that provisions must 
 be made for dc paging. 
 
 Each of the loopback systems de- 
 scribed above utilized a loopback switch 
 that during normal operation (no line 
 breaks) is left in the "open" position. 
 This loopback switch serves an important 
 function in any loopback system. For in- 
 stance, consider what would happen in 
 a loopbacked system with no loopback 
 switch, or if the switch is normally left 
 closed. No communication outages would 
 be experienced when the first line break 
 occurred because each phone would still 
 be connected, through one or the other 
 legs of the loop, to the system. The 
 problem is that unless someone under- 
 ground noticed the broken phone line, 
 everyone would assume that the system 
 was completely intact because no com- 
 munication difficulties were being exper- 
 ienced. The system could operate in this 
 mode for a long period of time. How- 
 ever, when a second line break occurred 
 communications to and from all phones be- 
 tween the two breaks would be lost. Note 
 also that each time the dispatcher talks 
 he hears himself on the loopback phone. 
 This feature alone assures that the phone 
 line is intact. 
 
142 
 
 5.3.2b Sectionalizing the Underground 
 Network 
 
 The desirability of selective area 
 paging and simultaneous conversation 
 capability along with the maximum possi- 
 ble use of two-wire transmission line 
 makes the use of a zoning or sectional- 
 ization of the mine telephone system at- 
 tractive. In this method, each zone or 
 section in the underground complex is 
 served by its own cable pair. 
 
 To see how the telephone sections 
 would be interconnected, consider the 
 simplified four-section system shown in 
 figure 5-18. Within each area, the pag- 
 ing telephones would operate normally. 
 That is , all phones in each 
 operate on a party-line basis, 
 tact with a phone outside the local area 
 is desired, connection to the area being 
 called would be made at an outside cen- 
 tral exchange. This type of system could 
 also be made more reliable by having two 
 different signal paths (loopbacks) avail- 
 able between each area and the central 
 exchange. 
 
 area would 
 When con- 
 
 yyr^T^ 
 
 FIGURE 5-18. - Sectional ization of a phone system. 
 
 5.3.3 Summary 
 
 Advanced planning is essential to 
 the successful design and installation 
 of any communication system. The design 
 plan should take into consideration 
 changes in system requirements to meet 
 communication demands throughout the en- 
 tire life of the mine. 
 
 Single-pair, multipair, and multi- 
 plex systems are the basic choices avail- 
 able once it has been determined that 
 a hardwired system will best meet the 
 communication requirements. A consider- 
 able percentage of the expense involved 
 in each of these systems is due to the 
 distribution (cable) network, and advance 
 planning is especially critical in this 
 area. Wire lines to meet telemetry re- 
 quirements for remote control and mon- 
 itoring of equipment and atmospheric 
 conditions should also be recognized. 
 Note that MSHA regulations may prohib- 
 it running two systems in a single 
 cable. 
 
 Methods also exist that allow im- 
 provement of systems already installed. 
 The performance of trolley carrier sys- 
 tems can be improved by removing or iso- 
 lating bridging loads on the trolley wire 
 that cause signal attenuation. Dedicated 
 lines or remote transceivers can also be 
 used to improve the quality of these 
 systems . 
 
 General maintenance and splicing 
 technique can have a large effect on the 
 quality of voice service over wire phone 
 systems. These systems can also be made 
 more reliable by providing loopback 
 paths so that each phone will remain con- 
 nected to the system in case of a line 
 break. 
 
BIBLIOGRAPHY 
 
 143 
 
 1. Aldridge, M. D. Analysis of Com- 
 munication Systems in Coal Mines. Bu- 
 Mines OFR 72-73, June 1973, 211 pp.; 
 available from NTIS PB 225 862. 
 
 2. Lagace, R. L. , W. G. Bender, J. D. 
 Foulk.es, and P. F. O'Brien. Technical 
 Services for Mine Communications Re- 
 search. Applicability of Available Mul- 
 tiplex Carrier Equipment for Mine Tele- 
 phone Systems. BuMines OFR 20(1 )-76, 
 July 1975, 95 pp.; available from NTIS PB 
 249 829. 
 
 3. Parkinson, H. E. Mine Pager to 
 Public Telephone Interconnect System. 
 BuMines RI 7976, 1974, 14 pp. 
 
 D. 
 
 4. Spencer, R. H. , P. O'Brien, and 
 Jeffreys. Guidelines for Trolley Car- 
 
 rier Phone Systems. 
 March 1977, 170 pp. 
 NTIS PB 273 479. 
 
 BuMines OFR 150-77, 
 available from 
 
144 
 
 CHAPTER 6. —INSTALLATION TECHNIQUES 
 
 6.1 The Basic Philosophy 
 
 The investment involved in any com- 
 munication system represents a consider- 
 able sum. Even though it is desirable 
 that the system work properly each and 
 every time it is called into use, some 
 failures are bound to occur. Most fail- 
 ures, however, and especially those that 
 occur most frequently, are due to poor 
 installation techniques. An extra hour 
 spent at an installation site can save 
 many maintenance trips and many frustrat- 
 ing hours of system troubleshooting. 
 
 Typical faults likely to cause com- 
 munication outrage are 
 
 Pager phone systems 
 
 Poor splices aggravated by corrosion. 
 
 Strain relief not provided. 
 
 Drip loop not provided. 
 
 Incorrect branch connections. 
 
 Overloading the circuit. 
 
 Poor battery connections. 
 
 Improper wire size or type. 
 
 Lightning strikes. 
 
 Improper placement of wire runs. 
 
 Carrier phones 
 
 Mounting transceiver near load resis- 
 tors or other sources of heat. 
 
 Tracks not electrically bonded. 
 
 Cable abrasion due to poor mounting 
 location. 
 
 Disconnected battery. 
 
 Poor mechanical installation. 
 
 Each installation should be well 
 planned. After an installation is 
 completed, the technician should ask 
 the question, "What can go wrong with 
 this unit or line?" Remember the adage, 
 "Whatever can go wrong, will." Preven- 
 tive measures taken during installation 
 will pay off in the long run. 
 
 6.2 Pager Phone Installation 
 
 The pager phones used in many under- 
 ground coal mines are battery-operated, 
 party-line telephones with provisions for 
 loudspeaker paging. The system is usual- 
 ly two-wire, nonpolarized, and operated 
 by self-contained batteries. Many of the 
 individual units are certified as 
 permissible. 
 
 6.2.1 Mounting 
 
 Pager phones are designed to be 
 mounted on an upright support at the 
 desired location. For convenience, the 
 phone should be mounted 5 feet above the 
 floor where there is no obstruction to 
 using the handset or removing the cabinet 
 front cover for servicing or battery re- 
 placement. In low-coal situations, a 
 suitable height for installation should 
 be selected convenient to the normal 
 operator's position at the site selected. 
 About 12 Inches of free space on each 
 side of the phone should be provided for 
 cabinet access. The phone should be pro- 
 tected against direct exposure to drip- 
 ping water and should not be allowed 
 to rest in a puddle of water. The mount- 
 ing location should be convenient to 
 a work location and have a safe, unob- 
 structed area for a worker to stand and 
 use the phone. The phone must be in a 
 location where the worker will not be in 
 the path of moving vehicles or falling 
 debris. Each telephone is normally well 
 insulated, but it is still good practice 
 to provide an insulating mat or dry 
 planking for the user to stand on. 
 
145 
 
 6.2.2 Connections 
 
 For handling convenience, the branch 
 line or connecting cable to each individ- 
 ual telephone can be a lighter wire gage 
 than the main cable. Each connection to 
 the main line should be a good electrical 
 and mecTianical joint, protected by a 
 careful double wrap of plastic electrical 
 tape. 
 
 Special care should be taken to in- 
 sure that each splice is a good electri- 
 cal and mechanical connection. Connec- 
 tions that are of poor or marginal 
 quality, or that are not adequately pro- 
 tected from moisture, will contribute to 
 poor performance. During periods when 
 humidity levels are high, especially dur- 
 ing the summer months, corrosion will 
 form on all exposed splices. As this 
 corrosion builds, audio levels decrease 
 and line noise increases until eventually 
 the entire system becomes useless. 
 
 Connections at the phone depend on 
 each manufacturer's design and on indi- 
 vidual state or local requirements. A 
 majority of the phones provide two ex- 
 posed spring-loaded terminals for attach- 
 ing the wires. For proper connection, it 
 is necessary to strip the installation 
 away from each conductor in the pair, 
 seal off the exposed area of the cable 
 with plastic electrical tape to keep out 
 moisture, and then insert one of the ex- 
 posed conductors into each of the cabinet 
 terminals. Some states, such as Pennsyl- 
 vania, do not allow the use of exposed 
 terminals at the face area of gassy 
 mines. For these applications, some 
 phones are equipped with twist-lock con- 
 nectors at the end of a short cable. 
 Each connector is mated with a similar 
 connector on the drop or branch line to 
 complete the installation. In either 
 type of installation, there should be a 
 drip loop below the cabinet to prevent 
 condensate from running down the cable 
 into the cabinet. 
 
 6.2.3 Batteries 
 
 Pager phones are usually operated by 
 one (or two) 12-volt, dry-cell batteries. 
 
 NEDA No. 923 or No. 926 (National Elec- 
 tronic Distributors Association) . To in- 
 stall batteries, it is necessary to open 
 the pager phone cabinet and inspect the 
 battery compartment. Remove the old bat- 
 tery by loosening the retaining clamp, 
 and either unscrew the battery terminals 
 to release the battery wires or remove 
 the battery plug, depending on the bat- 
 tery type. Remove the battery, and care- 
 fully wipe out the battery compartment to 
 remove dirt and moisture. Place a fresh 
 battery in the compartment, and secure 
 the retaining clamps tight enough to re- 
 strain the battery without crushing or 
 bending the battery case. Reconnect the 
 battery wires , being careful to observe 
 the polarity markings noted on the case. 
 If the plug-connector type is used, do 
 not force the connector. Correct polar- 
 ity is maintained when the larger connec- 
 tor pin fits in the larger hole. The 
 difference in pin sizes is not great, so 
 a mismatch can be forced. If the connec- 
 tor does not mate easily, reverse it and 
 try again without forcing. After replac- 
 ing the battery, close the cabinet and 
 mark the date of battery replacement 
 either on the outside of the cabinet or 
 in a log book. 
 
 CAUTION 
 
 Pager phone circuits are normal- 
 ly designed to provide sufficient 
 current limiting with the specified 
 battery. If other battery types are 
 used, such as the nickel-cadmium re- 
 chargeable type or one of the alka- 
 line, long-life, high-current vari- 
 eties, the circuit may not be able to 
 limit the available current to a safe 
 value. REPLACE WITH RECOMMENDED 
 BATTERY ONLY. 
 
 Battery life is not easy to predict, 
 because of the many operating variables 
 that affect the average current drain. 
 In general, the batteries in a telephone 
 system that is used many times a day may 
 have to be replaced every 4 to 6 weeks , 
 while a telephone system that is seldom 
 used may keep its batteries at usable 
 strength for 4 to 6 months. 
 
1A6 
 
 Each battery change should be re- 
 corded, either on the telephone cabinet 
 or in a central log. Experience gained 
 over a period of time will help predict 
 when a battery in a particular phone is 
 reaching the end of its useful life. 
 Periodic verification of battery status 
 at each phone should be made with a volt- 
 meter and recorded in the log. (Measure 
 battery voltage while under load; that 
 is, during paging.) When the battery 
 voltage drops to a value that is 75% to 
 80% of the installed level, it should be 
 replaced. For example, for a 12-volt 
 battery, the replacement level is about 
 8 to 9 volts. 
 
 6.2.4 Fuses 
 
 6.3 Phone Lines and Transmission Cables 
 
 6.3.1 Phone Lines 
 
 The cable used to interconnect un- 
 derground pager phones must be rugged 
 enough to withstand the underground envi- 
 ronment and also have the proper elec- 
 trical characteristics for requirements 
 of the pager system. Generally, the 
 cable used for this purpose is a twisted 
 pair of solid-conductor wires that has a 
 nonwater-absorbing, flame-retardant insu- 
 lation with a rating of 600 volts dc and 
 an outer abrasion-resistant covering. 
 The conductor used depends on the instal- 
 lation; recommended sizes are 19 AWG to 
 14 AWG. 
 
 Fuses are provided in pager tele- 
 phones as an added precaution against ex- 
 cessive current in the external circuit. 
 Current-limiting circuitry is normally 
 provided in the telephone, but the fuse 
 is an additional safeguard. No provision 
 is made in most phones to store a spare 
 fuse. It is good practice to tape two 
 additional spare fuses to the inside of 
 the cabinet when the phone is first in- 
 stalled. Then, the correct fuse will be 
 available at the phone if it is needed. 
 Make sure the fuses do not and cannot 
 short circuit any circuitry. 
 
 6.2.5 Amplifier Loudness 
 
 Each pager phone has a loudspeaker, 
 powered by its own internal amplifier, 
 that is switched on by the dc paging sig- 
 nal. The available audio power is about 
 5 watts, which is adequate to be heard 
 above most mine noises. Many telephones 
 have a volume control for the speaker. 
 During installation, the speaker should 
 be oriented, and the volume set, to in- 
 sure adequate coverage in the area. 
 
 During setup, someone should page 
 from another location to the phone being 
 installed. The volume control should be 
 set to the desired level during the pag- 
 ing. The telephone cabinet should be 
 positioned to direct maximum sound to 
 the work area. 
 
 Many telephones used underground, 
 particularly those used at the working 
 face, are subject to periodic relocation. 
 To allow for this , and to reduce the 
 problems associated with repeated cable 
 splicing, some convenient length of wire 
 (say, 500 feet) can be included as part 
 of the branch line. This extra wire can 
 be kept reeled, or neatly coiled in a 
 bundle and secured with a few wraps of 
 plastic electrical tape. The extra wire 
 should be hung near the telephone in a 
 place free of dripping water or water ac- 
 cumulation, and should be supported by an 
 insulated hanger that is isolated from 
 power or trolley wires. The practice of 
 coiling the cable is recommended, but 
 with certain restrictions. If the cable 
 is used to transmit monitor signals via 
 an RF (radio frequency) carrier imposed 
 on the two-wire pager phone line, the 
 coiled cable becomes an Inductor that 
 will impede the proper transmission of 
 the RF signal.'' 
 
 Methods and recommended techniques 
 for the permanent installation of phone 
 lines are presented in a Bureau of Mines 
 
 ^ The addition of equipment to a phone 
 system could violate intrinsic safety 
 standards; check with MS HA for detailed 
 application information. 
 
147 
 
 handbook (2^) ,2 including installation, 
 lightning protection, cable selection, 
 and splicing methods. Note that 30 CFR 
 specifies certain requirements for cable 
 installation. 
 
 6.3.2 Leaky Feeder Cable 
 
 Installation of leaky feeder cable 
 requires some special techniques. A typ- 
 ical installation of leaky feeder cable 
 is shown in figure 6-1. For installation 
 of the repeaters, refer to the manufac- 
 turer's installation guide. Hanging the 
 leaky feeder cable requires clamps such 
 as those used for conduits or other power 
 cables. In areas where corrosion may be 
 a problem, stainless steel or plastic 
 clamps should be used. Typical hangers 
 are shown in figure 6-2. The type of in- 
 sulated hanger shown supports the leaky 
 feeder cable from the messenger cable. 
 Leaky feeder cable should be supported at 
 intervals of 5 feet and is usually termi- 
 nated with an antenna. 
 
 6.4 Carrier Phone Installation 
 
 The primary function of the carrier 
 phone system is to provide a reliable 
 communication network over which the dis- 
 patcher can direct all tracked vehicle 
 
 ^Underlined numbers in parentheses re- 
 fer to items in the bibliography at the 
 end of this chapter. 
 
 
 REPEATEH 
 
 ^SPUTTER 
 
 
 flEPEATEH 
 
 
 ,...., 
 
 
 
 ^ 
 
 X 
 
 
 _^J^ 
 
 
 
 
 REPEATER 
 
 SPACING 
 
 
 
 
 
 AfiOOF 
 
 EET 
 
 
 
 traffic in the mine. The safety and pro- 
 ductivity of the mine depend, to a large 
 measure, on the ability of the dispatcher 
 to maintain direct contact with all mo- 
 tormen via the carrier phone system. For 
 this reason, the carrier phone installa- 
 tion should be carefully thought out, and 
 the workmanship should be of the highest 
 caliber. 
 
 CAUTION 
 
 Installation procedures in this 
 section are guidelines and not com- 
 prehensive technical instructions. 
 Procedures described in this section 
 must be performed by people thorough- 
 ly qualified to do such work. In- 
 stallations should comply with manu- 
 facturer's recommendations, good 
 safety procedures, and all applicable 
 codes and regulations. 
 
 FIGURE 6-1. - Typical installation. 
 
 6.4.1 The Dispatcher Location 
 
 The trend in modern coal mining is 
 to locate the dispatcher aboveground in a 
 separate building or a separate room in 
 the mine office complex. This location 
 provides a continuously manned communica- 
 tions center even if the mine must be 
 evacuated owing to emergencies or venti- 
 lation failures. Since 1974 the mining 
 laws of West Virginia have required that 
 the dispatcher be located on the surface 
 in all new mines and for existing mines 
 if the dispatcher is relocated (Article 
 22-2-37, Part T5). 
 
 Underground dispatchers' locations 
 vary greatly , depending on the mine lay- 
 out and growth. The two most common lo- 
 cations chosen are at the bottom of the 
 main shaft or near the physical center of 
 the mine. 
 
 INSULATED MESSENGER 
 CABLE HANGER 
 
 METAL HANGER 
 
 FIGURE 6=2. = Hanger hardware 
 
 The carrier phone equipment is usu- 
 ally installed on a panel which is 
 mounted on a wall adjacent to the dis- 
 patchers' desk. This panel provides one 
 convenient location for all the subassem- 
 blies that make up a carrier phone and 
 protects the interconnecting cables 
 from unnecessary flexing and stretching. 
 The panel should be made from at least 
 
148 
 
 1/8-inch-thick steel plate if the carrier 
 phone uses a power-conditioning unit or 
 resistor box that contains series- 
 dropping resistors. 
 
 CAUTION 
 
 Remove the electronic subassem- 
 blies from the mounting plate during 
 welding operations. Keep all elec- 
 trical cables and other nonmetallic 
 materials away from the welding area. 
 This will prevent the carrier phone 
 components from being damaged by heat 
 during welding operations. 
 
 The microphone-speaker assembly is 
 the only part of the carrier phone that 
 interfaces directly with the dispatcher; 
 therefore, it must be located within easy 
 reach. The speaker volume control should 
 also be within easy reach. 
 
 Locate the transceiver on the panel 
 at either side of the speaker assembly, 
 taking into consideration the location of 
 the interconnecting cables. Leave room 
 for the excess cable to be coiled up and 
 secured to the panel. 
 
 Temperature-sensitive electronic 
 circuits are located inside the trans- 
 ceiver assembly. Therefore, it should be 
 protected from the temperature extremes 
 produced by load resistor banks and room 
 heaters. For reliable operation, the am- 
 bient operating temperature range that 
 the transceiver is exposed to should be 
 restricted to -40° to +140° F. 
 
 transceiver electronics and to recharge 
 the battery. The circuit generally used 
 in this unit contains a large series- 
 dropping resistor that under normal oper- 
 ating conditions dissipates several 
 hundred watts. The high temperature as- 
 sociated with this power dissipation 
 would be harmful to the sensitive trans- 
 ceiver circuitry; therefore, it is a 
 separate unit that can be located where 
 it will not heat up the transceiver. 
 When only the series-dropping resistor is 
 contained in this unit, it is called a 
 resistor box. It is also referred to as 
 the battery charger by some manufactur- 
 ers; in this case, it would contain the 
 dropping resistors and the charging 
 circuits. 
 
 The main consideration when locating 
 this unit is its heat dissipation and its 
 relationship to the heat-sensitive trans- 
 ceiver. The heat is dissipated into the 
 ambient air and into the structure on 
 which it is fastened; therefore, it is 
 important to follow the manufacturer's 
 mounting instructions carefully. 
 
 The power unit should never be 
 mounted below the transceiver (heat 
 rises) or the speaker enclosure. Keep 
 the power unit a minimum of 6 inches away 
 from either side or the top of the trans- 
 ceiver. If mine personnel can come in 
 contact with the hot surfaces of the 
 power-conditioning unit, a protective 
 grille should be added. This grille 
 should be open at the top and bottom to 
 allow for proper air circulation. 
 
 Approximately 6 inches of clearance 
 should be left around all surfaces on 
 which the connectors and/or fuses are 
 mounted. If possible, the connector- 
 mounting surfaces should be protected 
 from dirt and moisture. Sufficient 
 clearance should be allowed to remove ac- 
 cess covers and open-hinged panels so 
 that adjustments can be reached and plug- 
 in modules can be changed. 
 
 The power-conditioning unit is used 
 to convert the trolley voltage (typical- 
 ly 300 or 600 volts dc) or the local 
 ac power to 12-volt dc power for the 
 
 A 12-volt lead-acid automotive-type 
 storage battery is most often used as an 
 external emergency power source with car- 
 rier phones. When locating this type of 
 battery, the prime considerations should 
 be the accessibility of the fill caps for 
 servicing and proper room ventilation to 
 handle the outgassing of hydrogen. The 
 battery should also be kept away from ma- 
 terials that are susceptible to corrosion 
 by sulfuric acid. 
 
 The ideal temperature range for the 
 battery is 60° to 80° F. Low tempera- 
 tures reduce capacity but prolong battery 
 
149 
 
 life; high temperatures give some addi- 
 tional capacity but reduce total battery 
 life. Temperatures above 125° F can ac- 
 tually damage some of the battery compo- 
 nents and cause early failure. 
 
 Once the various subassemblies have 
 been physically mounted to the panel, the 
 final installation task is to make the 
 electrical interconnections. This pro- 
 cedure consists primarily of inserting 
 cable-mounted connectors into the proper 
 receptacles on the subassemblies and con- 
 necting the signal and power cords into 
 the proper mine ectrical systems. 
 
 A block diagram of a typical carrier 
 phone interconnecting cable system is 
 shown in figure 6-3. The cable connected 
 to the trolley power and/or building pow- 
 er should be installed last. The other 
 cables may be installed in any order that 
 is convenient. 
 
 CAUTION 
 
 Clean and inspect all connectors 
 before mating. Study the keying ar- 
 rangement or polarization to prevent 
 jamming and misalinement. 
 
 Before connecting power, verify that 
 the RF signal common and the case and 
 chassis grounds are all connected. The 
 RF signal common should be an all- 
 metallic connection to the rail system, 
 even if the dispatcher is located above- 
 ground. Often the rails are bonded to 
 the steel structural members of the main 
 shaft to help establish a good earth 
 
 POWER 
 AND 
 SIGNAL 
 CABLE 
 
 BATTERY 
 
 CHARGER 
 
 AND 
 
 RESISTOR 
 
 BOX 
 
 » TRANSCEIVER 
 
 <^ 
 
 MICROPHONE 
 
 FIGURE 6-3. - Typical carrier phone intercon- 
 necting cable system. 
 
 ground for the mine. If this is the 
 case, the RF signal common can be wired 
 to the shaft structure at the surface or 
 the hoist house structure. A minimum 14 
 AWG insulated copper wire should be used 
 for this purpose. If the input power is 
 supplied by the trolley wire, then the RF 
 signal common and the power common should 
 be jumpered together. 
 
 Connect all chassis and case grounds 
 from the lugs or studs provided by the 
 manufacturer to earth ground. Do not 
 rely on the mechanical mounting of the 
 case for a ground connection; always run 
 a separate ground wire to the earth 
 ground. Refer to the Code of Federal 
 Regulations, Title 30, Part 75, Subpart 
 H, for explicit grounding requirements. 
 
 The earth ground connection or 
 building is generally made to a metallic 
 water supply pipe or to the structural 
 ironwork of the building. In either 
 case, the connection should be made close 
 to where the pipe or structure enters the 
 earth to insure a minimum resistance be- 
 tween the connection and the earth. 
 
 The input power to the dispatcher's 
 phone is supplied from either the trolley 
 wire (typically 300 to 600 volts dc) or 
 the local 115-volt ac power. If trolley 
 wire input power is to be used with the 
 dedicated line coupling method, then the 
 in-line fuse holder cable is connected to 
 only the hot input power terminal on the 
 phone. The other end of the cable is 
 connected to the trolley wire. Whenever 
 trolley wire input power is used, the 
 common power connection and the common 
 signal connection are jumpered together 
 and wired to the rails. 
 
 If the carrier phone is not located 
 adjacent to the haulageway, then a wall- 
 mounted fuse box should be used instead 
 of the in-line fuse. Terminate the wire 
 on the line side of the fuse block. Us- 
 ing the same type of wire, make a welded 
 connection to the rail and run this back 
 to the fuse box. Now, two-wire neoprene- 
 jacketed-type portable cable may be 
 used to supply power to the dispatcher's 
 phone. 
 
150 
 
 NOTE 
 
 The manufacturer's detailed in- 
 stallation instructions should be 
 carefully followed to make certain 
 the carrier phone is compatible with 
 the polarity of the mine trolley 
 power. 
 
 The 12-volt power fuse and the trol- 
 ley power fuse should be removed to per- 
 mit making the battery connections with- 
 out a load immediately being placed 
 across the battery. The grounded side of 
 the battery should be connected first. 
 If the mine has a positive trolley sys- 
 tem, then the negative side of the bat- 
 tery should be grounded. The battery 
 post is made of lead, as are the internal 
 connections between the post and the bat- 
 tery plates. If too great a torque is 
 applied to the clamping bolt, the inter- 
 nal connections can develop hairline 
 fractures that can cause an intermittent 
 connection. To avoid this condition, a 
 second wrench should be used to steady 
 the bolthead while tightening the nut. 
 
 Fuses provide an intentionally weak- 
 ened part of an electric circuit and 
 thereby act as a safety valve in the 
 event of dangerous overloads. This pro- 
 tects both personnel and equipment from 
 potential fire hazards due to overheating 
 of the carrier phone. 
 
 NOTE 
 
 Fuses do not provide protection 
 from dangerous high-voltage shocks. 
 
 Fuses come in many sizes, types, and 
 electrical ratings. Always use a re- 
 placement fuse that has the same rating 
 as specified by the carrier phone 
 manufacturers . 
 
 The last step in installation is to 
 connect the power wire. Two commonly 
 used installation methods are direct cou- 
 pling to the trolley wire, and single 
 dedicated line coupling. For the in- 
 staller's safety, the input power should 
 be connected last. 
 
 Direct coupling involves wiring the 
 hot RF signal connecting point directly 
 to the trolley wire with the in-line 
 holder cable provided with the phone. If 
 the dispatcher's office is remotely 
 located, then a fuse box adjacent to the 
 trolley wire should be used. If the in- 
 put power to the phone is to be supplied 
 by the trolley wire typically (300 to 600 
 volts dc) , then the hot power connection 
 is jumpered to the hot RF signal connec- 
 tion with a length of 14 AWG insulated 
 copper wire. Do not install the 3-ampere 
 in-line power fuse until all ground con- 
 nections are made up. 
 
 A second method of signal coupling 
 is to connect the dispatcher's phone to a 
 single conductor dedicated wire. This 
 wire would originate at the hot RF signal 
 connecting point. 
 
 6.4.2 Vehicle Installations 
 
 The carrier phone typically consists 
 of a transceiver assembly, a microphone- 
 speaker assembly, and power conditioning 
 units; these are sometimes an integral 
 part of the transceivers (fig. 6-4). 
 Carrier phone equipment is installed on 
 all types of tracked vehicles. Three 
 commonly used vehicles found on coal 
 haulage systems are locomotives, portal 
 buses, and utility cars. Each of these 
 vehicles has a different seating arrange- 
 ment for the driver (fig. 6-5). 
 
 The microphone-speaker assembly is 
 the only part of the carrier phone that 
 interfaces directly with the vehicle 
 operator. Thus, it must be located so 
 that it can be easily reached. If the 
 microphone hanger is not conveniently lo- 
 cated, it will not be used by the opera- 
 tor, and the microphone and cord will 
 suffer unnecessary damage from mistreat- 
 ment. The speaker volume control should 
 also be within easy reach, and the speak- 
 er should be pointed directly at the 
 operator to provide the best reception. 
 
 Vehicles without dual controls re- 
 quire the operator to assume two dif- 
 ferent positions in front of the same 
 
151 
 
 TRANSCEIVER 
 
 TRANSCEIVER 
 
 FIGURE 6-4. - Typical carrier phones. 
 
 controls so that he can observe the track 
 ahead of him. This further complicates 
 the positioning of the microphone-speaker 
 assembly. It is sometimes helpful to use 
 two microphone hangers for this type of 
 installation so that the microphone is 
 convenient no matter which way the vehi- 
 cle is traveling. 
 
 Entanglement of the microphone cord 
 with other vehicle controls, causing 
 an unsafe operating condition, should 
 also be considered when locating the 
 microphone-speaker assembly. The micro- 
 phone should also be mounted in an area 
 that will protect it from falling debris 
 and/or dripping water. 
 
 Once a suitable place for the 
 microphone-speaker assembly has been de- 
 termined, the transceiver location can be 
 considered. The first restriction on its 
 location is the length of the cables run- 
 ning between the different assemblies. 
 Temperature-sensitive electronic circuits 
 
 are located inside the transceiver assem- 
 bly. Therefore, it should be protected 
 from temperature extremes such as those 
 produced by load resistor banks and the 
 vehicle's drive motors. 
 
 It is important that installation of 
 the transceiver does not reduce the mini- 
 mum roof clearance of the vehicle. Ap- 
 proximately 6 inches of clearance should 
 be left between the vehicle and the sur- 
 faces on which the connectors and/or 
 fuses are mounted. If possible, the con- 
 nectors should be protected from dirt and 
 moisture. Sufficient clearance should be 
 allowed to permit removal of access cov- 
 ers and open-hinged panels so that ad- 
 justments can be reached and plug-in mod- 
 ules can be changed. 
 
 The main consideration when locating 
 the power conditioning unit is its heat 
 dissipation and its relationship to the 
 heat-sensitive transceiver. The heat is 
 dissipated into the ambient air and into 
 
152 
 
 SPEAKER 
 MICROPHONE 
 TRANSCEIVER 
 
 (B) UTILITY CAR 
 
 (-SPEAKER 
 
 I pMICROPHONE 
 
 (ROOF REMOVED 
 TO SHOW INTERIOR) 
 
 TRANSCEIVER 
 
 MICROPHONE 
 SPEAKER 
 
 (C) PORTAL BUS 
 
 WITH DUAL CONTROLS 
 
 FIGURE 6-5. - Typical mine vehicles. 
 
153 
 
 the structure on which it is fastened; 
 therefore, it is important to follow the 
 manufacturer's mounting instructions 
 carefully. The mounting surface should 
 be a massive structural part of the vehi- 
 cle that can absorb the heat transferred 
 from the unit. 
 
 If it is a horizontal surface, a 
 minimum of 3 inches should be allowed on 
 all sides; if possible, nothing should be 
 mounted above the unit. If it is a ver- 
 ticle surface, a minimum clearance of 3 
 inches above and below the unit should be 
 provided for proper air circulation. The 
 power unit should never be mounted below 
 the transceiver; if possible, a minimum 
 separation of 1 foot in all other direc- 
 tions should be provided. 
 
 A 12-volt lead-acid automotive-type 
 storage battery is most often used as an 
 external emergency power source with car- 
 rier phones. When locating this type of 
 battery, the prime considerations should 
 be the accessibility of the fill caps for 
 servicing and proper room ventilation to 
 handle the outgassing of hydrogen. The 
 battery should also be kept away from ma- 
 terials that are susceptible to corrosion 
 by sulfuric acid. 
 
 The ideal temperature range for the 
 battery is 60° to 80° F. Low tempera- 
 tures reduce capacity but prolong battery 
 life; high temperatures give some addi- 
 tional capacity but reduce total battery 
 life. Temperatures above 125° F can ac- 
 tually damage some of the battery compo- 
 nents and cause early failure. 
 
 Most carrier phone components are 
 supplied with mounting plates that can be 
 tack-welded to the vehicle. This pro- 
 vides a permanent mounting surface with 
 tapped holes or threaded studs onto which 
 the subassemblies are fastened. This ar- 
 rangement also provides an easy means of 
 interchanging subassemblies for mainte- 
 nance purposes. 
 
 CAUTION 
 
 Remove the subassembly from the 
 mounting plate during the welding 
 operation. Keep all electrical ca- 
 bles and other nonmetallic materials 
 away from the welding area. This 
 will prevent the carrier phone compo- 
 nents from being damaged by the heat 
 generated from the welding operation. 
 
 Procedures for making the electrical 
 connections between carrier system compo- 
 nents are similar to those for the dis- 
 patcher's installation (paragraph 6.4.1). 
 For the installer's safety, the trolley 
 shoe should be removed from the trolley 
 line. 
 
 Proper cable protection will reduce 
 the downtime of the communication system 
 and prevent accidents , such as loose ca- 
 bles tripping up mine personnel when en- 
 tering or leaving the vehicle. The in- 
 terconnecting cables should be located, 
 if possible, away from areas occupied by 
 mine personnel or supplies. This will 
 prevent cutting and crushing of the ca- 
 bles caused by shifting loads. 
 
 Heavy-duty plastic ties or cable 
 clamps should be used to lash the cord to 
 the frame of the vehicle. If possible, 
 the cable should be run under overhanging 
 parts of the frame to protect it from 
 falling debris and/or dripping water. 
 Enough slack should be left to form a 
 drip loop to prevent condensate from run- 
 ning down the cord and into the rear of 
 the connector. All holes in the frame 
 through which the cable runs should be 
 grommeted. The cable should not be run 
 over sharp edges that might abrade it. 
 Excess cable should be neatly coiled and 
 secured with plastic electrical tape or 
 cable ties and then clamped to the frame. 
 The cable should never be stretched be- 
 tween clamps ; this will leave it in ten- 
 sion, causing an elongation of the insu- 
 lation and the conductor. In addition. 
 
154 
 
 the jacket will lose a considerable part 
 of its resistance to mechanical damage, 
 making it vulnerable to cutting, tearing, 
 and abrasion. 
 
 6.5 Carrier Current Hoist Phone 
 
 Carrier current hoist phones utilize 
 existing physical conductors (the hoist 
 rope) for a transmission medium. Typical 
 hoist radio hardware is shown in figure 
 6-6. 
 
 6.5.1 Cage 
 
 The cage equipment consists of the 
 transceiver, which contains a speaker, 
 microphone, and push-to-talk switch, a 
 battery, generally of the lead-acid type, 
 a cage coupler, and the connecting ca- 
 bles. The transceiver is the only unit 
 that must be mounted within the cage, 
 where space is usually at a premium. For 
 that reason, it should be recessed in the 
 cage wall. The battery must be mounted 
 
 MICROPHONE 
 \ 
 
 PUSH-TO-TALK FOOT SWITCH 
 
 HEADFRAME 
 COUPLER 
 
 HOIST ROPE 
 
 POWER SUPPLY 
 
 SPEAKER 
 GRILL 
 
 MICROPHONE 
 PUSH-TO-TALK SWITCH 
 
 COUPLER CABLE- 
 FIGURE 6-6. - Hoist radio hardware. 
 
 CAGE COUPLER 
 
155 
 
 in an upright position either inside the 
 cage or on top. Mounting the battery on 
 the top of the cage provides early access 
 for charging or replacement. The cage 
 coupler is bolted to the hoist rope above 
 the cage with the conical section up to 
 act as a rock shield. It is suggested 
 that the battery be placed within a pro- 
 tective enclosure to prevent a short cir- 
 cuit which could be caused by debris. Be 
 sure the cable and unit connectors are 
 clean before mating them. Conductive 
 dust in a connector interface may cause 
 the equipment to malfunction. Cables be- 
 tween the battery, transceiver, and cou- 
 pler should be strategically placed to 
 avoid damage. Cable clamps should be 
 used to take up slack; a loose cable is a 
 hazard to personnel and equipment. It is 
 suggested that cables be run through 
 heavy-gage conduit. 
 
 6.5.2 Hoistroom and Headframe 
 
 The hoistroom equipment is shown in 
 the upper part of figure 6-6. The hoist- 
 room will contain the power supply, 
 transceiver, push-to-talk foot switch, 
 and microphone. The power supply and 
 transceiver may be wall mounted. It is 
 best to leave at least 6 inches between 
 the power supply and transceiver, and the 
 power supply should not be mounted below 
 the transceiver. The microphone should 
 be placed so that it can be within 2 
 inches of the operator's mouth while the 
 operator has both hands on the hoist con- 
 rols. The foot switch should be in easy 
 reach of the operator's foot while the 
 operator is at the controls. 
 
 The headframe coupler is located at 
 the top of the shaft. It may be clamped 
 
 to or suspended from the headframe struc- 
 ture. The coupler cable from the head- 
 frame coupler to the transceiver should 
 be run through conduit. 
 
 6 . 6 Summary 
 
 Tables 6-1 through 6-4 are basic 
 checklists for four types of installa- 
 tion: Pager phone, carrier phone, hoist 
 phone, and cable. It is evident that not 
 all criteria are covered in these basic 
 checklists. Additional items that are 
 peculiar to a specific installation may 
 be added. 
 
 There are some procedures associated 
 with any installation. They are 
 
 READ INSTRUCTIONS BEFORE STARTING! 
 
 DO IT SAFELY! 
 
 DO IT CAREFULLY! 
 
 CHECK IT THOROUGHLY! 
 
 SEEK HELP IF NECESSARY! 
 
 Short cuts in installation will 
 probably lead to equipment malfunction or 
 damage. All communications equipment 
 should be tested with transmissions to 
 and from another unit after installation. 
 
 A little extra time spent in the in- 
 stallation phase of a communication sys- 
 tem can mean the difference between a 
 reliable, well-managed system and an 
 undependable system requiring frequent 
 maintenance. 
 
156 
 
 TABLE 6-1. - Pager phone installation basic checklist 
 
 Item 
 
 Yes 
 
 No 
 
 Comments 
 
 1. Has the exposed area of the cable been sealed with electrical 
 tape to keep out moisture? 
 
 
 
 
 2. Is the drip loop positioned properly to keep condensate from 
 getting into phone? 
 
 
 
 
 3. Is the tension in the spring terminals sufficient for a good 
 connection between the wire and phone? 
 
 
 
 
 4. Is the twist-to-lock connector in the locked position? 
 
 
 
 
 5. Has the battery been tested? 
 
 
 
 
 6. Have spare fuses been provided? 
 
 
 
 
 7. Has the phone been called from a distant phone and been found 
 operable? 
 
 
 
 
 8. Is the volume satisfactory or has the amplifier been adjusted 
 for proper loudness? 
 
 
 
 
 9. Is the phone mounted at a proper height for convenience? 
 
 
 
 
 10. Is an insulating mat or dry planking provided on which the 
 user may stand? 
 
 
 
 
 11. Is all cabling secured and protected from passing machinery? 
 
 
 
 
 12. Are all splices QUALITY splices? 
 
 
 
 
 13. Are the cables heavy enough? 
 
 
 
 
 TABLE 6-2. - Carrier phone installation basic checklist 
 
 Item 
 
 Yes 
 
 No 
 
 Comments 
 
 1. Is the microphone-speaker assembly within easy reach? 
 
 
 
 
 2. Is the transceiver protected from temperature extremes (away 
 from power conditioning unit)? 
 
 
 
 
 3. Is the power conditioning unit mounted so personnel will not 
 come in contact with it? 
 
 
 
 
 4. Is the power conditioning unit covered by a protective grille? 
 
 
 
 
 5. Has the battery been tested under load? 
 
 
 
 
 6. Have mating surfaces of connectors been inspected and 
 cleaned? 
 
 
 
 
 7. Have all cable connectors been firmly joined to the units? 
 
 
 
 
 8. Are threaded connectors tightened? 
 
 
 
 
 9. Have all chassis and case grounds been wired to earth ground? 
 
 
 
 
 10. Can the microphone cord become entangled in the vehicle 
 controls? 
 
 
 
 
 11. Are cables protected from abrasion? 
 
 
 
 
 12. Are all components mounted low enough so that minimum roof 
 clearance has not increased? 
 
 
 
 
 13. Has sufficient clearance been given to allow easy removal of 
 access covers, hinged panels, etc.? 
 
 
 
 
 14. Are the bridging capacitors in place on all sectionalized 
 trolley lines? 
 
 
 
 
157 
 
 TABLE 6-3. - Hoist phone installation basic checklist 
 
 Item 
 
 Yes 
 
 No 
 
 Comments 
 
 1. Is the microphone of the transceiver at a proper height 
 (mouth level) for average person? 
 
 
 
 
 2. Is the battery accessible for charging or replacement? 
 
 
 
 
 3. Is the cage coupler firmly mounted? 
 
 
 
 
 4. Have mating surfaces of connectors been inspected and 
 cleaned? 
 
 
 
 
 5. Have cable connectors been firmly joined to the units? 
 
 
 
 
 6. Are threaded connectors tightened? 
 
 
 
 
 7. Are cables protected from damage? 
 
 
 
 
 8. Have any slack cables been tied down (clamped)? 
 
 
 
 
 9. Is headframe coupler firmly connected to or suspended from 
 headf rame? 
 
 
 
 
 10. Are microphone and push-to-talk switch near hoist controls? 
 
 
 
 
 11. Is the transceiver frame firmly attached to the cage? 
 
 
 
 
 12. Have all completed connections been sprayed with silicone or 
 other moisture-inhibiting spray? 
 
 
 
 
 TABLE 6-4. - Telephone cable installation basic checklist 
 
 Item 
 
 Yes 
 
 No 
 
 Comments 
 
 1. Has proper cable been selected according to system plan (type 
 and gage)? 
 
 
 
 
 2. Is the cable supported at the proper interval (approximately 
 10 feet for twisted pair or figure-8 cable and 5 feet for 
 leaky feeder)? 
 
 
 
 
 3. Do droplines have strain relief on main line and tap line? 
 
 
 
 
 4. Are splices mechanically sound and protected from moisture? 
 
 
 
 
 5. Is strain relief used at splices? 
 
 
 
 
 6. Have lightning arrestors been used according to code? 
 
 
 
 
 7. Has extra insullation been provided where the cable crosses 
 the trolley or other high-voltage lines? 
 
 
 
 
 8. Is the cable positioned out of the way of machinery and 
 secured in place? 
 
 
 
 
 9. Are all splices QUALITY splices? 
 
 
 
 
158 
 
 BIBLIOGRAPHY 
 
 1. Long, R. G. Guidelines for Instal- 
 lation, Maintenance and Inspection of 
 Mine Telephone Systems. BuMines OFR 116- 
 78, June 1975, 53 pp.; NTIS PB 287 641. 
 
 2. Long, R. G. , R. L. Chufo, and R. A. 
 Watson. Technical Guidelines for In- 
 stalling, Maintaining, and Inspecting 
 
 Underground Telephone 
 Handbook, 1978, 44 pp. 
 
 Systems. BuMines 
 
 3. Spencer, R. H. , P. O'Brien, and 
 D. Jeffreys. Guidelines for Trolley Car- 
 rier Phone Systems. BuMines OFR 150-77, 
 March 197, 170 pp.; NTIS PB 273 479. 
 
CHAPTER 7.— MAINTENANCE 
 
 159 
 
 7.1 General 
 
 7.2.2 Pager Phones 
 
 Preventive maintenance practices 
 are listed in the manufacturers' instruc- 
 tion books. In general, equipment that 
 requires frequent and extensive preven- 
 tive maintenance is generally the most 
 costly. The manpower spent on these fre- 
 quent trips to remote areas is such that 
 it is usually better to invest in a more 
 costly system which requires little pre- 
 ventive maintenance. The best preventive 
 maintenance for the system is a good 
 installation. 
 
 7.2 Preventive Maintenance and 
 Inspections 
 
 With any system, periodic inspection 
 is required because of the corrosive at- 
 mosphere and adverse conditions that ex- 
 ist in underground mines. These inspec- 
 tions can spot potential trouble in the 
 system. Repair or replacement at that 
 time averts the possibility of losing ef- 
 fectiveness of all or part of the system. 
 
 7.2.1 Cables 
 
 Approximately once per month all ca- 
 bles in the communications system should 
 be inspected for kinking, chafing, crack- 
 ing, wear, stretching, or other signs 
 of physical abuse. Particular attention 
 should be paid to cable glands at the en- 
 try or exit points to the various units 
 in the system, where the cable goes 
 around sharp corners , in the vicinity of 
 holding cleats which may be clamping the 
 cable too tightly causing potential dam- 
 age, and across the areas where the cable 
 is exposed to physical damage from out- 
 side sources, such as equipment or fall- 
 ing objects. If a cable is damaged, it 
 should be replaced as soon as possible. 
 
 It is mandatory that the ground 
 leads and connections to carrier current 
 phones be thoroughly inspected and main- 
 tained in good condition, since consider- 
 able hazard may exist to the operator 
 or equipment if a ground connection is 
 broken. 
 
 The most readily available test set 
 to determine if a pager telephone is 
 operating correctly is the pager phone 
 itself. The following physical check of 
 the system can be performed at any phone 
 station. 
 
 7.2.2a Listen Circuit 
 
 Remove the handset from its cradle 
 and listen to determine if the circuit is 
 functional, as indicated by the presence 
 of noise or conversation on the line. If 
 no noise or voice signals are present at 
 the handset receiver, take the following 
 corrective action: 
 
 1. Operate the handset press-to- 
 talk switch several times. Any corrosion 
 on the contacts of this switch may cause 
 a receiver to be temporarily inopera- 
 tive. Repeated operation may clear the 
 condition. 
 
 2. Open the cabinet and see if the 
 battery cables are properly connected and 
 are making firm contact. Check the hand- 
 set cable and its connections in the cab- 
 inet of the pager phone, and see if there 
 is any evidence of a break in the cable, 
 corroded contacts, or poor connections. 
 
 3. Remove the handset receiver ear- 
 piece by unscrewing it counterclockwise 
 (to the left), and remove the receiver 
 from the socket. Examine the handset 
 cavity; in some units, a patch of cotton 
 batting or floss is used as a barrier to 
 reduce acoustic feedback in the handset. 
 If the cotton patch has absorbed mois- 
 ture, remove it and replace with a 
 crumpled ball of soft rubber, stuffed 
 just far enough into the handset so it 
 will not touch the receiver or switch 
 terminals. 
 
 7.2.2b Page Circuit and Talk Circuit 
 
 Push the page switch, squeeze the 
 handset press-to-talk switch, and call 
 any other phone. Release the page switch 
 
160 
 
 and listen for a reply. If the back- 
 ground noise is too high, or if the re- 
 ceived signal is either too weak or too 
 garbled to be understood, then repairs 
 should be initiated to improve that par- 
 ticular telephone. The phone should be 
 replaced by an operable unit and repaired 
 by qualified personnel. Ask the answer- 
 ing party if the paging signal could be 
 understood, and also check the quality of 
 your received signal. Have the other 
 party page you to verify that your speak- 
 er works. Some of the most common prob- 
 lems with the pager phone system that can 
 be remedied by good installation and 
 maintenance practics are: very low voice 
 levels and very high noise levels. 
 
 In the environment of underground 
 mines, switch contacts are particularly 
 susceptible to erratic operation because 
 of corrosion or oxidation of the switch- 
 ing contacts. This is particularly true 
 of contacts that are used infrequently. 
 Repeated operation of each of the 
 switches in the telephone may aid in 
 clearing some of the corrosion and 
 restoring the phone to more reliable 
 operating condition. Cleaning individual 
 contacts should not be attempted with a 
 phone in service; it should only be done 
 by trained or experienced repair person- 
 nel, who have approved burnishing tools 
 specifically designed for use on switch 
 contacts. 
 
 Often, the sources of these problems 7. 2. 2d Battery Condition 
 
 are — 
 
 1 . Poor placement of the phone line 
 (near rectifiers, motors, etc.). 
 
 2. Using too light a gage phone 
 line. 
 
 3. Using the wrong kind of wire for 
 the phone line. The line should be of 
 the twisted two-wire type. Nontwisted 
 line of any gage is not acceptable. It 
 is the twist that provides a great deal 
 of noise immunity. 
 
 4. Poorly made splices. 
 
 These 
 
 cause high resistance and leaky joints in 
 the line that lower the signal and in- 
 crease the noise. (Note that the phone 
 systems always are worst in the summer 
 months. This is because the high humid- 
 ity is affecting the splices.) 
 
 7.2.2c General Comments 
 
 If any of the signals is erratic, 
 low in signal level, has exceptionally 
 high noise levels, or is unintelligible, 
 check whether other phones in the system 
 are having similar problems. If not, re- 
 place the defective phone with a good 
 one. If the other phones are not operat- 
 ing properly, it is possible that the 
 problem is in the line. A cable may be 
 short-circuited, improperly spliced, or 
 running too close to noise-producing 
 power or trolley lines. Such conditions 
 should be corrected. 
 
 The battery condition of a pager 
 phone can be approximately checked by 
 pushing the page button and calling some 
 other phone to determine whether or not 
 the paging signal is sufficiently strong 
 to energize all relays within the system. 
 Of particular importance is whether or 
 not the battery has sufficient voltage to 
 energize the paging relay of the tele- 
 phone farthest from the phone being 
 tested. Therefore, one of the most dis- 
 tant phones should be called. Batteries 
 can also be checked with a voltmeter to 
 judge if they are near the end of their 
 life or in a marginal state. There are 
 several methods of measuring the avail- 
 able battery voltage as noted in the fol- 
 lowing section. 
 
 7.2.2e Battery Testing 
 
 Most pager phones are powered by one 
 or two 12-volt batteries of the NEDA 923 
 or 926 drycell type. These are 12-volt, 
 metal-cased batteries that measure 2-3/4 
 inches wide, 5-1/4 inches long, and 4-3/8 
 inches high. The difference between the 
 two types is that the 926 has two screw 
 terminals for lead attachment and the 923 
 has a two-prong connector system for lead 
 attachment. For those phones using a 
 24-volt system, two batteries are 
 connected in a series. In consideration 
 of intrinsic safety, it is common to find 
 some means of current limiting, such as a 
 50- to 100-ohm resistor and a fuse in se- 
 ries with the battery system, to limit 
 
161 
 
 the maximum current flow. Batteries are 
 approaching the end of their useful life 
 in a system when the available voltage at 
 the terminals has dropped 25% from the 
 rated value measured under load condi- 
 tions. In a 12-volt system, this is ap- 
 proximately 8 to 9 volts; in a 24-volt 
 system, it is 16 to 18 volts. 
 
 Measurement of the battery voltage 
 can be made by connecting a dc voltmeter 
 across the battery terminals, pressing 
 the page switch, and reading the battery 
 voltage. Measurement of battery voltage 
 on the line will not give a true measure 
 of the battery condition, because of 
 the added voltage drop in the current- 
 limiting resistor. 
 
 Remember that it is useless to mea- 
 sure the output of a battery not under 
 load. Under these conditions, even the 
 poorest battery will still maintain its 
 rated terminal voltage. 
 
 It is not always practical to carry 
 a voltmeter into all sections of a mine, 
 and checking a battery requires that the 
 phone enclosure be opened. The following 
 scheme can minimize such difficulties. 
 
 A voltmeter can be permanently in- 
 stalled at some convenient location 
 aboveground, such as in a repair or main- 
 tenance shop.^ The meter is connected 
 across the line so that it continuously 
 indicates any dc voltage on the line. A 
 listing of voltage readings is made from 
 each remote phone at this reference sta- 
 tion, when the individual phones are pag- 
 ing with new batteries installed. A 
 chart is then made of the allowable re- 
 duction in voltage for each phone by es- 
 timating a 20% to 25% reduction from the 
 new battery condition. Reference to this 
 chart can give advance warning of the 
 approximate condition of each battery 
 and will provide guidance for planned 
 prelacement. A periodic check can be 
 made of each phone by requesting a page 
 from each of the phones and maintaining a 
 
 ^This could violate MSHA intrinsic 
 safety standards; check with MSHA for ap- 
 plication details. 
 
 log of the voltage readings. This will 
 assist in maintaining an up-to-date sta- 
 tus of the battery condition at individ- 
 ual phones. This procedure will remain 
 valid as long as the phone system is 
 configured as it was when the original 
 listing was made. Substantial change in 
 the phone system could require making a 
 new chart. 
 
 7.2.3 Carrier Phones 
 
 CAUTION 
 
 Some of the procedures discussed 
 in this manual are undertaken with 
 the in-mine trolley wire energized 
 and are therefore very hazardous. 
 Extreme caution must be exercised 
 to avoid accidental electrocution. 
 Fuses used in test leads protect only 
 the equipment and do not provide any 
 protection from shock hazard for the 
 operator. Do not attempt any of the 
 electrical tests or installations de- 
 scribed in this manual unless you are 
 qualified for such work and are thor- 
 oughly familiar with electrical work 
 on trolley wires. 
 
 Each of the carrier phone units 
 should be examined for any external phys- 
 ical damage. All fixing screws must be 
 tight. All connectors and externally ac- 
 cessible fuses should be checked for 
 proper seating. 
 
 7.2.3a Microphone 
 
 The carrier phone microphone is a 
 delicate piece of equipment and is most 
 prone to abuse by handling or dropping. 
 The microphone should be examined for 
 evidence of physical abuse. The action 
 of the transmit relay can be observed by 
 pressing the transmit button and listen- 
 ing for the transmit relay inside the 
 transmitter (in units where such a relay 
 is used) to produce a sharp click. The 
 microphone quality can then be assessed 
 by transmitting a test count to a remote 
 unit ; the operator of the remote unit 
 will judge the quality of the voice he 
 receives and report back to the unit 
 being tested so that the receiving 
 
162 
 
 quality of the unit being tested may also 
 be assessed. 
 
 7.2.3b Batteries 
 
 Two different types of battery sys- 
 tems are used in carrier phones. One is 
 a conventional car battery type or wet 
 lead-acid cell, and the other is a gelled 
 electrolyte battery. Both types should 
 be tested once a month to insure a proper 
 state of charge, and the electrolyte 
 should be checked in wet lead-acid bat- 
 teries, if possible. The gelled electro- 
 lyte battery is also of a lead-acid con- 
 struction; however, its so-called dry 
 electrolyte system cannot be changed 
 since the cell is sealed to prevent any 
 electrolyte loss. Overcharging of either 
 of the battery types causes considerable 
 electrolyte loss, and both types of bat- 
 tery can be ruined if overcharged for a 
 long period of time. 
 
 CAUTION 
 
 Electrolyte loss also happens to 
 a lesser extent during a normal 
 charge cycle and results in the emis- 
 sion of hydrogen and oxygen from the 
 cells in a ratio which is explosive. 
 Slow emission of this hydrogen and 
 oxygen gas mixture in the enclosures 
 using a gelled electrolytic battery 
 can create a hazardous mixture of 
 gases inside the units. Some units 
 are vented to prevent a pressure 
 buildup inside the enclosure, but 
 this is insufficient ventilation to 
 prevent the possible buildup of a 
 hazardous atmosphere inside the box. 
 Thus, transceivers using a gelled 
 electrolyte battery should only be 
 opened in a well-ventilated area 
 where there are no possible sources 
 of ignition of the hydrogen and oxy- 
 gen mixture before it is sufficiently 
 diluted by the surrounding air to be- 
 come harmless. Wet lead-acid batter- 
 ies should be placed in a well- 
 ventilated area in the vehicle to 
 prevent buildup of pockets of danger- 
 ous hydrogen-oxygen mixture. 
 
 7.2.3c Wet Cell Maintenance 
 
 If a car-type wet cell lead-acid 
 battery is used, it should be installed 
 in a well ventilated area easily accessi- 
 ble for routine maintenance. Each week 
 the level of the electrolyte in each cell 
 should be checked and restored to its 
 proper level by the addition of distilled 
 water. The electrolyte should read a 
 specific gravity of approximately 1.275 
 on a battery-testing hydrometer when the 
 battery is fully charged. The voltage 
 for each cell should be between 2.2 and 
 2.4 volts. Since in normal operation the 
 battery is under continuous charge, the 
 specific gravity and voltage of a battery 
 in good condition should be around the 
 stated values. Values significantly less 
 are sjmiptoms of problems with either the 
 battery or the battery charger and should 
 be investigated. If electrolyte is lost 
 from the battery due to spillage, then 
 electrolyte premlxed to the same specific 
 gravity should be used to refill the bat- 
 tery to its normal level. 
 
 Terminal posts on lead-acid batter- 
 ies should be examined and cleaned each 
 month. Petroleum jelly may be used to 
 coat these posts to prevent corrosion. 
 Also, any corrosion of the battery box 
 should be scraped clean, and petroleum 
 jelly should be applied to prevent any 
 further corrosion. 
 
 If a vehicle equipped with a carrier 
 phone is to be taken out of service 
 for some time, then both battery leads 
 should be disconnected to prevent dis- 
 charge of the battery while the unit re- 
 mains in standby mode. Again, petroleum 
 jelly should be applied to the battery 
 posts and the terminals to prevent any 
 corrosion. 
 
 Any battery found to be in a weak 
 condition should be removed for recharg- 
 ing and replaced by a fully charged bat- 
 tery. If a particular vehicle has re- 
 peated battery problems , the battery 
 charger in that vehicle should be removed 
 for checkout. 
 
163 
 
 7.2.3d Gelled Electrolyte Battery 
 
 It is not possible to service the 
 electrolyte in a gelled electrolyte bat- 
 tery since it is sealed at the factory. 
 However, these batteries do vent small 
 amounts of hydrogen and oxygen during the 
 charging process , which will increase to 
 larger amounts if the battery is over- 
 charged. Normally, the battery should be 
 charged by a taper-charge process. This 
 means that when the battery is in a dis- 
 charged condition, the battery charger 
 can apply a comparatively large amount of 
 current to build the charge up in the 
 battery quickly. However, as the battery 
 charge increases , the charging rate 
 should decrease. When the battery is al- 
 most fully charged, the charging current 
 should fall to zero or maintain a very 
 small charge. Each of the two types of 
 carrier phones using batteries of this 
 type have a taper-charge-type battery 
 charger built in to maintain the cells at 
 a fully charged state, without the hazard 
 of overcharging. 
 
 The important parameter to measure 
 for proper gelled electrolyte battery 
 maintenance is the battery voltage. A 
 nominal 12-volt gelled electrolyte bat- 
 tery is fully charged when it reads 13.8 
 volts across the terminals. This should 
 be the voltage reading when the battery 
 has been fully charged by the operation 
 of its battery charger. Any voltage 
 higher than 13.8 is an indication that 
 the battery is being overcharged, thereby 
 suffering a considerable loss of life due 
 to the drying out of the electrolyte. 
 This also causes generation of dangerous 
 quantities of hydrogen and oxygen gas 
 mixtures as the cell vents. If this is 
 the case, the battery charger should be 
 examined for malfunction. 
 
 discharged and stored in this condition 
 without being recharged, the battery may 
 develop a condition where it cannot be 
 recharged and should be replaced. 
 
 7.2.3e Troubleshooting on the Vehicle 
 
 When an operator reports a malfunc- 
 tion carrier phone, initial diagnosis of 
 a problem can be carried out using only 
 the equipment suggested in table 7-1. 
 The repairman may either take his equip- 
 ment to the faulty vehicle , or the faulty 
 vehicle may be returned to the test and 
 maintenance area. 
 
 First, the battery voltage should be 
 checked to make sure it has not become 
 discharged. If it is found to be good, 
 all external fuses in the unit should be 
 checked. If a faulty fuse is found, it 
 should only be replaced with a fuse of 
 the proper rating. If the fuse blows 
 again, then the unit is probably faulty. 
 It is possible that replacing the blown 
 fuse with a new one will cause the unit 
 to operate properly since a momentary 
 overload could have caused the original 
 fuse to blow. 
 
 Sometimes the phone itself can pro- 
 vide valuable information on the nature 
 of a problem. Use of the carrier phone 
 will generally isolate the problem into 
 one of the following three categories. 
 
 1. Cannot transmit to others or re- 
 ceive from others : 
 
 a. Check the main fuse. 
 
 b. Check the ground connection. 
 
 c. Check all connectors for 
 corroded contacts. 
 
 Alternatively, if the carrier phone 
 has been left on for an extended time 
 without any battery charging from the 
 trolley wire, it is possible for the bat- 
 tery to become moderately or deeply dis- 
 charged. A moderately discharged battery 
 can be removed for recharging and gener- 
 ally will not suffer any significant 
 harm. However, if the battery is deeply 
 
 d. Check all cables for breaks. 
 
 e. Check the battery condition. 
 
 f. If cause cannot be readily 
 located, replace with spare unit and 
 take the malfunctioning unit in for 
 bench maintenance. 
 
164 
 
 TABLE 7-1. - Suggested test equipment 
 
 Item 
 
 Type 
 
 Use 
 
 Multimeter. 
 
 Various, 
 
 Fuses. 
 
 .do. 
 
 Substitute units. 
 
 Same as used in the 
 mine. 
 
 Hydrometer. 
 
 Battery type. 
 
 Distilled water. . 
 Petroleum jelly.. 
 Battery charger. . 
 
 Any, 
 
 Any applicable (for 
 wet battery) or 
 special battery 
 charger for gelled 
 electrolyte 
 battery. 
 
 Meter can be used for measuring voltages in 
 and around the unit, power consumption, 
 power output, and fuse checking. In order 
 to give useful results for transmitter power 
 output measurements, the meter should be 
 capable of operating with frequencies of at 
 least 100 kHz. 
 
 Fuses provide an intentionally weakened part 
 of an electric circuit, and thereby act as a 
 safety valve in the event of dangerous over- 
 loads. This protects both personnel and 
 equipment from potential fire hazards due to 
 overheating of the carrier phone. 
 
 A blown fuse generally indicates that some 
 part of the circuit of the carrier phone has 
 become defective. Occasionally a temporary 
 external overload condition can cause a fuse 
 to blow; hence it is a useful practice to 
 change a blown fuse one time to see if the 
 unit can be brought back into service. 
 Should the fuse blow again, then a more 
 detailed trouble-shooting process should be 
 attempted. 
 
 Each carrier phone consists of a number of 
 different units interconnected by cables. 
 To facilitate troubleshooting on the vehi- 
 cle, a fully operational spare set of the 
 type used in the mine should be maintained 
 so that initial trouble-shooting can be per- 
 formed by substitution of the individual 
 units. 
 
 The hydrometer measures the charge-discharge 
 condition of the battery electrolyte. 
 
 A battery with a low level of electrolyte 
 will require an addition of distilled water. 
 
 Coating the battery terminals with petroleum 
 jelly aids in preventing corrosion. 
 
 The battery charger is used to recharge bat- 
 teries that have become discharged. 
 
 NOTE. — Insure equipment is suitable for 
 desired application. 
 
165 
 
 2. Can hear others but cannot ap- 
 parently transmit: 
 
 a. Check cables and connectors 
 (especially the microphone for cor- 
 roded contacts or breaks). 
 
 b. Replace the microphone. 
 
 c. If neither the above is at 
 fault, the problem is probably in 
 the transmitter; take the malfunc- 
 tioning unit in for bench 
 maintenance. 
 
 3. Cannot hear others but they can 
 hear your transmission: 
 
 a. Check the volume control 
 setting. 
 
 b. Check the cables and connec- 
 tors for breaks and corrosion. 
 
 c. Replace the speaker assembly 
 by substitution. 
 
 d. Check the squelch setting. 
 
 e. If none of these measures 
 solve the problem, it is probably 
 in the receiver; replace the trans- 
 ceiver with a spare unit and take 
 the malfunctioning unit in for bench 
 maintenance. 
 
 If all these steps fail to make the 
 unit operational, then repair by substi- 
 tution is usually the quickest way of 
 getting the unit into operation again. 
 Substitution should be in the order of 
 items considered to be more or less vul- 
 nerable. Unless it is obvious which unit 
 is faulty, the process should be carried 
 out in the following order: 
 
 1. Change the microphone assembly 
 and test for normal operation. 
 
 2. Change the transceiver assembly 
 and check for normal operation. 
 
 3. Change the loudspeaker unit and 
 check for normal operation. 
 
 4. Where relevant, change the bat- 
 tery charger box and check for normal 
 operation. 
 
 When the faulty unit has been iso- 
 lated and replaced, it should be returned 
 to the repair area for a more detailed 
 examination, including an overall per- 
 formance checkout after the fault has 
 been isolated and repaired. 
 
 CAUTION 
 
 The following procedure is un- 
 dertaken with the trolley wire ener- 
 gized; therefore, it is extremely 
 hazardous. Extreme caution must be 
 exercised to avoid potentially lethal 
 shock. Only personnel thoroughly 
 familiar with electrical work on 
 trolley wires should conduct the test 
 procedures. Equipment used must be 
 appropriate for this application. 
 
 On 300-volt systems, a test can be 
 made of the transmitter power output onto 
 the trolley line. A simple method of 
 measurement in the field makes use of the 
 multimeter with the range selector switch 
 set to the 50-volt-ac scale. The black 
 meter lead should be plugged into the 
 column (-) terminal of the meter and the 
 free end connected to the ground. The 
 red lead must be plugged into the meter 
 output jack and connected to the trolley 
 wire. The trolley pole must be in con- 
 tact with the wire. A reading of 15- 
 volts or more when the transmitter is 
 keyed indicates normal operation provided 
 the test is made at least 200 feet from 
 the nearest power rectifier that supplies 
 the trolley wire. This test cannot be 
 performed on 600-volt dc systems since 
 this voltage will overstress some compo- 
 nents inside the multimeter. In this 
 case, the unit should be returned to the 
 repair shop for a standard bench test. 
 It should be noted that the meter will 
 respond to ripple present on the trolley 
 wire; thus a base reading of up to 10 
 volts will be shown even with the trans- 
 mitter off. 
 
166 
 
 7.2.3f Mapping Signal Levels 
 
 The maintenance of trolley carrier 
 phone systems requires not only the main- 
 tenance of the equipment involved, but 
 the maintenance of the transmission line 
 (trolley wire-rail) used to transmit the 
 signals (refer to paragraph 5.3.1). Evi- 
 dence accumulated over the years indi- 
 cates that this signal path is subject to 
 many loads that impede the propagation of 
 carrier signals. 
 
 One of the most useful ways of de- 
 termining the state of the overall 
 transmission system is to map the signal 
 and noise strengths at various points 
 throughout the mine. Such mapping re- 
 quires a tuned signal-measuring device. 
 
 The mapping is preferably carried 
 out by measuring the signal produced 
 by the dispatcher's transmitter at vari- 
 ous points along the rail haulage system 
 where vehicles operate. A satisfactory 
 way of conducting the measurements is 
 to place a suitable tuned voltmeter 
 aboard a mine vehicle (such as a jeep) , 
 and at appropriate places along the rail 
 haulage — for example, at 2,000- or 3,000- 
 foot intervals — measure the received dis- 
 patcher's signal and background noise. 
 These values should be noted on a mine 
 map for future reference as the mine ex- 
 pands, or as carrier phone problems oc- 
 cur. Except under extremely unusual con- 
 ditions, the signal-strength map produced 
 in this manner will also indicate the 
 level of signal that a vehicle transmit- 
 ter at the measuring position would pro- 
 duce at the dispatcher's place. A por- 
 tion of a mine map with signal and noise 
 readings is shown in figure 7-1. 
 
 The equipment for making such a 
 signal-strength map must be battery oper- 
 ated, easily portable, and easy to use 
 and read. Two such units commercially 
 available are shown in figure 7-2. These 
 tuned voltmeters are general-purpose, 
 battery-operated instruments appropriate 
 
 
 
 
 ffi 
 
 •'i'^ 
 
 
 
 
 rzprs 
 
 
 
 
 FIGURE 7-1. - Example of signal level map. 
 
 for many tasks other than the mapping of 
 trolley carrier signal levels. For this 
 reason careful attention must be paid to 
 the tuning of the instrument to the pre- 
 cise frequency, attenuator settings, and 
 meter indications. Table 7-2 gives spec- 
 ifications for these tuned voltmeters. 
 
 The simple straightforward procedure 
 of measuring the dispatcher's signal lev- 
 el from a jeep or vehicle moving about 
 the mine can best be accomplished by con- 
 necting the trolley wire voltage on board 
 the vehicle to the input of the tuned 
 voltmeter. Because of the hazards asso- 
 ciated with the high voltage of the trol- 
 ley wire, either voltmeter has to be 
 properly Isolated so that personnel oper- 
 ating the instrument are not subjected to 
 this voltage through error in operation. 
 Therefore, it is important that a capaci- 
 tor and a fuse be connected in the series 
 with the instrument to insure that the 
 potentially lethal voltage of the trolley 
 wire does not inadvertently reach an 
 operator. Figure 7-3 shows a possible 
 way of connecting the instruments. 
 
TABLE 7-2. - Key specifications of tuned voltmeters 
 
 167 
 
 Specification 
 
 Frequency range kHz , 
 
 Accuracy: 
 
 Frequency kHz. 
 
 Level dB. 
 
 Selectivity (standard 250 Hz), Hz: 
 
 3-dB bandwidth 
 
 35-dB bandwidth , 
 
 60-dB bandwidth , 
 
 Ranges (full scale) , 
 
 Intermediate frequencies, kHz: 
 
 1st , 
 
 2nd 
 
 Power requirements 
 
 Voltage , 
 
 Battery life (zinc carbon) ... .hours, 
 
 Temperature range °C. 
 
 Dimensions, inches: 
 
 Width 
 
 Height 
 
 Depth , 
 
 Weight pounds. 
 
 NAp Not applicable. 
 
 Sierra 
 
 127C 
 
 
 
 Ry 
 
 =om 3115 
 
 
 
 2-350 
 
 
 
 3-200 
 
 
 
 ±1 
 ±1 
 
 
 
 ±1 
 ±1 
 
 
 1 mV 
 
 250 
 
 600 
 
 1,000 
 
 to 10 V 
 
 -37 
 mV 
 
 to 
 to 
 
 1,000 
 NAp 
 
 4,000 
 +13 dB (3.7 
 1.65 V) 
 
 6 zinc-carbon 
 rechargeable 
 
 1,305 
 
 330 
 
 or 7 NiCd 
 
 D-size cells 
 
 9 (nominal) 
 
 100 
 
 -10-50 
 
 
 5 
 
 NAp 
 
 NAp 
 
 2 gel cells 
 
 Globe 610 
 
 12 
 
 (continuous) 
 
 -10-55 
 
 
 
 12 
 7-1/2 
 7-1/2 
 
 15 
 
 
 
 7-1/4 
 
 5-1/4 
 
 7-3/4 
 
 6 
 
 WARNING 
 
 Disconnect the instrument when 
 the vehicle is moving. Transients 
 from the vehicle motor can cause 
 damage. 
 
 CAUTION 
 
 The following procedure is un- 
 dertaken with the trolley wire ener- 
 gized; therefore, it is extremely 
 hazardous. Extreme caution must be 
 exercised to avoid potentially lethal 
 shock. The fuses used in the test 
 leads serve only to protect equipment 
 and do not in any way reduce the 
 shock hazard to personnel. Only per- 
 sonnel thoroughly familiar with elec- 
 trical work on trolley wires should 
 conduct the test procedures. Equip- 
 ment used must be appropriate for 
 this application. 
 
 To make a measurement , the vehicle 
 is moved to the desired location in the 
 mine and stopped. The operator then asks 
 
 the dispatcher to "key" his transmitter 
 for a 5-second transmission of unmodu- 
 lated carrier. The response on the indi- 
 cating meter is noted, together with any 
 attenuator setting, so that an absolute 
 value of voltage (in volts rms) can be 
 noted on the corresponding position on 
 the mine map. It may be necessary when 
 starting measurements to switch the range 
 knobs of the instruments to make sure 
 that the instrument's response is on 
 scale rather than high and off the scale. 
 In this event, perhaps two transmissions 
 will be required before on-scale readings 
 are obtained. After the transmission 
 from the dispatcher is recorded, the sen- 
 sitivity of the instrument should be in- 
 creased and the noise level at the par- 
 ticular position noted again in volts or 
 millivolts rms. 
 
 The signal-level map will reveal re- 
 gions of the mine where the dispatcher 
 signals are weak which may cause diffi- 
 culties in carrier communication. The 
 mine map will also reveal regions where 
 
168 
 
 V ^ -• 
 
 RYCOM 3115 
 
 FIGURE 7-2. - Tuned voltmeters. 
 
 TROLLEYWIRE 
 
 SIGNAL LEAD 
 
 GROUND TO 
 NEAREST NUT 
 OR BOLT HEAD 
 
 FIGURE 7-3. " Instrument connections. 
 
 excessive noise is the main cause of poor 
 communications. In this event, it is im- 
 portant to locate the source of the of- 
 fending noise and to take measures to al- 
 leviate the problem. 
 
 The signal-level map will also be 
 extremely useful should carrier communi- 
 cations deteriorate with time, with the 
 installation of new equipment, or with 
 the advancement of the mine. Reference 
 can be made back to the original signal 
 levels to determine if and why communica- 
 tions have been degraded. 
 
 7 . 3 Summary 
 
 Good preventive maintenance and pe- 
 riodic inspection practices are the key 
 to successfully maintaining any communi- 
 cation system. Common problems that can 
 affect communications include: 
 
 Corrosion or conductive dust on bat- 
 tery terminals. 
 
 Cable abrasion and line breaks. 
 
 Corrosion on switch contacts or in 
 cable splices. 
 
 Weak batteries. 
 
 Blown fuses. 
 
 Weak or broken springs on spring- 
 loaded connectors. 
 
 Poor splicing techniques. 
 
 In addition to problems that develop 
 owing to normal system usage and environ- 
 mental conditions, trolley carrier sys- 
 tems may be affected by characteristics 
 of the trolley wire and rail itself. 
 Poor signal strength may result because 
 of bringing loads across the trolley 
 wire-rail or high signal attenuation 
 rates in the trolley wire. Methods of 
 compensating for the effect of bridging 
 across the wire-rail are given in section 
 5.3.1. 
 
BIBLIOGRAPHY 
 
 169 
 
 1. Long, R. G. Guidelines for Instal- 
 lation, Maintenance and Inspection of 
 Mine Telephone Systems. BuMines OFR 116- 
 78, June 1975, 53 pp.; NTIS PB 287 641. 
 
 2. Long, R. G. , R. L. Chufo, and R. A. 
 Watson. Technical Guidelines for In- 
 stalling, Maintaining, and Inspecting 
 
 Underground Telephone 
 Handbook, 1978, 44 pp. 
 
 Systems. BuMines 
 
 3. Spencer, R. H. , P. O'Brien, and 
 D. Jeffreys. Guidelines for Trolley Car- 
 rier Phone Systems. BuMines OFR 150-77, 
 March 1977, 170 pp.; NTIS PB 273 479. 
 
170 
 
 APPENDIX A. —COMMUNICATION SYSTEM EXAMPLESi 
 
 A.l INTRODUCTION 
 
 Because no two mines are identical, 
 there is no "one best system" that can be 
 defined to meet the requirements of all 
 mines. The following examples of system 
 installations are presented to indicate 
 how some mines have adapted available 
 equipment to meet their particular 
 requirements. Selection of examples were 
 based on the goal of obtaining the widest 
 possible range and cross section of the 
 following characteristics: 
 
 Type of mine 
 
 Age and size of mine 
 
 Electrical power usage 
 
 Haulage methods 
 
 Existing communications 
 
 Usefulness of present communications 
 
 Examples A through F are of coal 
 mines utilizing various combinations of 
 magneto, pager, and conventional trolley- 
 carrier-type phone systems. Example G is 
 of a magnetite ore block-caving operation 
 where a radiating cable and radio system 
 is used. Example H indicates how a dial- 
 page phone system can be utilized in a 
 coal mine. Example I presents a multi- 
 channel (multiplexed carrier) system 
 presently in use in a deep metal mine. 
 
 A. 2 MINE A 
 
 Mine Description 
 
 Mine A is part of a connected four- 
 mine complex. This particular mine is 
 approximately 20 years old, and although 
 there are some new working sections, the 
 major coal extraction is from retreat 
 mining where pillars are being pulled. 
 Personnel entry is achieved through a 
 
 ^Use of company names is for identifi- 
 cation purposes only and does not imply 
 endorsement by the Bureau of Mines. 
 
 vertical shaft approximately 545 feet 
 deep. Coal is removed from the face area 
 by shuttle cars and placed in a set of 
 six tracked haulage cars. When full, 
 the sets of cars are combined into trains 
 and brought to the surface through a 
 slope entry. Average coal production is 
 4,000 to 5,000 tons per day for 240 work- 
 ing days, setting yearly production at 
 approximately 1 million tons. 
 
 The mine size is currently 2.4 miles 
 north and south by 3.9 miles east and 
 west with overburden from 545 feet to 
 1,000 feet. All tunnels and haulageways 
 are typically 6.5 feet high by 14 to 
 15 feet wide. An average working section 
 is 425 feet by 300 feet long, and 10-foot 
 roof bolts are used. The mine has only 
 one borehole, which is used to supply the 
 mine with water. 
 
 Currently the mine has six working 
 sections of which five are worked every 
 shift. The shifts run from 8 a.m. to 
 4 p.m. , 4 p.m. to 12 p.m. , and 12 p.m. to 
 8. a.m. A typical working section cycle 
 starts with the continuous miner cutting 
 coal and filling a shuttle car. When the 
 shuttle car is full, the driver moves the 
 coal load to the tracks and transfers the 
 coal to one of the six haulage cars posi- 
 tioned on the side track. The shuttle 
 car then returns to the continuous miner 
 to repeat the cycle. Excluding mechani- 
 cal trouble, the continuous miner will 
 cut a block of coal 5 feet high, 15 feet 
 wide, and 16 feet long in 1 hour, and a 
 section can mine five blocks this size in 
 an 8-hour shift. The mine typically has 
 100 men underground per shift. 
 
 Mine Equipment and Power 
 
 The prime power for the mine is 
 550 volts brought in on a feeder cable. 
 In the mine the trolley wire is run par- 
 allel to the feeder cable. At the work- 
 ing section the continuous miner, shuttle 
 cars, and car pull are run off the 550- 
 volt-dc trolley line fed at nip stations. 
 Compressed air is used to run the roof- 
 bolting machine. 
 
171 
 
 The equipment at each working sec- 
 tion includes one continuous miner, two 
 shuttle cars, one roof -bolting machine, 
 and one car pull. Other equipment 
 includes 3 bottom-loading machines, 
 
 2 minor-type cutting machines, and 
 12 pumps. 
 
 The tracked vehicles include 3 dual 
 locomotives or tandems, 24 Jeeps, and 
 
 3 portal buses. 
 
 Present Mine Communications 
 
 The present communication consists 
 of a carrier phone system and a magneto- 
 phone system. All vehicles are equipped 
 with FEMCO carrier phones, and all active 
 working sections, along with selected 
 underground positions, have Western Elec- 
 tric magnetophones. 
 
 Telephone System 
 
 The heart of this mine's communica- 
 tion system is a central dispatcher 
 located at the bottom of the main shaft. 
 
 Eight party-line magnetophone cir- 
 cuits terminate at a simple switchboard 
 in the dispatcher's office. Each of 
 these 8 circuits has several of the 
 41 telephones wired in parallel. Calls 
 between circuits must be made through the 
 dispatcher and his or her switchboard, 
 whereas calls within a circuit need not. 
 The dispatcher can connect any two phone 
 circuits together and can make two of 
 these connections, generating two inde- 
 pendent phone circuits for two-channel 
 operation. 
 
 Since this magnetophone system oper- 
 ates with a bell ringer rather than a 
 loudspeaker, the rings are coded to indi- 
 cate certain places or individuals. The 
 dispatcher communicates through a single 
 headset, and selection of either the mine 
 phone or the carrier phone is made using 
 a two-position switch. Other switches 
 connect and disconnect the various mine 
 telephone circuits. 
 
 This dispatcher controls all vehicle 
 traffic and serves as a telephone 
 operator. Operator duties include 
 
 answering phone calls, switching phone 
 circuits, personnel calling and location, 
 and taking and relaying messages. 
 
 Because the dispatcher is more 
 likely to contact a working section 
 through the motor and an associated car- 
 rier phone in that section, the mine 
 phone is used relatively little conqjared 
 with the carrier phone. 
 
 Based on the observed traffic den- 
 sity and on the number of phones in the 
 system, the probability of a busy signal 
 on the magnetophone system is 5%. 
 
 Trolley Carrier Phone System 
 
 Vehicle-mounted carrier phone usage 
 during a typical shift is shown in figure 
 A-1. 
 
 During a first shift survey, there 
 were 182 dispatching calls, 20 calls 
 relating to personnel location, and 58 
 calls relating to placing empty and 
 loaded cars. 
 
 Communications Requirements — ^Users ' 
 Viewpoint 
 
 Evidence of this mine's interest in 
 communications is shown by the expression 
 of one foreman that "their production 
 would be cut in half if they lost either 
 telephone or carrier phone communica- 
 tions." An important comminications. 
 requirement as defined by the management 
 of this mine concerned safety. They 
 strongly felt that a secure channel was 
 needed where only the persons calling and 
 called could hear the conversation. 
 There are two reasons for this: First, 
 anyone seeking aid for an injured miner 
 tends to belittle the seriousness of the 
 injury because he knows that friends and 
 relatives of the injured miner, and those 
 just curious, will be listening to the 
 conversation. The problem is not unique 
 to this mine. Secondly, the phones of 
 these eavesdroppers load the line to the 
 extent that the emergency conmiuni cat ions 
 are impaired. 
 
 Based on this realistic situation, a 
 basic communications requirement is a 
 
172 
 
 £ 
 d 
 
 UJ 
 
 CO 
 
 < 
 
 O 
 
 I- 
 
 8-9 
 
 9-10 
 
 10-11 
 
 11 a.m.- 
 
 Noon- 
 
 1-2 
 
 2-3 
 
 3-4 
 
 a.m. 
 
 a.m. 
 
 a.m. 
 
 noon 
 
 1 p.m. 
 
 p.m. 
 
 p.m. 
 
 p.m 
 
 FIGURE A-1. = Vehicle-mounted carrier phone use in typical shift. 
 
 private line, selective calling channel 
 over which the person attending an 
 injured person can privately call, at his 
 or her discretion, the mine foreman, the 
 dispatcher, the safety foreman, or the 
 nearest hospital or ambulance service. 
 Note that a conventional private dial 
 system meets this requirement. Mine C 
 (described in section A-4) has a dial 
 phone adjacent to each pager phone. This 
 met the need for a secure channel 
 for both management and emergency 
 communications . 
 
 Communications Requirements — Based 
 on Survey Analysis 
 
 Although the personnel interviewed 
 felt the quality of their communications 
 was adequate, analysis indicates that 
 excessive noise and distortion were pres- 
 ent. Therefore, a requirement that 
 applies to this mine as well as to all 
 communication systems is that of reason- 
 able signal-to-noise ratio for good 
 intelligibility. 
 
 The fact that the chance of getting 
 a busy signal is 5% is proof that addi- 
 tional channel capacity is needed. Add- 
 ing additional channels to a wired system 
 appears to be an acceptable solution 
 since these extra channels will minimize 
 the telephone duties the dispatcher now 
 performs and will eliminate the communi- 
 cation system blocking problem. Calcula- 
 tions indicate that a minimum of five 
 communication channels are needed for 
 this mine. Furthermore, making one of 
 the five channels a private line will 
 fulfill the requirement for private 
 communications . 
 
 Also, from observing the mine opera- 
 tion and talking to various personnel it 
 appears that section foremen, like the 
 foremen in most industrial operations, 
 are overworked, and yet are the key to 
 improving productivity. Therefore, wire- 
 less communication is needed for at least 
 the section foremen along with vari- 
 ous other supervisors and maintenance 
 personnel. 
 
173 
 
 From this brief analysis of the mine 
 and its current communications, the fol- 
 lowing is a list of minimum communication 
 requirements for Mine A. 
 
 a. Reasonable communication channel 
 signal-to-noise level. 
 
 b. At least five independent voice 
 channels. 
 
 c. At least one secure voice chan- 
 nel, which may be included in the five 
 voice channels. 
 
 d. Some form of wireless communica- 
 tion to select individuals on the working 
 section or roving in haulageways. 
 
 A. 3 MINE B 
 
 Mine Description 
 
 Mine B consists of adjacent (No. 1 
 and No. 2) low-coal mines. The No. 1 
 Mine employs longwall and continuous min- 
 ing. The No. 2 Mine employs conventional 
 and continuous mining and is preparing 
 for its first longwall operation. Both 
 mines employ belt coal haulage to closely 
 located drift entrances. Men and sup- 
 plies enter the No. 1 Mine by a 400-foot 
 shaft remotely located from the No. 2 
 Mine drift entrance. From the two mines, 
 8,000 tons of coal per day are mined by 
 about 600 union men under the supervision 
 of about 60 officials. 
 
 The No. 1 and No. 2 Mines each cur- 
 rently employ one longwall mining unit 
 and conventional working sections of Lee 
 Norse continuous miners. For the No. 1 
 Mine's longwall mining, coal is moved by 
 an armored face conveyor to a stage 
 loader at one end of the longwall, to an 
 extendable belt, and finally to a con- 
 ventional belt. 
 
 In addition to longwall mining, the 
 No. 2 Mine employs a full-dimension unit, 
 a conventional mining unit, and continu- 
 ous miners for seven working sections per 
 shift. 
 
 The equipment used in conventional 
 mining consists of a cutting machine, a 
 
 loader, and two shuttle cars. With a 
 full-dimension system, the shuttle cars 
 are replaced by an extendable belt. 
 
 Coal is brought out of the two mines 
 by conveyor belt, and men and supplies 
 are moved by track. The coal is moved by 
 belt from the two mines to a screening 
 house having 1,250 tons of storage capac- 
 ity. To cope with slacks and overflows, 
 coal can be automatically diverted to a 
 12,000-ton-capacity storage pile. 
 
 Ac power is brought into the mines 
 at 12,470 volts. Two rectifiers are 
 positioned at every 6,000 feet of track, 
 each with a capability of 300 kW, to 
 supply 300-volt-dc power to the trolley 
 wires and their feeders. In addition to 
 supplying locomotives with power, the 
 trolley lines supply power, at nip points 
 along the line, for the operation of the 
 300-volt-dc shuttle cars. Where needed, 
 the 12,470 volts ac is transformed to 
 600 volts to provide ac power for rock 
 dusters, conveyor belt drives, miners, 
 roof bolters, and belt feeders. 
 
 Present Mine Communications 
 
 The equipment used in each of the 
 two mines includes paging-type party line 
 telephones, trolley carrier phones, the 
 fire sensor tape recording that would 
 automatically be patched into the phone 
 system should there be a belt fire, and 
 the fan sensors that utilize the phone 
 lines. 
 
 The No. 1 Mine's communications sys- 
 tems are independent of the No. 2 Mine's, 
 but generally of the same size and equip- 
 ment types. The No. 2 Mine's chief 
 electrician's office has a No. 1 Mine 
 phone, as does the No. 2 Mine's foreman's 
 office. 
 
 Telephone System 
 
 Since both mines have similar com- 
 munication equipment, only the No. 2 Mine 
 will be described. The No. 2 Mine has 
 31 underground loudspeaking telephones. 
 The underground phones are all in a 
 single network. The following seven sur- 
 face phones are also in this network: 
 
174 
 
 Outside mechanics shanty — 1 
 
 Outside shop — 1 
 
 Auditorium — 1 
 
 Chief electrician — 1 
 
 Cleaning plant — 2 
 
 Double breaker switchhouse — 1 
 
 The paging phones used in these 
 mines use 6 volts for normal phone use 
 and 22.5 volts during paging. With these 
 phones, pressing the paging button at any 
 station permits the operator to broadcast 
 through the loudspeakers on the remaining 
 37 telephones. On releasing the paging 
 button the operator can converse with 
 anyone who picks up the handset on any 
 other phone. 
 
 Tape recordings were taken of both 
 the No. 1 and the No. 2 Mine's party line 
 pager phone system and the No. 1 Mine's 
 carrier phone system. Analysis of these 
 recordings revealed that for the No. 2 
 Mine, based on hour intervals, the most 
 the system is used is about 50% of the 
 time, between 9 and 10 a.m. But, based 
 on 15-minute intervals, the phones are 
 used nearly 90% of the time around 3:30 
 in the afternoon. 
 
 This heavy usage occurs during the 
 last hour of the shift when section fore- 
 men are making their end-of-shift reports 
 on production status, supplies on hand, 
 supply requests, and maintenance work 
 requests. The fact that the phone system 
 is used 90% of the time signifies that 
 other calls that could improve production 
 efficiency must either be delayed or not 
 made at all. 
 
 Carrier Phone System 
 
 A second means of voice communica- 
 tions is the carrier phone system that 
 uses the dc trolley wire as a carrier of 
 88-kHz (No. 2 Mine) and 100-kHz (No. 1 
 Mine) FM. In each mine five carrier 
 phones are used: One as a base station 
 
 at the inside mechanics shanty, 
 utility jeeps, and two on motors. 
 
 two on 
 
 One shortcoming of the present car- 
 rier system is that there is no way for 
 personnel with carrier phones to communi- 
 cate with working sections. Mine per- 
 sonnel would like some way of patching 
 the carrier and pager phone systems 
 together. 
 
 Longwall Communications 
 
 Five permissible loudspeaking tele- 
 phones are spaced at 125-foot intervals 
 along the 500-foot longwall system. 
 These five phones are connected together 
 to form an independent communications 
 system. Near the phones at either end of 
 the longwall system are phones of the 
 overall telephone network. Though not 
 interconnected, the two phone networks 
 are physically close to each other. 
 
 The five phones are identical to 
 those of the main telephone system except 
 that the paging mode is permanently wired 
 into all five phones as a safety measure. 
 Anything said into any one of the five 
 handsets will be broadcast over all five 
 loudspeakers, thus alerting all nearby 
 personnel of activity on the longwall 
 section. 
 
 Belt Maintenance Communications 
 
 Along the belt lines (every 
 2,000 feet) and at the belt heads are 
 located phones of the telephone communi- 
 cations system. The belt heads are the 
 only spots where belt mechanism fires are 
 likely to occur. 
 
 Belt Fire Alarm System 
 
 Although these mines have never had 
 a belt fire, their fire alarm system is 
 better than required by law. A tape 
 player is positioned underground and when 
 activated will broadcast a warning over 
 all telephone and carrier phone loud- 
 speakers. The recorded message warns 
 all personnel of the alarm condition, 
 specifies the location of the tripped 
 
175 
 
 alarms , and advises personnel of safety 
 precautions to be taken. 
 
 Fan-Stop Alarm 
 
 In the event a fan stops, provision 
 is made for utilizing the phone systems 
 to insure that proper action is taken. 
 At the No. 1 Mine, where there is a phone 
 at the fan site and where personnel are 
 within earshot of an audible alarm, the 
 person responding to the alarm can use 
 the normal telephone system in seeking 
 help. The fans for the No. 2 Mine are 
 remote from any mine personnel so the 
 alarm is automatically sent over a com- 
 mercial phone line to No. 2 Mine's 
 lamp house. 
 
 Communications Requirements — User's 
 Viewpoint 
 
 Through interviews and discussions 
 with those who use, plan, and maintain 
 the communications systems , communica- 
 tions requirements were determined that 
 would aid production at these mines. 
 These requirements dealt directly with 
 mine operations not using a dispatcher, 
 with operations where coal haulage is by 
 belt only, and with operations involving 
 low-coal and longwall mining. 
 
 The first suggestion made by mine 
 personnel was that they needed someone to 
 perform the communications and informa- 
 tion center tasks often performed by the 
 dispatcher in other mines. Presently 
 they have no way of relaying messages 
 between, or interlinking, the independent 
 telephone and carrier phone systems. 
 They also would like someone to monitor 
 belt line sensors, from a center, in 
 order to coordinate troubleshooting, 
 maintenance, and repair of all belt 
 lines. Thus, a requirement for a com- 
 munications center operator (communica- 
 tions coordinator) would resolve the two 
 immediate problems as well as many 
 others. 
 
 Low coal and longwall mining com- 
 bine in determining a requirement for 
 fixed communications terminals to be 
 close to all classes of foremen, and a 
 
 requirement for personal hands-off- 
 operation radios of insignificant weight 
 and bulk. Coordinating the operation and 
 and repair of a 500-foot longwall miner 
 is difficult, especially in low coal. 
 
 Mine personnel felt that having 
 nothing would be better than having a 
 simple radio pager where a section fore- 
 man might have to crawl 700 feet to the 
 nearest phone to find out that it really 
 wasn't that important. If the section 
 foremen are given anything for mobile 
 coimnuni cat ions , it must be small, light, 
 and two-way. In this mine they would 
 like the section foremen to be able to 
 easily contact a general assistant fore- 
 man for supplies and repairs. The need 
 for small portable two-way communications 
 is shown by the case where someone at the 
 mine, on his own initiative, tried some 
 two-way units he had borrowed from a 
 local hospital. 
 
 Communication Requirements — Based 
 on Survey Analysis 
 
 An analysis of both the No. 1 and 
 the No. 2 Mine survey indicates that the 
 communications systems noise levels were 
 unacceptably high and that communications 
 capability is on the verge of becoming 
 unacceptable. Improved communication and 
 improved mining operation would result 
 merely by improving the signal-to-noise 
 ratio of the present communications 
 systems. 
 
 Since the current phone traffic 
 makes the chance of getting a busy signal 
 between 350 and 450 times greater than 
 most industries find acceptable, addi- 
 tional channel capacity is needed to 
 reduce the chance of blocking to the 1% 
 level. Although blocking is still 
 10 times greater than industrial stan- 
 dards, it appears to be a reasonable 
 selection for mine communications. 
 
 Also, from observing the mine opera- 
 tion and talking to various personnel, it 
 appears that section foremen and select 
 longwall personnel need some form of 
 two-way wireless communication of minimum 
 size and weight. All personnel expressed 
 
176 
 
 a negative attitude toward any one-way 
 type of communication. 
 
 Furthermore, the mine personnel felt 
 they needed a location where the daily 
 production activity could be monitored. 
 This location can evolve into a communi- 
 cation center, since as the mine expands 
 and more vehicles are equipped with 
 trolley carrier phones, a combination 
 dispatcher, call monitor, and production 
 monitor can be financially justified. 
 
 Specifically for these mines, the 
 following represents a minimum for future 
 communication requirements: 
 
 a. Reasonable communication channel 
 signal-to-noise ratio. 
 
 b. At least five independent voice 
 channels to replace the present one 
 channel. 
 
 transferred by shuttle cars to a set of 
 6 haulage cars; 2 of these combinations 
 of 6 cars are attached to make a 12-car, 
 train, which removes the coal from the 
 mine. 
 
 The B seam is currently 1.7 miles 
 north and south by 3.2 miles east and 
 west; the C seam is 0.35 mile north and 
 south by 0.45 mile east and west. The 
 overburden ranges from at the slope 
 entry to 2,000 feet. All tunnels are 
 typically 18 feet wide. The average 
 C seam tunnel is 9 feet high, and the 
 typical B seam height is 15 feet. Since 
 the B seam has coal 22 feet thick in some 
 places, the top level is mined first; 
 they return to mine the bottom coal for 
 maximum yield. Roof bolt length is typi- 
 cally 6 feet with variations from 4 to 
 12 feet. These bolts are positioned 
 on 5-foot centers in the B seam and on 
 4-foot centers in the C seam. 
 
 c. Small, lightweight wireless two- 
 way communicator units for foremen and 
 select personnel. 
 
 d. A communication center. 
 A. 4 Mine C 
 
 Mine Description 
 
 Mine C has been operational since 
 1903 with production originally estimated 
 for 100 years. Coal is being mined in 
 the B and C seams, and there is 36 to 
 60 feet of vertical displacement between 
 these seams. The mine employs continuous 
 mining techniques, and personnel enter 
 the mine through a slope entry. Coal 
 production is approximately 1 million 
 tons per year and is removed by a com- 
 bination of belt and haulage cars. The 
 B seam has two active working sections, 
 and each section transfers coal from 
 shuttle cars to a small feeder belt. A 
 longer mother belt then takes the coal 
 to a main loader head. This loader head 
 has the capacity for 18 cars; when 12 
 cars are filled, these cars are assembled 
 into a 12-car train 240 feet in length 
 for main line haulage. The C seam has 
 only 1 working section, and coal is 
 
 The mine has one borehole into the 
 B seam, which was used at one time for a 
 phone line and another time for pumping 
 water out of the mine. A second borehole 
 in the C seam is used to pump methane out 
 of the mine. 
 
 Currently the mine has two coal- 
 producing sections in the B seam and one 
 coalproducing section in the C seam. 
 Furthermore, the B seam has one large 
 cleanup section and two smaller cleanup 
 or rehabilitation sections. A typical 
 working section is 320 feet square. The 
 mine has two production shifts and one 
 maintenance shift, and they run from 
 8 a.m. to 4 p.m. , 4 p.m. to 12 p.m. , and 
 12 p.m. to 8 a.m. The mine has 71 men 
 underground for the first production 
 shift, 55 men underground for the second 
 production shift, and 45 men underground 
 for the last or maintenance shift. 
 
 Mine personnel typically require 
 20 minutes to get from the portal to 
 their working sections, and they take 
 30 minutes for lunch some time between 
 11 a.m. and 1 p.m. These lunch periods 
 are staggered between working sections. 
 On the B seam the work cycle starts with 
 the continuous miner cutting coal and 
 
177 
 
 filling a shuttle car. When the shuttle 
 car is full, the driver transfers the 
 coal load to one of the 36-inch 550-fpm 
 belts run to the section. The belts then 
 remove the coal to the south loader head, 
 where it is loaded into cars that will 
 make up the main line haulage train. The 
 shuttle car round trip takes approxi- 
 mately 7 minutes to complete a work cycle 
 on the B seam. Except for the shuttle 
 cars dumping directly into haulage cars, 
 the C seam has the same type of work 
 cycle. Furthermore, the C seam has no 
 belt haulage and uses tracked haulage for 
 coal removal. 
 
 Mine Equipment and Power 
 
 Three surface substations convert 
 44,000 volts three-phase to 4,160 volts 
 three-phase, the 4,160 volts is carried 
 underground to various 440-volt-ac and 
 275- to 300-volt-dc power stations. 
 
 At the working sections, the shuttle 
 cars are powered either by 440 volts ac 
 or 275 to 300 volts dc; the continuous 
 miner is powered by 440 volts ac, and the 
 roof bolting machines are powered by 275 
 to 300 volts dc. The dc voltage can be 
 obtained by either a trolley nip point or 
 an ac-dc load center. 
 
 All trolleys are powered by a 275- 
 to 300-volt-dc trolley line, which is a 
 common ring bus fed by 300-kW recti- 
 fiers and two 500-kW rectifiers. The 
 quantity and type of trolleys or vehicles 
 follow: 
 
 Man-trip cars — 7 
 
 Mechanics' jeeps — 2 
 
 27-ton motor — 6 
 
 13-ton motor — 2 • 
 
 Present Mine Communications 
 
 This mine utilizes a combination of 
 loudspeaking paging phones, dial tele- 
 phones, and 88-kHz vehicle-mounted car- 
 rier phones. The carrier phone system is 
 tied electrically to the loudspeaking 
 
 paging phones by a trolley coupler. This 
 type application should not be used with 
 intrinsically safe phones. To improve 
 communication coverage, auxiliary speak- 
 ers are sometimes used with the loud- 
 speaking paging phones. The following 
 tabulation shows the number of phones and 
 their general location: 
 
 Phone type 
 
 B 
 
 C 
 
 Sur- 
 
 Stor- 
 
 Vehi- 
 
 
 seam 
 
 seam 
 
 face 
 
 age 
 
 cle 
 
 Dial 
 
 
 
 
 
 
 telephone. . . 
 
 12 
 
 1 
 
 16 
 
 
 
 
 
 Loudspeaking 
 
 
 
 
 
 
 pager phone. 
 
 5 
 
 1 
 
 1 
 
 1 
 
 
 
 Carrier phone 
 
 1 
 
 
 
 1 
 
 
 
 17 
 
 In general, vehicle operators and 
 supervisory personnel use the trolley 
 carrier phones, and mine section foremen, 
 maintenance personnel, and supervisory 
 personnel use the dial telephones and 
 pager phones. 
 
 Dial Telephone System 
 
 The mine has purchased from the tel- 
 ephone company dial telephones, telephone 
 environmental enclosures, associated 
 PABX, and 25-pair telephone cable with 
 wire size No. 19. All underground tele- 
 phone equipment and wire was installed by 
 mine personnel and has been in service 
 for the last 10 years. Standard dial 
 telephones are mounted in environmental 
 enclosures. 
 
 The biggest problem the mine has had 
 with the dial telephones was dust and 
 moisture getting into the dial mechanism. 
 This was understandable since it was 
 found that the majority of the under- 
 ground telephone enclosures had been left 
 wide open and were liberally rock dusted. 
 Of the eight underground telephones 
 checked, seven were in good working order 
 and one had rock dust in its dial mechan- 
 ism contacts. Another problem, not 
 related directly to the telephone equip- 
 ment, was that of acoustic noise from 
 mine machinery. For example, a telephone 
 is required near the 3d south loader 
 head, and mine personnel find comminica- 
 tion difficult when the loader head is 
 in operation. Mine officials have 
 
178 
 
 considered building a telephone enclosure 
 that will shield this telephone from 
 external acoustic noise. 
 
 Other than leaving the door to the 
 environmental enclosures open, the mine 
 personnel appeared to operate the tele- 
 phone system properly. The telephone was 
 mostly used to call from underground sta- 
 tions to surface stations or call out of 
 the mine. Most calls were for supplies, 
 maintenance, and location of personnel. 
 This telephone system was also used as a 
 backup system when commini cat ions were 
 bad. For example, once personnel were 
 contacted using the trolley carrier phone 
 or pager phone and extended conversation 
 was needed, the person would be 
 instructed to go to the nearest dial 
 extension telephone and reestablish con- 
 tact to complete the communication. 
 
 Over the history of the mine, only 
 one major emergency has occurred, a 
 destructive fire. This fire destroyed 
 the telephone cable, and the underground 
 dial telephone system could not be used 
 for emergency personnel evacuation. This 
 points out the basic weakness that tele- 
 phone systems without loopback paths have 
 during a real disaster situation. 
 
 Loudspeaking Paging Phone — Carrier Phone 
 System 
 
 Carrier phones installed at the mine 
 include six 10-year-old units and thir- 
 teen 14-month-old units, all with 88-kHz 
 center frequency. The mine personnel 
 plan to replace the older carrier models 
 with new models in the near future. The 
 carrier phone to paging phone coupler ( an 
 application that cannot be used with an 
 intrinsically safe phone system ) is of 
 standard manufacture, and auxiliary 
 speakers are used with the paging phones 
 where the need arises. The loudspeaking 
 paging phones communicate through a pair 
 of wires from the 25-pair telephone 
 cable, and the carrier phones use the dc 
 trolley wires for their signal paths. 
 
 With the exception of service prob- 
 lems with the older carrier phones, all 
 carrier and paging phones are of good 
 
 quality, are holding up well, and are 
 apparently being properly used by working 
 personnel. However, there is a problem 
 associated with the carrier phones com- 
 municating from certain dtad zones in the 
 mine to the surface. Another problem 
 with the carrier phones was that the 
 battery had to be serviced every 30 days 
 and mine personnel said this was excess- 
 ive. Also, the mine officials indicated 
 that the ringfed trolley rectifiers added 
 receiver noise and that additional rec- 
 tifier line filtering helped but did 
 not eliminate the local problem when 
 the trolley was near the rectifier 
 stations. 
 
 External audio noise and replacing 
 the internal battery approximately every 
 90 days were the most annoying problems 
 associated with the loudspeaking paging 
 phones. 
 
 Although the two systems are elec- 
 trically tied together, the loudspeaking 
 paging phone was primarily used to reach 
 the working section and the carrier phone 
 was used for right of way, placing loads 
 and empties, personnel location, and 
 requesting supplies. When the trolley- 
 mounted carrier phone was used for self- 
 traffic control, the operator would twice 
 give his location and destination and 
 then proceed to his final destination. 
 Although this method of traffic control 
 worked for this mine, an improvement 
 could be seen using a dispatcher. 
 
 Tape recordings for a first 8-hour 
 work shift survey show that the most fre- 
 quent call made was concerned with right 
 of way. Also, it was noted that out of 
 the total 296 calls on the trolley and 
 pager system, only 8 were on the pager. 
 By listening to tape recordings of both 
 the pager and the carrier phone simulta- 
 neously, it was discovered that 78 calls 
 out of the 296 were not heard on the out- 
 side trolley carrier phone. It was also 
 found that the trolley-to-pager hookup 
 failed on 12 occasions. The actual fail- 
 ure of the coupler to function on some 
 signals and the propagation dead zones 
 were major problems associated with this 
 system. 
 
179 
 
 During the fire previously men- 
 tioned, the carrier phones were the only 
 communication that worked through the 
 evacuation. The telephone wire was 
 fused, and communication was not possible 
 using the dial system. However, the car- 
 rier phones could and did operate, using 
 their internal batteries, through the 
 fire. Although the loudspeaking phones 
 were not in widespread service at the 
 time of the fire, their line would have 
 also been fused, making them inoperative 
 if and when needed. 
 
 Communications Requirements — User 's 
 Viewpoint 
 
 Mine personnel indicated that the 
 most urgent communication requirement was 
 the elimination of dead zones in their 
 trolley carrier phone system. 
 
 Communication between the shuttle 
 cars and from the shuttle cars to the 
 continuous miner was also thought to be 
 useful. However, there was apparently no 
 great need or requirement for this type 
 of comminication. 
 
 Portable two-way wireless communica- 
 tion for the maintenance foreman, fire 
 boss, miners on the weekend inspection, 
 and working section foreman was noted as 
 a possible requirement. If portable two- 
 way wireless equipment costs were high, 
 the maintenance foreman, roving super- 
 visors i and key personnel could use a 
 one-way pager. However, mine personnel 
 did not consider equipping a working sec- 
 tion with a one-way pager since a working 
 section foreman mostly communicates 
 out from his location and is seldom 
 called from other sections or surface 
 locations. 
 
 A requirement existed for battery- 
 operated portable emergency communica- 
 tions that could be moved with the miners 
 as the working section moved. This 
 requirement became evident during the 
 fire, when it would have been useful dur- 
 ing the emergency and recovery efforts. 
 Also noted was the fact that if the num- 
 ber of working sections increased. 
 
 the mine may economically justify a 
 dispatcher. 
 
 Communication Requirements — Based 
 on Survey Analysis 
 
 This mine is unique in that it has a 
 dial telephone system, a pager system, 
 and a carrier phone system. The mine, as 
 configured, has no need for additional 
 channels, private channels, or the capa- 
 bility to interconnect to the public 
 phone since the dial telephone has all 
 these capabilities. 
 
 From the traffic density seen on the 
 pager phone and trolley carrier phone 
 system, only two additional channels can 
 be justified to get the probability of 
 blocking to the 1% level. However, only 
 3% of this communications is from the 
 pager phone system, and the vehicle com- 
 munication system must be single-channel 
 operation for safety reasons. Therefore, 
 there exists no justification for addi- 
 tional channels for the pager phone. 
 Using this analysis and the needs gener- 
 ated by the mine personnel, the following 
 list was developed to represent commini- 
 cation requirements for this mine. 
 
 a. Reliable two-way 
 communication. 
 
 vehicle 
 
 b. A dispatcher with comminication 
 center if the mine increases appreciably 
 in size. 
 
 c. Portable two-way wireless com- 
 munication for working section foreman, 
 maintenance foreman, and key personnel. 
 
 d. Portable battery-operated com- 
 munication equipment for mine-to-surface 
 emergency two-way communication. 
 
 A. 5 Mine D 
 
 Mine Description 
 
 Mine D was opened in 1892. Even 
 though the mine is old, they are still 
 developing in some areas. At present 
 they have eight sections on development 
 
180 
 
 and five on retreat. It is estimated 
 that there are 15 years of mining left. 
 
 The mine is entered through one of 
 two drift mouths. A vertical shaft is 
 available but is seldom used. The mine 
 produces approximately 4,500 tons of coal 
 a day by the conventional, continuous, 
 and longwall mining methods in the fol- 
 lowing percentages: 
 
 First shift: 
 
 Conventional 18 
 
 Continuous 35 
 
 Longwall 10 
 
 Other shifts 37_ 
 
 Total 100 
 
 The coal is removed from the mine by 
 track to the cleaning plant about 2 miles 
 from the mine. The mine has an annual 
 production of about 1,300,000 tons. 
 
 is taken to the same cleaning plant as is 
 the coal mined underground. This 
 requires that dispatcher 1 dispatch 
 right-of-way outside as well as 
 underground. 
 
 The mine employs a total of 675 men 
 and works three 8-hour shifts each day, 
 with the major production done on the 
 first shift. The start and end of each 
 shift and the total men working follow: 
 
 First shift: 
 
 6:45 a.m. to 3 p.m. 
 308 men 
 
 Second shift: 3:30 p.m. to 
 
 11:45 p.m. — 216 men 
 
 Third shift: 
 
 12: 15 a.m. to 
 8:30 a.m. — 151 men 
 
 Mine Equipment and Power 
 
 The mine is 7 miles by 5 miles and 
 has overburden from 300 feet to 800 feet. 
 All haulage tunnels are at least 6 feet 
 high by 18 feet. There is no average 
 size for a working section; some are as 
 much as 3,000 feet in length. This 
 requires that coal be removed from the 
 working face to the track by belt. The 
 belt is 36 inches in width, and the mine 
 has approximately 25,000 feet of belt. 
 If the distance is short (less than 
 500 feet), it is possible to dump coal 
 from the shuttle car directly into the 
 coal car. 
 
 Eight 37-ton locomotives are used to 
 remove the coal. The mine has 30 miles 
 of track underground at present. Eight 
 boreholes are used to provide access for 
 13,800-volt-ac, three-phase, cables to 
 the mine. Roof control is obtained by 
 the use of roof bolts ranging in length 
 from 42 inches to 8 or 9 feet. 
 
 Owing to the amount of track and 
 layout of the mine, it is necessary to 
 have two dispatchers. This mine is also 
 engaged in strip mining at various loca- 
 tions directly above the mine. This coal 
 
 Power is provided by the power com- 
 pany at 138,000 volts ac. This is then 
 stepped down to 13,800 volts ac and dis- 
 tributed to eight boreholes, where it is 
 taken underground. At some point under- 
 ground it is converted to 250 volts dc, 
 550 volts dc, or 440 volts ac three- 
 phase, depending on the equipment being 
 used on the section and location in the 
 mine. One of the reasons for this is 
 that from the outside to dispatcher 2, 
 the mine uses 550 volts dc on the 
 trolley. Then branching out from dis- 
 patcher 2, the mine uses 250 volts dc on 
 the trolley. The 440 volts ac is used in 
 both areas. 
 
 The power provided to sections var- 
 ies according to the type of equipment 
 used. There are cases where, on the same 
 section, 250 volts dc or 550 volts dc 
 must be provided for the shuttle cars, 
 and 440 volts ac provided for the miner. 
 Both longwalls require 440 volts ac, as 
 do some of the newer continuous miners. 
 There are also battery-powered scoops on 
 some sections. They are used for clean- 
 ing the section and hauling supplies. 
 
181 
 
 The mine at present uses the follow- 
 ing equipment: 
 
 Continuous miners — 8 
 
 Loading machines — 5 
 
 Longwalls (1 plow, 1 shear) — 2 
 
 Cutting machines — 3 
 
 Shuttle cars — 29 
 
 Present Mine Communications 
 
 At present, the mine communication 
 system consists of a magnetophone system, 
 carrier phone system, and loudspeaking 
 phones. 
 
 The loudspeaking phones are used 
 only on the longwalls. Carrier phones 
 are placed on most of the track vehicles. 
 At the cleaning plant a 60-watt amplifier 
 is used as a public address system call- 
 ing 14 stations, each of which has a 
 microphone and speaker. 
 
 Each dispatcher is responsible for 
 carrier phone and magnetophone control in 
 his area of the mine. The two dispatch- 
 ers must consult with each other when 
 routing traffic toward each other. Typi- 
 cally, the telephone network having dis- 
 patcher 2 as its control point has 
 heavier traffic of a more varied nature 
 than that of dispatcher 1. 
 
 During the busiest period of the 
 shift, the fourth hour, the busier trunk 
 was used 70% of the time. This three- 
 channel traffic intensity implies that 
 there is a 30% chance of getting a busy 
 signal on any given call. This is con- 
 siderably worse than the one chance in a 
 thousand of commercial telephone stan- 
 dards. A six-channel network would be 
 required to bring the system to commer- 
 cial standards. 
 
 There have been no major emergencies 
 at the mine to test the existing system. 
 It is possible that a roof fall could 
 break the phone line and cut off communi- 
 cations to the outside for some areas of 
 
 the mine. In the case of an accident the 
 section notifies the dispatcher, who 
 in turn calls mine management on the 
 outside. 
 
 Telephone System 
 
 The mine has 77 magneto telephones, 
 60 of which are underground. These 
 phones are approximately 30 years old. 
 They use simple twisted-pair. No. 14 wire 
 for the phone circuit. 
 
 The phones are usually mounted on 
 wood that is connected, in some manner, 
 to the roof. They are placed at loca- 
 tions along the main haulage. Phones on 
 the section are located at the head and 
 tailpiece of the belt. These phones are 
 not permissible. 
 
 The dispatchers are the heart of the 
 phone system. Dispatcher 1 is respon- 
 sible for the outside phones and for 
 underground phones 1 to 20. Dispatcher 2 
 has phones 20 to 60. If a person wished 
 to call outside from say phone 57, he 
 would have to ring dispatcher 2. Dis- 
 patcher 2 would then call dispatcher 1, 
 who would ring outside, then connect the 
 lines. 
 
 Since the phones are a ringer type, 
 each station must have a certain ring. 
 It should be kept in mind that the cir- 
 cuits for the two dispatchers are sepa- 
 rated. Therefore, each dispatcher could 
 use the same ring combinations. 
 
 Recordings and corresponding analy- 
 sis of the traffic on the phone system 
 shows that there are periods when the 
 system is used 70% of the time and that 
 the system is overloaded at times. 
 
 Trolley Carrier Phone System 
 
 The carrier phones are mounted on 
 most of the track vehicles. The fact 
 that the mine uses 250 volts dc and 
 550 volts dc on the trolley requires the 
 use of two different carrier phones. 
 They have twelve 250-volt, 163-kHz 
 trolley phones and twenty-eight 550- 
 volt, 100-kHz phones. Both tube and 
 
182 
 
 transistorized versions are used. The 
 tube type is 20 years old, and the tran- 
 sistorized type is 12 years old. The 
 transistorized units are equipped with a 
 12-volt battery, so that they will still 
 operate should the power in the mine go 
 off. 
 
 Pager Phones 
 
 Pager phones are used on the long- 
 walls and on the outside of the mine. 
 The pager phones are mounted on J-hooks 
 from the roof support jacks. Wires are 
 hung from the roof supports for the 
 phones. Rocks falling between the jacks 
 have caused the line to break, causing a 
 potential safety hazard due to inter- 
 rupted communication. The reason for 
 this is there is no way of hearing a ring 
 on the pager system. There are 10 pagers 
 on the plow and 5 on the shear. The 
 phones are 10 years old. The mine per- 
 sonnel felt that the phones were mis- 
 treated by man and environment, and that 
 was the reason for failures. 
 
 Fan Monitors 
 
 The size of the mines requires that 
 the fans be located at great distances 
 from the maintenance shop. The fans are 
 monitored by sending a signal over the 
 high-voltage lines, which is monitored at 
 the outside shop. The five frequencies 
 used (one for each fan) are 39, 116, 47, 
 61, and 33 kHz. 
 
 Communication Requirements — User's 
 Viewpoint 
 
 The phone system performed very 
 well considering its age. However, the 
 changes in humidity caused some problems. 
 There was also a problem with having to 
 walk long distances. The phones are not 
 permissible; this limits how close to the 
 working face they can be placed and often 
 requires that an individual walk as much 
 as 500 feet to reach one. 
 
 individuals on the section. The mine 
 personnel were of the opinion that com- 
 munications to those two men would be 
 desirable. 
 
 The maintenance foreman and master 
 mechanic felt that portable two-way com- 
 munications would decrease the time 
 needed to locate them. Portable communi- 
 cations are also desired for the super- 
 visory personnel (superintendent, mine 
 foremen, and maintenance foremen). At 
 the same time a private line was 
 requested for the phone system for super- 
 visory personnel. 
 
 The safety department personnel sug- 
 gested that remote monitoring of the mine 
 conditions would help increase safety for 
 the entire mine. It was suggested that a 
 private channel directly to the outside 
 for emergency use would decrease the time 
 required to get help from the outside. 
 This private channel would also insure 
 that the occurrence of an accident would 
 not be heard by men on other sections. 
 
 There should be a secure channel 
 open at all times, from any phone under- 
 ground, to some central communications 
 center aboveground. It is not necessary 
 that this line be connected to the com- 
 mercial phone system. Since the . mine 
 management are the first to be notified 
 in case of an emergency, they in turn can 
 call whomever is needed. In a mine this 
 size the time saved by placing the call 
 from underground to the commercial system 
 for assistance, then notifying manage- 
 ment, would be of little help. 
 
 Communications Requirements — Based 
 en Survey Analysis 
 
 This mine has some communication 
 problems that are related only to the 
 extreme age of the equipment used. How- 
 ever, problems due to the large size of 
 the mine may be typical of other large 
 mines. 
 
 Mine personnel felt that wireless 
 communications of some type would be of 
 help on a section. The foreman and the 
 mechanic are the two most sought after 
 
 Signal-to-noise ratio (SNR) causes 
 problems when talking great distances (5 
 to 10 miles). A new system must start 
 by improving SNR on long-distance 
 
183 
 
 converations , which may be typical of 
 many large mines. 
 
 An analysis of the telephone traffic 
 density indicates that three more chan- 
 nels (total six channels) would make the 
 system comparable to an estimated mine 
 standard of 1 in a 100 chances of getting 
 a busy signal. 
 
 is the "stall machine" used at the tail 
 end of the plow longwall to give better 
 roof control. This machine is a limited 
 travel shear that leaves a cleaner end on 
 the longwall than the plow. About one- 
 third of the mining is by longwall, one- 
 third is conventional, and the last third 
 is continuous. The number of working 
 sections for each type of mining follows: 
 
 A dial system is recommended for 
 this mine. A multipair or multiplex 
 system would help to lessen the load of 
 the dispatcher and could also provide the 
 capability for conference calls. These 
 systems also provide the channel privacy 
 requested by mine management. For safety 
 reasons, the trolley carrier phone system 
 should remain one channel. 
 
 Using the above analysis and the 
 suggestions of mine management, the fol- 
 lowing list of improvements was derived: 
 
 a. Reliable two-way vehicle system. 
 
 b. A total of at least six channels 
 to meet minimum standards. 
 
 c. At least one secure channel. 
 
 d. Portable two-way wireless com- 
 munications for certain key personnel. 
 
 e. Battery-operated communications 
 equipment that will work during an 
 emergency. 
 
 f . A communications center located 
 at dispatcher. 
 
 A. 6 Mine E 
 
 Mine Description 
 
 Mine E has a drift entry in a 5.5- 
 foot soft coal seam. The working sec- 
 tions are presently 3.5 miles in from the 
 entrance, with the possibility of eventu- 
 ally working at twice this distance. 
 Mining at the present rate gives the mine 
 a life of from 30 to 40 years. The mine 
 operates two longwalls about 500 feet 
 wide and will travel a range of 
 3,500 feet. New to mining in this area 
 
 1st 
 
 2d 
 
 shift 
 
 shift 
 
 2 
 
 2 
 
 2 
 
 2 
 
 1 
 
 1 
 
 Conventional. 
 Continuous. . , 
 Longwall 
 
 Only a small amount of mining is 
 done during the maintenance or third 
 shift. The mine is small enough that 
 there is no underground maintenance shop. 
 Hence, repairs that cannot be made at 
 the site of the failure must be made 
 outside. 
 
 Coal is moved from the face by 
 shuttle car except on the longwalls, 
 where it goes directly to belt. Local 
 belt haulage is used between the shuttle 
 cars and tracked cars on the main line. 
 The longest belt run is 4,500 feet. 
 
 During the first shift there are no 
 idle sections so there is no need for 
 maintenance crews when each section crew 
 has its own mechanic. Extra mechanics 
 work along the main line during this 
 shift and help section mechanics when 
 needed. 
 
 During the third shift, when few 
 sections are working, there are three 
 maintenance crews whose specific job 
 is to work on equipment at the idle 
 sections. 
 
 Mine Equipment and Power 
 
 Power is fed to the mainline trolley 
 wire at 250 volts dc by four rectifiers. 
 There are deadblocks between the four 
 sections of trolley wire so that each 
 rectifier supplies power to only one 
 short length of wire. All face-mining 
 equipment is ac operated so there is no 
 
184 
 
 need to have nip points from the trolley 
 line. 
 
 Rather than utilizing power bore- 
 holes, 7,200-volt ac power is brought in 
 along the mainline, up to transformers at 
 the working sections. There the voltage 
 is stepped down to 440 volts. Thus as 
 the sections advance, the transformers 
 must be moved to follow. 
 
 The only power outages have been due 
 to storms or hunters shooting transform- 
 ers on the power company's distribution 
 system. Outside there are two high lines 
 feeding the mine's single substation. 
 
 Should there be a power interruption 
 within the mine, the substation attendant 
 will check by telephone with the sections 
 before reenergizing the distribution 
 system. 
 
 Present Mne Communications 
 
 Telephone System 
 
 Pager phones are used in a network 
 of 11 phones along the track throughout 
 the mine, plus a phone in the dis- 
 patcher's outside office and one at the 
 communication repair station in the shop. 
 Tape recordings made during an 8-hour 
 shift indicate that there were 160 dis- 
 cussions concerning the location and 
 movement of empty and loaded coal cars. 
 For the next most common topic, there 
 were nearly 80 discussions concerned with 
 the production of mined coal. Collec- 
 tively the other subjects (reporting, 
 personnel location, maintenance, etc.) 
 add up to about 80, so no one of them is 
 a significant user of channel capacity. 
 Analysis showed that early in the shift, 
 and just before the end, the phones are 
 used as much as 50% of the time. This 
 places the probability of a potential 
 caller finding a busy line at one chance 
 in two during these periods. 
 
 The loudspeaker telephones have an 
 average age of about 4 years. Rock dust 
 does seep in through their cases, but the 
 users and maintenance men report there 
 are few failures and these fall in no 
 
 consistent pattern. The people inter- 
 viewed could give no suggestions on how 
 the phone system — the one following the 
 track — could be improved. There are sev- 
 eral reasons that could be contributing 
 to their satisfaction: 
 
 a. The phones are relatively new. 
 
 b. The phone network is not large. 
 
 c. The time and talent spent on 
 maintaining the system are great. 
 
 d. The equipment supplier gave them 
 much help in setting up their systems. 
 
 e. The characteristics of the phone 
 lines are good. 
 
 The telephone network is such that 
 anyone on a working section is never more 
 than 350 feet from a phone. They feel 
 this is adequate and that having a phone 
 any closer would not really be of more 
 value. Other phone locations are the 
 boom and tailpiece of every belt, plus 
 four in the escapeway. No allowance has 
 been made for emergency usage in the 
 sense that, should a telephone line be 
 broken, there is no loopback to carry the 
 signal. 
 
 Carrier Phone System 
 
 The carrier phone is a 72-kHz system 
 that uses the trolley power line to carry 
 the signals. Even though this is a 
 fairly small mine, they did experience 
 dead zones of unacceptably low signal 
 strength in certain areas. 
 
 The dispatcher has an outside dial 
 phone as well as a speaker phone, so he 
 serves as a message relay center and 
 information center as well as a dis- 
 patcher. The communications maintenance 
 area of the shop also has a trolley car- 
 rier phone to aid the maintenance people 
 in servicing the trolley carrier phone 
 system. The carrier phones do not have 
 storage battery backup. If there is a 
 failure of trolley power, carrier phone 
 communications are lost. 
 
185 
 
 The mine has had a dispatcher for 
 only the last 2 years. Before that, 
 motor operators controlled the track for 
 themselves. At that time the mine tried 
 tying the telephone system in with the 
 trolley phone system (this type applica- 
 tion cannot be used with an intrinsically 
 safe telephone system) but found it only 
 added confusion to have those not near 
 the main line hear all the discussions of 
 the motormen. 
 
 To get the carrier signal around 
 the deadblocks, 2-yF capacitors are used. 
 
 To keep rectifier "hash" out of the 
 72-kHz system, L-filters are used at each 
 rectifier (paragraph 5.3.1a). The filter 
 consists of a SO-pF capacitor across a 
 rectifier's output, with a 10-turn, 
 heavy -cable coil, the coil having an 
 approximate diameter of 2 feet. The 
 manufacturer tuned the filter to reject 
 72-kHz interference. 
 
 CAUTION 
 
 Installation of equipment in a 
 mine should be done only by people 
 thoroughly qualified to do such work. 
 Installations should follow proce- 
 dures recommended by the equipment 
 manufacturer and should comply with 
 good safety practices. All installa- 
 tions should also comply with appli- 
 cable codes and regulations. 
 
 Longwall Communication System 
 
 The longwall miner has its own com- 
 munications system consisting of seven 
 loudspeaking telephones, one at each end 
 and the other five equally spaced along 
 the 500-foot longwall. These loudspeak- 
 ing telephones have no handset and thus 
 operate in the pager mode only, with the 
 loudspeaker serving as a microphone when 
 the push-to-talk button is pressed. The 
 telephone lines lie in the troughs that 
 carry the hydraulic lines. At one time 
 the wires were hung under the top plate 
 of the jacks, but slate falling between 
 the jacks kept breaking the wires. 
 
 Signal lights are positioned along 
 the longwall miner so the operators can 
 coordinate their efforts if the phone 
 system fails. The quality of speech 
 reproduction for the phones was good, and 
 the loudspeaker volume was adequate in 
 spite of the high ambient noise of the 
 miner. 
 
 The only complaint the personnel had 
 was that the push-to-talk button failed 
 often. This button is mounted on the 
 recessed front face of the unit. The 
 phones at the ends of the longwall are 
 mounted horizontally so the recessed 
 panel acts as a catch basin for the 
 watered-down coal dust. Evidently the 
 directional properties of the speaker are 
 such that this mounting is necessary. 
 
 Communications Requirements — 
 User's View 
 
 Except for correction of the minor 
 problems already presented, the mine per- 
 sonnel had little to suggest about new 
 communications systems that would make 
 their work safer and more effective. 
 This may be due to their present system 
 being new and seemingly adequate for this 
 size mine, or due to their not having 
 time to visualize how a higher capability 
 system might profitably be utilized. 
 
 The one desire expressed at this 
 mine was for a secure channel for seeking 
 aid for an accident victim. As in other 
 mines, when an accident is being 
 reported, everyone who knows the phone is 
 being used for this purpose will listen 
 if he can. This lowers the productivity 
 of the eavesdropper; takes his mind off 
 his work, making him more accident prone; 
 and worse, loads the telephone system so 
 that the dispatcher can no longer clearly 
 understand the report. It is not essen- 
 tial that the conversation cannot be 
 listened to, just that personnel not 
 become aware that someone in a panic 
 is calling the dispatcher. Personnel 
 seldom listen in on run-of-the-mill 
 conversations . 
 
186 
 
 Communications Requirements — Based 
 on Survey Analysis 
 
 The exceptional high quality and the 
 unusual amount of care given to the tele- 
 phone and carrier phone systems leave 
 little to suggest as to improving these 
 communications means in mines similar to 
 this one. 
 
 This mine, like some others visited, 
 has a need for a secure channel as an aid 
 in effectively handling injury problems, 
 and it would be desirable to have a 
 secure management channel. 
 
 Better communications capability 
 would increase productivity in the long- 
 wall mining sections. Fast, effective 
 hands-free communication is needed by 
 operating personnel during both operation 
 and repair of the miner. Because of its 
 high production rate — and thus the high 
 cost of downtime — and because of the 
 almost impossible working conditions, it 
 seems essential that all longwall workers 
 have their own wireless communications 
 network with each having small, light- 
 weight equipment, including speakers and 
 bone-conducting microphones mounted in 
 helmets. The communication center oper- 
 ator should also be able to monitor this 
 network. 
 
 A. 7 Mine F 
 
 Mine Description 
 
 Mine F has been operational since 
 1963 with production originally estimated 
 for 25 years. Coal is being mined 
 from the Mammoth Seam in the Cherokee 
 Group. Seam thickness is approximately 
 60 inches. 
 
 This mine is the only nonunion mine 
 surveyed. As a result, some of the oper- 
 ations are notably different from those 
 seen at the other mines examined. 
 
 The mine employs conventional mining 
 techniques and employs tracked haulage to 
 remove the coal. Personnel entry and 
 coal removal are through a single shaft. 
 
 Coal production is approximately 
 250,000 tons per year. There Is one rain- 
 ing section, operating one shift. Coal 
 is mined via the room and pillar method 
 with activity rotating through six active 
 rooms . 
 
 The mine will ultimately be approxi- 
 mately 1-1/4 miles square. Mining activ- 
 ity is currently occurring about 3/4 mile 
 from the shaft. 
 
 The overburden at the shaft is 
 157 feet, increasing gradually to the 
 working face. Tunnels are typically 
 12 feet wide and range from 4 to 6 feet 
 high. Four- and 6-foot roof bolts are 
 Installed on 5-foot centers. 
 
 There are no boreholes into the 
 mine. The fresh air entrance serves as 
 the emergency exit and is located about 
 500 feet from the main shaft. The main 
 shaft serves as the air exhaust. 
 
 Mine Management 
 
 Since there is only one raining 
 section, the mine operates with very 
 few management personnel, as follows: 
 General manager, chief engineer, super- 
 intendent, and foreman. 
 
 Management personnel do quite a bit 
 of filling in as necessary; however, the 
 chief engineer normally tends to topside 
 operations while the superintendent 
 stands by at the bottom of the shaft. 
 The foreman remains at the face. The 
 mine has 25 men underground during the 
 shift. 
 
 There are five mining operations 
 rotating continuously through the six 
 active rooms at the face. A cycle starts 
 with the cutter undercutting the coal 
 face. This is followed by the coal 
 driller drilling holes for the charges. 
 After the driller moves on, the charges 
 are set and fired by the shot flrer. 
 After a delay for the air to clear, the 
 loader is moved in to begin loading 
 shuttle cars, which transfer coal to the 
 haulage cars. When a room has been 
 
187 
 
 cleaned of the loose coal, the roof bolt- 
 ers move in to extend the supported sec- 
 tion of the roof. 
 
 Loaded haulage trains are pulled to 
 the shaft where the cars are dumped into 
 a skip, one car to a skipload. The skip 
 is lifted up the shaft and dumped into 
 the crusher. Crushed coal is conveyed 
 into semitrailer trucks that are used 
 exclusively to haul the mine's output. 
 
 The maintenance philosophy of this 
 mine results in a large amount of nonpro- 
 ductive machine time. There is a com- 
 plete operating spare for each type of 
 machine in the mine. As a result of this 
 philosophy, however, there is virtually 
 no downtime for equipment maintenance. A 
 minor failure is repaired on the spot; in 
 case of a major failure, the spare 
 machine is placed in service while the 
 broken one is fixed. 
 
 There is no fixed shop location. 
 The maintenance personnel travel with the 
 mining crew. The presence of spare 
 machinery permits repairs and maintenance 
 to be performed thoroughly without slow- 
 ing production. 
 
 Supplies are delivered via the haul- 
 age cars. Just before the end of each 
 working day the foreman calls his list of 
 supplies to the hoisting engineer. These 
 are placed at the top of the shaft and 
 delivered to the face either at the end 
 of the day or the beginning of the next 
 one. Repair parts are delivered during 
 the day via a return trip of the haulage 
 cars. 
 
 The mine has a single man-trip car. 
 This is sufficient to carry the entire 
 crew, so only one man-trip is made, morn- 
 ing and evening. Administration of the 
 mine operation is quite informal. The 
 general manager oversees all operations 
 and assists the topside personnel as 
 necessary. All management personnel 
 assist when and where needed. 
 
 Ventilation is via a single fan, 
 blowing into the escape shaft and 
 exhausting through the main shaft. 
 Within the mine, water sprays are used to 
 keep dust down. There has never been any 
 problem with excessive water, so the only 
 water-handling gear is that used to con- 
 trol dust. 
 
 Mine Equipment and Power 
 
 The following pieces of mining 
 equipment are in use at the mine: 
 
 Shuttle cars — 3 Roof bolter — 1 
 Loader — 1 Coal drill — 1 
 Cutter — 1 Locomotives — 3 
 
 In addition to the equipment in use, 
 there is one operating spare of each type 
 of machine. In case of major breakdown 
 the spare is placed in operation while 
 the broken unit is repaired. 
 
 Primary power comes into the mine 
 through the main shaft. A 2,300-volt, 
 three-phase line is run to the two 
 transformer-rectifier sets used. One 
 transformer feeds the trolley for the 
 haulage system; the other powers all 
 machinery at the face. All machines in 
 the mine run off 280 to 300 volts dc. 
 
 Present Mine Communications 
 
 The mine currently has a combination 
 of three independent voice communication 
 systems. The loudspeaking phone system 
 uses two units, one located at the hoist- 
 ing engineer's position, and the other at 
 the working face. A two-station intercom 
 connects the top and bottom of the shaft. 
 Another intercom connects the hoisting 
 engineer and the mine office. Two spare 
 loudspeaking pagers serve as backup and 
 permit a third station to be patched in 
 if work is being done a long way from the 
 face. The hoisting engineer serves as 
 "communications central," tying the three 
 systems together. In addition to the 
 internal communication systems, an exten- 
 sion of the outside telephone line is 
 
188 
 
 located at the chief engineer's desk. 
 The pager at the face is kept mounted 
 near the power sled, so the two are moved 
 together. Nothing else is moved. 
 
 All equipment has been holding up 
 well. Perhaps twice a year, one of the 
 pagers will quit operating. Whenever 
 this happens, the bad unit is removed and 
 sent to a commercial repair station. 
 
 In normal system use, all calls are 
 made from a remote point to the hoisting 
 engineer "communication central." As 
 long as calls are being made in this man- 
 ner, the system functions well. A pos- 
 sible exception might occur in an emer- 
 gency situation at the face. The pager 
 at this point is 50 to 100 feet from the 
 nearest working room, and on the other 
 side of air-diverter flaps. It is con- 
 ceivable that an accident could occur in 
 which the phone would not be accessible. 
 The other possibility involving an acci- 
 dent situation would involve the phone 
 cable. There is a single run with no 
 backup or loopback path. This cable is, 
 however, protected in being mounted on 
 vertical timbers and is of armored 
 construction. 
 
 When calls are made from the "com- 
 munication central" position to other 
 parts of the mine, the system does not 
 work so well. A complaint was made that 
 if the superintendent leaves the bottom 
 of the hoist it may take a half hour to 
 get a message to him. It appears very 
 unlikely that a call to the pager at the 
 face would find anyone near enough to 
 hear it. The fastest route to the face 
 appears to involve relaying a message to 
 a motorman at the hoist and having him 
 deliver it to the face when he returns. 
 
 In this mine, communications ef- 
 ficiency would be improved by replac- 
 ing the three independent two-station 
 phone systems with a single multistation. 
 
 multichannel system. The system should 
 have a minimum of seven stations. 
 
 Their locations would be as follows: 
 
 Mine office 
 
 Hoisting engineer's position 
 
 Shaft bottom 
 
 Working face 
 
 Bottom of emergency shaft 
 
 Midway between bottom and face along 
 inbound haulageway 
 
 Midway between bottom and face along 
 outbound haulageway. 
 
 Other stations that might be con- 
 sidered include — 
 
 Topside storage or shop area 
 
 Chief engineer's desk 
 
 Crusher 
 
 Most of the added stations would be 
 concerned more with safety than with 
 production. As things stand, it is 
 possible to be blocked from a phone sta- 
 tion or to be a long walk from one. In 
 addition to the fixed stations, the man- 
 agement personnel should have radio com- 
 municators. This would eliminate the 
 existing situation in which a critical 
 man can be out of touch when others need 
 a decision or information. 
 
 An expanded system needs no more 
 than two general channels plus a private 
 channel. Radio communicators operate 
 best if they can access all three chan- 
 nels, but could operate with access only 
 to one of the general channels. 
 
189 
 
 The cable into the mine should be a 
 continuous loop of armored wire for maxi- 
 mum reliability and protection. By using 
 a multiplex system, all channels plus 
 monitors could be included on a single 
 cable. 1 
 
 A. 8 Mine G 
 
 Mines that do not employ rail haul- 
 age systems powered from a trolley wire 
 face unique problems in establishing sat- 
 isfactory communications between haulage- 
 way vehicles. Because common trolley 
 carrier phones cannot be utilized, some 
 other form of radio system must be used 
 to establish the required voice link 
 
 ^Approved and nonapproved systems may 
 not share the same cable; check with MSHA 
 for details. 
 
 between motormen and/or motormen and a 
 central dispatcher. This description 
 illustrates how one mine in this category 
 solved its haulageway communication 
 requirements using a unique system of UHF 
 and VHF repeaters combined with a "leaky 
 coaxial" transmission line. 
 
 This mine was a magnetite ore block 
 caving operation. Surface access to the 
 Mine (fig. A-2) was by two vertical 
 shafts to the No. 6 production level, 
 2,500 feet, with mining occurring at a 
 depth of 2,500 to 2,800 feet. Diesel- 
 powered, rubber-tired loading equipment 
 was used to transport ore to the crush- 
 ers. A conveyor belt ran between the 
 crusher rooms and the ore skip storage 
 bins where the ore was automatically 
 loaded into 20-ton skip cars and hoisted 
 to the surface. 
 
 No. 2 crusher 
 
 Fan hole drill operator 
 
 No. I crusher 
 
 Shop 
 
 °^*"'' 4 shaf? 
 
 B shaft 
 
 ; 
 
 FIGURE A-2. - Underground map of mine, 6th level 
 
190 
 
 Personnel underground included rov- 
 ing miners in production, haulage, and 
 shop areas, fan-hole drill operators 
 working alone, and maintenance vehicle 
 and ambulance operators. To satisfy the 
 objective to communicate between these 
 personnel and the surface, a guided wire- 
 less communication system utilizing 
 equipment available from Motorola, Inc. , 
 and Andrew Corp. was selected. Portable 
 HT-220 radios and industrial dispatcher 
 mobile transceivers were chosen for 
 personnel and vehicles, respectively. 
 Andrew Radiax cable, a special type of 
 cable that allows for leakage of signals 
 out of and into itself, was installed 
 throughout the major areas of the mine. 
 Because the total cable length exceeded 
 2 miles, it was necessary to install two 
 repeaters. Although the system did not 
 require a dispatcher or an operator, a 
 communications center was established at 
 an underground crusher room. Personnel 
 could be selectively paged from the con- 
 sole, and an evacuation alarm could be 
 activated from either the console or an 
 alternate monitor station at the shaft 
 bottom. The monitor station was wired to 
 the surface where a remote control unit 
 provided surface access to the system. 
 During a power failure, the system would 
 operate for 24 hours on backup battery 
 
 power. A telltale beep in the system 
 signaled that the system had reverted to 
 emergency power. The communication sys- 
 tem utilized off-the-shelf, readily 
 available communications equipment and 
 installation hardware. In addition, the 
 system was compatible with the installa- 
 tion and maintenance capabilities of the 
 mine personnel. 
 
 Figure A-3 shows the extent of the 
 radiating cable installation. There was 
 11,000 feet of RX5-1R 7/8-inch Andrew 
 Radiax in the system. The cable attenua- 
 tion was 1.2 dB/100 ft; thus, two re- 
 peaters were required to compensate for 
 signal loss as well as provide adequate 
 power levels for future system expansion. 
 
 The cable specifications state that 
 it must be supported every 5 feet. To 
 avoid installing 2,000 anchors in the 
 rock, a 3/16-inch steel messenger wire 
 was attached at 20-foot intervals to 
 roof -bolt-supported T-bars. The cable 
 was then strapped to the messenger wire 
 with standard cable ties. 
 
 Some areas were so far away from the 
 installed cable that radio transmissions 
 could not be established. This was over- 
 come by inserting a stub cable with one 
 
 3 — Disporcher console 
 ^epeorer A 
 
 LEGEND 
 i&^ Power divider 
 >- Antenna 
 O Repeater 
 A Fixed station 
 
 Scale, ft 
 
 Alrernare 
 
 control console 
 
 'I 
 
 Shaft A 
 
 FIGURE A-S. - Leaky feeder cable layout. 
 
191 
 
 end connected through a power divider to 
 the main cable; the other end terminated 
 in an antenna located within several hun- 
 dred feet of the desired working area. 
 
 The repeaters, composed of a unique 
 combination of UHF and VHF units, were 
 bolted together and mounted on pallets 
 for ease of transport within the mine. 
 The UHF and VHF units were interconnected 
 by a squelch and audio interface. Vehi- 
 cular and personnel communications took 
 place over the leaky coaxial cable on the 
 UHF repeater frequencies, while the con- 
 trol between repeaters, located some 
 2,000 feet apart, used VHF over the same 
 cable. A 5-MHz UHF transmit and receive 
 frequency separation allowed connection 
 to the common Radiax through a duplexer. 
 
 The repeaters were prewired, and the 
 system was assembled on the surface where 
 it underwent several months of burn-in. 
 This procedure eliminated the frustra- 
 tions of troubleshooting and testing the 
 system underground. 
 
 The fan-hole drill operator, 
 equipped with a portable radio attached 
 either to his belt or to a chest pack as 
 he preferred, was also equipped with an 
 accessory noise-reducing earrauff and mi- 
 crophone. The receiver audio was routed 
 to small loudspeakers inside his ear pro- 
 tectors, while the microphone and push- 
 to-talk switch were installed in a simi- 
 lar "earmuff which the fan hole drill 
 operator placed over his mouth when he 
 wished to make a radio transmission. All 
 portable radios in the mine were equipped 
 with the provision to use an external 
 speaker microphone accessory so that the 
 radio need not be detached from the oper- 
 ator's belt and raised to his ear or 
 mouth. In noisy locations the use of a 
 noise-reducing speaker-microphone is a 
 necessity. 
 
 Two types of vehicular radios were 
 used. The Industrial Dispatcher had all 
 controls, the microphone plug, and 
 speaker located on the transceiver pack- 
 age; this necessitated locating the 
 transceiver within the vehicle operator's 
 reach, which is nearly impossible on some 
 
 mining vehicles. A better radio for this 
 application was the motorcycle version of 
 the Industrial Dispatcher. All controls 
 and the microphone plug were located on 
 the small, rugged loudspeaker enclosure. 
 The loudspeaker can be mounted in a 
 convenient location, and the larger 
 transceiver unit can be mounted in a more 
 protected location. The antennas are 
 either 1/4-wave whips or Sinclair low- 
 profile blade antennas. The radome 
 version of the blade antenna appears 
 to be the most suitable for mining 
 applications. 
 
 A dispatcher control console at the 
 No. 2 crusher could be either manned or 
 unmanned; no operator was necessary for 
 system operation. Paging could be initi- 
 ated from an encoder at the console to 
 send private messages to pager-equipped 
 radios. Equipping the fan-hole drill 
 operator with a pager-encoded radio pre- 
 vents the nuisance of his listening to 
 general system traffic. He would only 
 hear messages directed specifically to 
 him. The console also had the provision 
 for sending a warning signal to all 
 radios in the mine. This wailing siren- 
 like signal could be used for mine evacu- 
 ation in an emergency. 
 
 The alternate control station pro- 
 vided an additional access point to the 
 communications network. This station 
 monitored the activity of the fan-hole 
 drill operator. Also, this station was 
 connected by wire line to an intercom 
 unit in the surface guardhouse. The 
 guard could access the system through 
 remote control. This feature was espe- 
 cially desirable during maintenance peri- 
 ods when the underground stations are 
 unmanned or during a mine emergency, 
 to coordinate evacuation and rescue 
 operations. 
 
 The mine had outfitted an under- 
 ground radio shop with the necessary 
 service equipment to maintain the commu- 
 nications system. A full-time Federal 
 Communication Commission second-class 
 licensed radioman was trained in system 
 installation, operation, and maintenance. 
 Reliability of the UHF-VHF system was 
 
192 
 
 excellent with negligible downtime. This 
 wireless communications system demon- 
 strated that the objectives of personnel 
 and vehicular underground mine communi- 
 cations can be satisfied. Worker and 
 management acceptance of the system was 
 excellent. 
 
 A. 9 Mine H 
 
 To meet changing communication 
 requirements in many of its mines, one 
 utility company has installed a new mul- 
 tichannel mine dial-page phone system at 
 its underground operations. The first of 
 its type, this fully permissible comiaini- 
 cation system combines the paging capa- 
 bility of current mine page phones with 
 the advantages of conventional tele- 
 phones. Manufactured by GAI-Tronics 
 Corp. , the Mine Dial-Page Phone System 
 (MDP) has the following features: 
 
 1. Each underground station is on a 
 separate circuit ready for instant use 
 depending upon the availability of an 
 open channel in the central switch. 
 
 2. When connected to a telephone 
 switchboard through a 12- to 48-volt in- 
 terface circuitry card provided for each 
 line, any underground station can call 
 another underground station directly, or 
 call any standard telephone at a surface 
 location. Also, any underground station 
 can be called from any aboveground stan- 
 dard telephone. 
 
 3. Selective paging capability to 
 any single, specific underground station. 
 
 4. A dial-access, all-station pag- 
 ing capability to call personnel not at 
 their normal location, or to alert all 
 underground personnel. 
 
 5. Automatic switching to a push- 
 button-operated, all-page-partyline mode 
 in the event of a telephone switchboard 
 power failure or severance of the cable 
 interconnecting the switchboard and the 
 interface cabinet, one of the key compon- 
 ents of the MDP system. 
 
 6. Plug-in electronic assem- 
 blies, wherever possible, to facilitate 
 
 maintenance and adaptation to changes in 
 mine operations. 
 
 The MDP system (fig. A-4) consists 
 of individual phone stations placed at 
 selected sites within the mine, one or 
 more interface cabinets located on the 
 surface, a telephone switchboard, and the 
 necessary raulticonductor interconnecting 
 cable. (One pair of conductors is 
 required for each private line.) 
 
 Each phone station is contained 
 within a bright yellow, molded polyester- 
 fiberglass-reinforced housing. This 
 material, coupled with the use of stain- 
 less steel hardware, gives a corrosion- 
 free enclosure. The station includes a 
 handset, a telephone dial, a loudspeaker, 
 an all-solid-state plug-in amplifier, and 
 a self-contained battery of the standard 
 12-volt mine page phone type. 
 
 In addition, since some power for 
 system operations is supplied from 
 the surface, the mine phones are designed 
 to have individual power to permit 
 emergency communications in the event of 
 a power cutoff. This is accomplished 
 by a standard 12-volt phone battery, 
 while the surface equipment is provided 
 with a 12-volt rechargeable battery, re- 
 quired only in the event that volt- 
 age to the dc power supply should be 
 lost. 
 
 Located outside each cable entry 
 into the mine is an interface cabinet. 
 The circuitry that converts the telephone 
 switchboard voltages (ac and dc) to per- 
 missible levels is contained in this 
 cabinet on a separate plug-in inter- 
 face card for each telephone line. 
 The cabinet also contains the 12-volt 
 rechargeable battery. This battery auto- 
 matically powers the system if there is a 
 power failure at the switchboard or in 
 the connecting cable between the switch- 
 board and the cabinet. 
 
 The switchboard itself is an impor- 
 tant link in the MDP operation. 
 
 This mine's initial installation 
 uses a private automatic branch exchange 
 provided and installed by the local 
 
193 
 
 FIGURE A.4. - Typical mine dial-page phone system. 
 
194 
 
 telephone company in the main office 
 building at the mining operation. 
 
 An incoming call triggers the inter- 
 face card circuitry in the cabinet, which 
 begins with the activation of a timed 
 holding circuit that completes the dc 
 loop of the telephone line and halts the 
 incoming ringing signal. The timing cir- 
 cuit holds the line for approximately 
 40 seconds and initiates additional cir- 
 cuitry which produces a distinctive 
 warble ring tone on the appropriate MDP 
 phone. The ring tone is applied for a 
 4-second period, and the balance of the 
 40 seconds is held for the calling party 
 to page a specific person or make an 
 announcement. At the end of this period, 
 if the called station has not answered, 
 the lines are automatically disconnected. 
 
 When the station answers before the 
 end of the 40-second hold period, the 
 timing circuitry is returned to its 
 original standby state and the loud- 
 speaker is muted. The party called 
 responds by taking the phone handset from 
 its holder and squeezing a press bar 
 located in the center of the handle. 
 Holding of the telephone line is accom- 
 plished by a circuit not associated with 
 the timing circuit, and the connection is 
 held as long as both parties are pressing 
 their respective press bars. 
 
 For outgoing calls, the user of the 
 MDP phone simply removes the handset from 
 the holder and presses the press bar. 
 When the familiar dial tone is heard, he 
 can dial his call. Release of the 
 press bar terminates the connection. A 
 delay circuit is provided to maintain 
 the line connection during any brief 
 (2-second maximum) release of the press 
 bar, such as to change hands. 
 
 The aforementioned procedures allow 
 one party to call another at a specific 
 location. A separate feature is provided 
 to page a person when his location is 
 unknown. By dialing a special number, a 
 separate amplifier and electronic source 
 within the interface cabinet activate the 
 loudspeaker at each MDP station to pro- 
 vide one-way paging communication. Such 
 
 a page call will be heard in the handset 
 receiver by all parties engaged in calls 
 to, from, and between MDP phones, but it 
 will not interrupt these conditions; the 
 conversation can continue at the con- 
 clusion of the page. The person being 
 paged, however, must dial the person 
 initiating the page to carry on a regular 
 conversation. 
 
 The interface cabinet contains a 
 separate fail-safe system to maintain 
 conmiunications in the event of an acci- 
 dental disconnection of the cable between 
 the cabinet and the telephone switch- 
 board, or if there is a failure of the 
 switchboard's power. A second circuit 
 network, controlled by a switchboard mon- 
 itor, automatically ties all of the 
 interface cards together in the event of 
 such failure. In this mode, two-way pag- 
 ing and handset conversation can be 
 carried out in a manner similar to that 
 of presentday mine page phones. A push- 
 to-page button is provided for paging in 
 this mode, with each phone having its own 
 battery to provide power for both normal 
 and this alternate-mode operation. 
 
 Ability to dial outside calls — 
 including direct-dial long-distance 
 calls — and to receive similar calls is 
 limited only by the telephone switch- 
 board. That is, an MDP phone station can 
 be used to dial any telephone or receive 
 any incoming call that a conventional 
 telephone connected to the same line 
 could handle. 
 
 A. 10 Mine I 
 
 Mine Description 
 
 Mine I is a silver mine centrally 
 located in the Couer d'Alene mining dis- 
 trict in Idaho. The mine was first 
 opened in 1884 and presently employs over 
 500 persons, 400 of whom work under- 
 ground. Main access to the mine is 
 through a 200-foot-long adit to shaft A 
 at the western edge of the mine. A miner 
 proceeds down that shaft to the 3100 and 
 3700 levels and then eastward through 
 5,000-foot-long drifts to shaft B, which 
 is collared at the 3,100-foot level. 
 
195 
 
 Miners must then go down that shaft to 
 the active working levels (fig. A-5) . 
 
 Shaft B is bottomed just below the 
 6,000-foot level. Ore is being produced 
 on the 4000, 4200, 4400, 4600, 4800, 
 4000, 5200, and 5400 levels. Level 
 development is in progress on the 
 5600 level, and shaft station development 
 is in progress on the 5800 level. 
 
 The A and B shafts are each provided 
 with electric-powered double-drum hoists 
 and electric-powered single-drxim chippy 
 hoists. The double-drum hoists on both 
 shafts are used primarily for hauling ore 
 and waste materials. The chippy hoist on 
 shaft A is used for moving men and 
 materials to all levels as far down as 
 the 4000 level and for hoisting ore from 
 the 4000 level to the 3100 level. The 
 shaft B chippy hoist is on the 3700 level 
 
 
 
 < 
 < 
 
 X 
 
 
 -J 
 
 g 
 
 1900 
 
 
 
 
 L 
 
 
 
 
 
 il 
 
 
 3100 
 
 3000 
 
 / 
 
 
 
 
 / 
 
 
 
 y 
 
 
 
 
 
 I 
 'J 
 
 d 
 
 
 
 
 
 
 3400 
 
 
 
 gl 
 
 
 
 
 
 3700 
 
 
 
 y 
 
 
 
 
 
 3700 
 
 y 
 
 
 
 
 
 
 
 
 
 
 
 4000 
 
 
 
 
 
 
 40O0 
 
 
 . 
 
 
 
 
 
 
 4200 
 
 O 
 
 I 
 
 
 
 
 
 
 i 
 
 IT 
 O 
 
 
 
 
 
 
 
 
 1 4400 
 
 ^ 
 
 
 
 
 
 
 O 
 
 z 
 
 
 
 
 
 
 
 4600 
 
 4800 
 
 
 
 
 
 
 4800 
 
 
 
 
 
 
 5000 
 
 
 1 
 
 
 
 
 5200 
 
 
 
 
 
 5400 
 
 LEGEND 
 
 
 
 
 
 I ■ ESCAPE ROUTES 
 
 
 
 
 
 anno best wanway 
 
 
 
 _5600 
 
 
 
 
 
 
 
 
 
 
 
 
 
 z 
 
 _5800 
 
 
 
 
 FIGURE A-5. - Mine I, mine map. 
 
 and is equipped with a four-deck man cage 
 with a total capacity of 48 men. It 
 is used for servicing all levels below 
 3700. 
 
 Airflow for the mine is dependent 
 upon pressures developed by fans located 
 underground (series ventilation). All of 
 the intake air for ventilation of the 
 mine is coursed down shaft A to the 3100 
 and 3700 levels. The air is split 
 between these two levels and travels lat- 
 erally to shaft B. The air is then 
 forced down the shaft B to the lower lev- 
 els. The return air flows back through 
 ventilation raises and exhaust airways to 
 the surface. 
 
 The ore deposits occur as long, gen- 
 erally narrow veins containing sulfides 
 of silver, copper, lead, and antimony in 
 a carbonate quartz gangue. The vein dips 
 range from 45° to 90° and are generally 
 to the south. Strike lengths on the 
 major ore shoots range up to a known max- 
 imum of 2,200 feet and are normally ex- 
 ceeded twofold or threefold vertically 
 along the dip of the structure. The 
 true vein width varies considerably 
 but generally averages between 2 and 
 5 feet. 
 
 The steeply dipping fissure veins 
 are mined by the horizontal cut and sand- 
 fill method by either breasting down or 
 back stoping. Stopes are developed a 
 maximum of 100 feet along the strike of 
 the vein. Level intervals are 200 feet. 
 A raise climber is used to drive the 
 6- by 6-foot raises between levels. 
 
 All underground transportation is 
 accomplished using either the hoists or 
 battery-powered locomotives on narrow- 
 gage tracks. The mined ore is trans- 
 ported to a muck pocket on the associated 
 haulage level. This ore, or muck as it 
 is called, is then dumped onto the 
 shaft B hoist skips and transported to 
 the 3100 level. The muck is then trans- 
 ported by locomotive to shaft A and 
 hoisted 3,100 feet to the headframe stor- 
 age bins. 
 
 Surface facilities include an office 
 area, warehouse electric shop, machine 
 shop, hoist and compressor house, garage. 
 
196 
 
 carpenter shop, mine and mill changehouse 
 for employees, dispensary, and tailing 
 ponds. Engineering personnel are also 
 located at the mine to provide facility 
 planning and better control progress of 
 the mining operations. 
 
 Present Mine Communications 
 
 The telephone permissibility re- 
 quirements are not applicable to metal 
 and nonmetal mines such as this mine. 
 A high-dc voltage on the carrier pair, 
 although a potential safety problem, is 
 of much less severity in a metal or non- 
 metal mine. Therefore, an Anaconda S6A 
 system was installed to provide telephone 
 service underground. 
 
 The Anaconda S6A is a seven-channel, 
 frequency division multiplex system. The 
 following items are worth noting in 
 regard to its suitability for mine 
 applications : 
 
 1. The system provides a suitable 
 number of channels (seven) on a single 
 wire pair. 
 
 2. The mechanical and environmental 
 specifications indicate the ability to 
 operate under the severe conditions found 
 in the mine. 
 
 3. The system allows branches to 
 individual conventional dial telephones 
 at any point on the system. 
 
 4. Remote units (at the telephones) 
 have batteries that are trickle-charged 
 over the wire pair. This enables the 
 system to be freestanding and not con- 
 nected to 110-volt power underground. 
 
 5. Carrier levels require no 
 adjustment, as the system has automatic 
 gain control circuitry. 
 
 The Anaconda S6A system is designed 
 to interface a central office at one end 
 and conventional telephones at the other. 
 It was designed as a transparent sub- 
 stitute for copper pairs connecting the 
 telephone company office to subscriber 
 telephones. To perform its signaling 
 
 functions, the system receives central 
 office signals at one end (such as the 
 ringing voltage generated by the central 
 office to ring the telephones) and repro- 
 duces them at the other end (it remotely 
 generates ringing voltages to ring the 
 bells as needed). Conversely, the system 
 can receive only dial pulses from the 
 telephone end, which it passes to the 
 central office. When used in this way, 
 the system is a transparent substitute 
 for copper pairs; that is, users cannot 
 tell whether the S6A system or copper 
 pairs are being used. 
 
 The central switching function of 
 the phone system is handled by a small 
 private branch exchange. System require- 
 ments were carefully examined before 
 choosing a location for this PBX. A 
 spare single twisted pair was available 
 from the shaft A surface to deep within 
 the mine. Any additional wiring in the 
 shafts was to be avoided. An air- 
 conditioned room was available in the 
 shaft B area at the 3,700-foot level that 
 met all environmental requirements of the 
 PBX. Additionally, this location was 
 approximately centered with respect to 
 the number of telephones desired in the 
 system. The single twisted pair was 
 opened at this point, thereby forming two 
 independent wire pairs. Carrier termi- 
 nals were then installed on each pair, 
 and these two independent carrier systems 
 were then connected to the PBX circuits. 
 This provided up to 14 private channels 
 for communication within the mine. One 
 channel in each carrier system was desig- 
 nated for use in a monitor-control sys- 
 tem. Of the remaining 12 channels, 5 in 
 each carrier system are used to connect 
 phones to the PBX, and the additional 
 channel is reserved as a spare. Addi- 
 tional phones for critical locations and 
 functions in the 3700 level shaft B area 
 are directly connected to PBX line cir- 
 cuits to provide them with private line 
 service. This minimizes the possibility 
 of getting a busy signal for these 
 phones . 
 
 Each phone has battery backup that 
 will allow operation for 24 hours. 
 
APPENDIX B.— FEDERAL REGULATIONS 
 
 197 
 
 The following sections of the U.S. 
 Code of Federal Regulations, Title 30, 
 Mineral Resources, Chapter 1 — Mine Safety 
 and Health Administration are presented 
 to assist planners of communication sys- 
 tems in insuring that all requirements 
 are being satisfied. 
 
 It should be noted that some States 
 have enacted laws that further regu- 
 late the use of communications, control, 
 and monitoring equipment in underground 
 mines. Check State and local regulations 
 before proceeding with the installation 
 of new or redesigned equipment. 
 
 PART 57— SAFETY AND HEALTH 
 STANDARDS— METAL AND NON- 
 METALLIC UNDERGROUND MINES 
 
 • 
 
 § 57.1 Purpose and scope. 
 
 The regulations in this part are pro- 
 mulgated pursuant to section 6 of the 
 Federal Metal and Nonmetallic Mine 
 Safety Act (30 U.S.C. 725) and pre- 
 scribe health and safety standards for 
 the purpose of the protection of life, 
 the promotion of health and safety, 
 and the prevention of accidents in un- 
 derground metal and nonmetallic 
 mines which are subject to that Act. 
 Each standard which is preceded by 
 the word "Mandatory" is a mandatory 
 standard. The violation of a manda- 
 tory standard will subject an operator 
 to an order or notice under section 8 
 of the Act (30 U.S.C. 727). Those regu- 
 lations in each subpart appearing 
 under the heading "General— Surface 
 and Underground" apply both to the 
 underground and surface operations of 
 underground mines; those appearing 
 under the heading "Surface Only" 
 apply only to the surface operations of 
 underground mines: those appearing 
 under the heading "Underground 
 Only" apply only to the underground 
 operations of underground mines. 
 
 57.11-54 Mandatory. Telephone or other 
 voice communication shall be provided be- 
 tween the surface and refuge chambers and 
 such systems shall be independent of the 
 mine power supply. 
 
 i 57.18 Safety programs. 
 
 General— Surface and Underground 
 
 57.18-12 Mandatory. Emergency tele- 
 phone numbers shall be posted at appropri- 
 ate telephones. 
 
 57.18-13 Mandatory. A suitable coimnu- 
 nlcation system shall be provided at the 
 mine to obtain assistance in the event of an 
 emergency. 
 
 § 57.19 Man hoisting. 
 
 1 57.11 Travelways and escapeways. 
 Travelways 
 general— surface and underground 
 
 Hoisting Procedures 
 
 57.19-55 Mandatory. When a manually 
 operated hoist is used, a qualified hoistman 
 shall remain within hearing of the tele- 
 phone or signal device at all times while any 
 person is underground. 
 
198 
 
 Signaling 
 
 57.19-90 Mandatory. There shall be at 
 least two effective approved methods of sig- 
 naling between each of the shaft stations 
 and the hoist room, one of which shall be a 
 telephone or speaking tube. 
 
 57.19-91 Mandatory. Hoist operators 
 shall accept hoisting instructions only by 
 the regular signaling system unless it is out 
 of order. In such an event, and during other 
 emergencies, the hoist operator shall accept 
 instructions to direct movement of the con- 
 veyances only from authorized persons. 
 
 57.19-92 Mandatory. A method shall be 
 provided to signal the hoist operator from 
 cages or other conveyances at any point in 
 the shaft. 
 
 57.19-93 Mandatory. A standard code of 
 hoisting signals shall be adopted and used 
 at each mine. The movement of a shaft con- 
 veyance on a "one bell" signal is prohibited. 
 
 57.19-94 Mandatory. A legible signal code 
 shall be posted prominently In the hoist 
 house within easy view of the hoistmen, and 
 at each place where signals are given or re- 
 ceived. 
 
 57.19-95 Mandatory. Hoisting signal de- 
 vices shall be positioned within easy reach 
 of persons on the shaft bottom or constant- 
 ly attended by a person stationed on the 
 lower deck of the siniing platform. 
 
 57.19-96 Mandatory. Any person respon- 
 sible for receiving or giving signals for cages, 
 skips, and mantrips when men or materials 
 are being transported shall be familiar with 
 the posted signaling code. 
 
 Mine Classification 
 
 57.21-1 Mandatory. A mine shall be 
 deemed gassy, and thereafter operated as a 
 gassy mine, if: 
 
 (a) The State in which the mine is located 
 classifies the mine as gassy; or 
 
 (b) Flammable gas emanating from the 
 orebody or the strata surrounding the ore- 
 body has been ignited in the mine; or 
 
 (c) A concentration of 0.25 percent or 
 more, by air analysis, of flammable gas ema- 
 nating only from the orebody or the strata 
 surrounding the orebody has been detected 
 not less than 12 inches from the back, face, 
 or ribs in any open workings; or 
 
 (d) The mine is connected to a gassy mine. 
 
 57.21-2 Mandatory. Flammable gases de- 
 tected only while unwatering mines or 
 flooded sections of mines or during other 
 mine reclamation operations shall not be 
 used to permanently classify a mine gassy. 
 During such periods that any flammable gas 
 is present in the mine, the affected areas of 
 the mine shall be operated in accordance 
 with appropriate standards in this Section 
 57.21. 
 
 Ventilation 
 57.21-20 Mandatory. Main fans shall be: 
 
 § 57.20 Miscellaneous. 
 
 General— ScitPACE and Underground 
 
 (f) Provided with an automatic signal 
 device to give warning or alarm should the 
 fan system malfunction. The signal device 
 shall be so located that it can be seen or 
 heard by a responsible person at all times 
 when persons are underground. 
 
 57.20-32 Mandatory. Telephones or other 
 two-way communication equipment with 
 instructions for their use shall be provided 
 for communication from underground oper- 
 ations to the surface. 
 
 [34 FR 12517, July 31, 1969, as amended at 
 35 FR 3677, Feb. 25, 1970; 42 FR 29424, June 
 8, 1977; 42 FR 57044, Oct. 31, 1977; 44 FR 
 31919, June 1, 1979; 44 FR 48535, Aug. 17, 
 1979] 
 
 § 57.21 Gassy mines. 
 
 Gassy mines shall be operated in accord- 
 ance with all mandatory standards in this 
 part. Such mines shall also be operated in 
 accordance with the mandatory standards 
 in this section. The standards in this section 
 apply only to underground operations. 
 
 57.21-29 Mandatory. Booster fans shall 
 be: 
 
 (a) Provided with an automatic signal 
 device to give warning or alarm should the 
 fan system malfunction. The signal device 
 shall be so located that it can be seen or 
 heard by a responsible person at all times 
 when persons are underground. 
 
 (b) Equipped with a device that automati- 
 cally deenergizes the power in affected 
 active workings should the fan system mal- 
 function. 
 
 (c) Provided with air locks, the doors of 
 which open automatically should the fan 
 stop. 
 
 (d) Equipped with two sets of controls ca- 
 pable of starting, stopping, and reversing, 
 the fans. One set of controls shall be located 
 at the fans. A second set of controls shall be 
 at another location remote from the fans. 
 
199 
 
 PART 75— MANDATORY SAFETY 
 STANDARDS— UNDERGROUND 
 COAL MrNES 
 
 § 75.321 Stoppage of fans, plans. 
 
 [Statutory Provisions] 
 
 Each operator shall adopt a plan on 
 or before May 29, 1970, which shall 
 provide that when any mine fan stops, 
 immediate action shall be taken by the 
 operator or his agent (a) to withdraw 
 all persons from the working sections, 
 (b) to cut off the power in the mine in 
 a timely manner, (c) to provide for res- 
 toration of power and resumption of 
 work if ventilation is restored within a 
 reasonable period as set forth in the 
 plan after the working places and 
 other active workings where methane 
 is likely to accumulate are reexamined 
 by a certified person to determine if 
 methane in amounts of 1.0 volume per 
 centum or more exists therein, and (d) 
 to provide for withdrawal of all per- 
 sons from the mine if ventilation 
 cannot be restored within such reason- 
 able time. The plan and revisions 
 thereof approved by the Secretary 
 shall be set out in printed form and a 
 copy shall be furnished to the Secre- 
 tary or his authorized representative. 
 
 §75.516-2 Communication wires and 
 cables; installation; insulation; support. 
 
 (a) All communication wires shall be 
 supported on insulated hangers or in- 
 sulated J-hooks. 
 
 (b) All communication cables shall 
 be insulated as required by § 75.517-1, 
 and shall either be supported on insu- 
 lated or uninsulated hangers or J- 
 hooks, or securely attached to messen- 
 ger wires, or buried, or otherwise pro- 
 tected against mechanical damage in a 
 manner approved by the Secretary or 
 his authorized representative. 
 
 (c) All communication wires and 
 cables installed in track entries shall, 
 except when a communication cable is 
 buried in accordance with paragraph 
 (b) of this section, be installed on the 
 side of the entry opposite to trolley 
 wires and trolley feeder wires. Addi- 
 tional insulation shall be provided for 
 communication circuits at points 
 where they pass over or under any 
 power conductor. 
 
 (d) For purposes of this section, com- 
 munication cable means two or more 
 insulated conductors covered by an ad- 
 ditional abrasion-resistant covering. 
 
 t38 FR 4975, Feb. 2.3, 1973] 
 
 § 75.517 Power wires and cables; insula- 
 tion and protection. 
 
 [Statutory Provisions] 
 
 Power wires and cables, except trol- 
 ley wires, trolley feeder wires, and 
 bare signal wires, shall be insulated 
 adequately and fully protected. 
 
 § 75.517-1 Power wires and cables; insula- 
 tion and protection. 
 
 Power wires and cables installed on 
 or after March 30, 1970, shall have in- 
 sulation with a dielectric strength at 
 least equal to the voltage of the cir- 
 cuit. 
 
 § 75.508-1 Mine tracks. 
 
 When mine track is used as a con- 
 ductor of a trolley system, the location 
 of such track shall be shown on the 
 map required by § 75.508, with a nota- 
 tion of the number of rails and the 
 size of such track expressed in pounds 
 per yard. 
 
 § 75.521 Lightning arresters; ungrounded 
 and exposed power conductors and 
 telephone wires. 
 
 Each ungrounded, exposed power 
 conductor and each ungrounded, ex- 
 posed telephone wire that leads under- 
 ground shall be equipped with suitable 
 lightning arresters of approved type 
 within 100 feet of the point where the 
 circuit enters the mine. Lightning ar- 
 resters shall be connected to a low 
 resistance grounding medium on the 
 surface which shall be separated from 
 neutral grounds by a distance of not 
 less than 25 feet. 
 
 [38 FR 4975, Feb. 23, 1973] 
 
200 
 
 §75.701-4 Grounding wires; capacity of 
 wires. 
 Where grounding wires are used to 
 ground metallic sheaths, armors, con- 
 duits, frames, casings, and other me- 
 tallic enclosures, such grounding wires 
 will be approved if: 
 
 (a) The cross-sectional area (size) of 
 the grounding wire is at least one-half 
 the cross-sectional area (size) of the 
 power conductor where the power con- 
 ductor used is No. 6 A.W.G., or larger. 
 
 (b) Where the power conductor used 
 is less than No. 6 A.W.G., the cross- 
 sectional area (size) of the grounding 
 wire is equal to the cross-sectional 
 area (size) of the power conductor. 
 
 § 75.1003-1 Other requirements for guard- 
 ing of trolley wires and trolley feeder 
 wires. 
 
 Adequate precaution shall be taken 
 to insure that equipment being moved 
 along haulageways will not come in 
 contact with trolley wires or trolley 
 feeder wires. 
 
 § 75.1003-2 Requirements for movement 
 of off-track mining equipment in areas 
 of active workings where energized 
 trolley wires or trolley feeder wires are 
 present; pre-movement requirements; 
 certified and qualified persons. 
 
 and such electric power can be sup- 
 plied only from inby the equipment 
 being moved or transported, power 
 may be supplied from inby such equip- 
 ment provided a miner with the means 
 to cut off the power, and in direct 
 communication with persons actually 
 engaged in the moving or transporting 
 operation, is stationed outby the 
 equipment being moved. 
 
 (2) The settings of automatic circuit 
 interrupting devices used to provide 
 short circuit protection for the trolley 
 circuit shall be reduced to not more 
 than one-half of the maximum cur- 
 rent that could flow if the equipment 
 being moved or transported were to 
 come into contact with the trolley 
 wire or trolley feeder wire; 
 
 (3) At all times the unit of equip- 
 ment is being moved or transported, a 
 miner shall be stationed at the first 
 automatic circuit breaker outby the 
 equipment being moved and such 
 miner shall be: (i) In direct commimi- 
 cation with persons actually engaged 
 in the moving or transporting oper- 
 ation, and (ii) capable of communicat- 
 ing with the responsible person on the 
 surface required to be on duty in ac- 
 cordance with § 75.1600-1 of this part; 
 
 (4) Where trolley phones are utilized 
 to satisfy the requirements of para- 
 graph (f)(3) of this section, telephones 
 or other equivalent two-way communi- 
 cation devices that can readily be con- 
 nected with the mine communication 
 system shall be carried by the miner 
 stationed at the first automatic circuit 
 breaker outby the equipment being 
 moved and by a miner actually en- 
 gaged in the moving or transporting 
 operation; and. 
 
 (f) A minimum vertical clearance of 
 12 inches shall be maintained between 
 the farthest projection of the unit of 
 equipment which is being moved and 
 the energized trolley wires or trolley 
 feeder wires at all times during the 
 movement or transportation of such 
 equipment; provided, however, that if 
 the height of the coal seam does not 
 permit 12 inches of vertical clearance 
 to be so maintained, the following ad- 
 ditional precautions shall be taken: 
 
 (l)(i) Except as provided in para- 
 graph (f)(l)(ii) of this section electric 
 power shall be supplied to the trolley 
 wires or trolley feeder wires only from 
 outby the unit of equipment being 
 moved or transported, (ii) Where 
 direct current electric power is used 
 
 § 75.1402 Communication between shaft 
 stations and hoist room. 
 
 [Statutory Provisions] 
 
 There shall be at least two effective 
 methods approved by the Secretary of 
 signaling between each of the shaft 
 stations and the hoist room, one of 
 which shall be a telephone or speaking 
 tube. 
 
 § 75.1402-1 Communication between shaft 
 stations and hoist room. 
 
 One of the methods used to commu- 
 nicate between shaft stations and the 
 hoist room shall give signals which can 
 be heard by the hoisting engineer at 
 all times while men are underground. 
 
201 
 
 § 75.1402-2 Tests of signaling systems. 
 
 Signaling systems used for conununi- 
 cation between shaft stations and the 
 hoist room shall be tested daily. 
 
 Subpart Q — Communications 
 
 § 75.1600 Communications. 
 
 [Statutory Provisions] 
 
 Telephone service or equivalent two- 
 way communication facilities, ap- 
 proved by the Secretary or his author- 
 ized representative, shall be provided 
 between the surface and each landing 
 of main shafts and slopes and between 
 the surface and each working section 
 of any coal mine that is more than 100 
 feet from a portal. 
 
 § 75.1600-1 Communication facilities; 
 
 main portals; installation require- 
 ments. 
 A telephone or equivalent two-way 
 communication facility shall be locat- 
 ed on the surface within 500 feet of all 
 main portals, and shall be installed 
 either in a building or in a box-like 
 structure designed to protect the facil- 
 ities from damage by inclement weath- 
 er. At least one of these commimica- 
 tion facilities shall be at a location 
 where a responsible person who is 
 always on duty when men are under- 
 ground can hear the facility and re- 
 spond immediately in the event of an 
 emergency. 
 
 [38 FR 29999, Oct. 31, 1973] 
 
 § 75.1600-2 Communication facilities; 
 
 working sections; installation and 
 maintenance requirements; audible or 
 visual alarms. 
 
 (a) Telephones or equivalent two- 
 way communication facilities provided 
 at each working section shall be locat- 
 ed not more than 500 feet outby the 
 last open crosscut and not more than 
 800 feet from the farthest point of 
 penetration of the working places on 
 such section. 
 
 (b) The incoming communication 
 signal shall activate an audible alarm, 
 distinguishable from the surrounding 
 noise level, or a visual alarm that can 
 be seen by a miner regularly employed 
 on the working section. 
 
 (c) If a communication system other 
 than telephones is used and its oper- 
 ation depends entirely upon power 
 from the mine electric system, means 
 shall be provided to permit continued 
 communication in the event the mine 
 electric power fails or Is cut off; pro- 
 vided, however, that where trolley 
 phones and telephones are both used, 
 an alternate source of power for the 
 trolley phone system is not required. 
 
 (d) Trolley phones connected to the 
 trolley wire shall be grounded in ac- 
 cordance with Subpart H of this part. 
 
 (e) Telephones or equivalent two- 
 way communication facilities shall be 
 maintained in good operating condi- 
 tion at all times. In the event of any 
 failure in the system that results in 
 loss of communication, repairs shall be 
 started immediately, and the system 
 restored to operating condition as soon 
 as possible. 
 
 [38 PR 29999, Oct. 31, 1973] 
 
 § 75.1713-2 Emergency communications; 
 requirements. 
 
 (a) Each operator of an underground 
 coal mine shall establish and maintain 
 a communication system from the 
 mine to the nearest point of medical 
 assistance for use in an emergency. 
 
 (b) The emergency communication 
 system required to be maintained 
 under paragraph (a) of this § 75.1713-2 
 may be established by telephone or 
 radio transmission or by any other 
 means of prompt commxinication to 
 any facility (for example, the local 
 sheriff, the State highway patrol, or 
 local hospital) which has available the 
 means of communication with the 
 person or persons providing emergen- 
 cy medical assistance or transporta- 
 tion in accordance with the provisions 
 of § 75.1713-1. 
 
202 
 
 APPENDIX C. —EQUIPMENT SUPPLIERS 
 
 Pager Phones (And Associated Equipment) 
 
 Appalachian Electronics 
 801 West Monroe Ave. 
 Ronceverte, WV 24970 
 
 ComTrol Corp. 
 500 Penna. Ave. 
 Irwin, PA 15642 
 
 CSE Mine Service Co. 
 600 Seco Rd. 
 Monroevllle, PA 15146 
 
 Fairmont Supply Co. 
 
 Box 501 
 
 Washington, PA 15301 
 
 FEMCO (See National Mine Service Co.) 
 
 Gal-Tronlcs Corp. 
 P.O. Box 31-T 
 Reading, PA 19603 
 
 Harrison R. Cooper Systems, Inc. 
 
 AME Box 22014 
 
 Salt Lake City, UT 84122 
 
 JABCO (See Schroeder Brothers Corp.) 
 
 Mine Safety Appliances Co. 
 600 Penn Center Blvd. 
 Pittsburgh, PA 15235 
 
 National Mine Service Co. 
 4900/600 Grant St. 
 Pittsburgh, PA 15219 
 
 Prelser/Mlneco 
 Jones & Oliver Sts. 
 St. Albans, WV 25177 
 
 Pyott-Bonne, Inc. 
 P.O. Box 809 
 Tazewell, VA 24651 
 
 Schroeder Brothers Corp. 
 
 Nlchol Ave. 
 
 Box 72 
 
 McKees Rocks, PA 15136 
 
 Wlnster Engineering Ltd. 
 Manners Ave. 
 Ilkeston, Derbyshire 
 United Kingdom 
 
 Carrier Phones 
 
 American Mine Research, Inc. 
 P.O. Box 1628 
 Bluefleld, WV 24701 
 
 ComTrol Corp. 
 500 Penna. Ave. 
 Irwin, PA 15642 
 
 CSE Mine Service Co. 
 600 Seco Rd. 
 Monroevllle, PA 15146 
 
 Fairmont Supply Co. 
 
 Box 501 
 
 Washington, PA 15301 
 
 FEMCO (See National Mine Service Co.) 
 
 Harrison R. Cooper Systems, Inc. 
 
 AMF Box 22014 
 
 Salt Lake, UT 84122 
 
 Mine Safety Appliances Co. 
 600 Penn Center Blvd. 
 Pittsburgh, PA 15235 
 
 National Mine Service Co. 
 4900/600 Grant St. 
 Pittsburgh, PA 15219 
 
 Hoist Communications 
 
 ComTrol Corp. 
 500 Penna. Ave. 
 Irwin, PA 15642 
 
 Fairmont Supply Co. 
 
 Box 501 
 
 Washington, PA 15301 
 
203 
 
 FEMCO (See National Mine Service Co.) 
 
 Harrison R. Cooper Systems, Inc. 
 
 AMF Box 22014 
 
 Salt Lake City, UT 84122 
 
 Mine Safety Appliances Co. 
 600 Penn Center Blvd. 
 Pittsburgh, PA 15235 
 
 National Mine Service Co. 
 4900/600 Grant St. 
 Pittsburgh, PA 15219 
 
 Republic Wire and Cable 
 P.O. Box 352 
 Flushing, NY 11352 
 
 Winster Engineering Ltd. 
 Manners Ave. 
 Ilkeston, Derbyshire 
 United Kingdom 
 
 PABX and Multiplex Equipment 
 
 Winster Engineering Ltd. 
 Manners Ave. 
 Ilkeston, Derbyshire 
 United Kingdom 
 
 Intercoms 
 
 ComTrol Corp. 
 500 Penna. Ave. 
 Irwin, PA 15642 
 
 Executone, Inc. 
 
 Dept. TR-77 
 
 Long Island City, NY 11101 
 
 FEMCO (See National Mine Service Co.). 
 
 Mine Safety Appliances Co. 
 600 Penn Center Blvd. 
 Pittsburgh, PA 15235 
 
 National Mine Service Co. 
 4900/600 Grant St. 
 Pittsburgh, PA 15219 
 
 Anaconda Telecommunications 
 305 North Muller 
 Anaheim, CA 92801 
 
 Essex Group 
 
 800 East Garfield Ave. 
 
 Decatur, IL 62525 
 
 Executone, Inc. 
 
 Dept. TR-77 
 
 Long Island City, NY 11101 
 
 Phelps Dodge Communication Co. 
 5 Corporate Park Dr. 
 White Plains, NY 10604 
 
 Pulsecom Div. 
 Harvey Hubbell, Inc. 
 5714 Columbia Pike 
 Falls Church, VA 22041 
 
 Reliable Electric Co. 
 11333 West Addison 
 Franklin Park, IL 60131 
 
 Til Industries, Inc. 
 100 North Strong Ave. 
 Lindenhurst, NY 11757 
 
 Winster Engineering Ltd. 
 Manners Ave. 
 Ilkeston, Derbyshire 
 United Kingdom 
 
 Radio Pocket Pagers 
 
 Executone, Inc. 
 
 Dept. TR-77 
 
 Long Island City, NY 11101 
 
 FEMCO (See National Mine 
 Service Co.) 
 
 General Electric Co. , Mobile Radio Dept. 
 P.O. Box 4197 
 Lynchburg, VA 24502 
 
 National Mine Service Co. 
 4900/600 Grant St. 
 Pittsburgh, PA 15219 
 
 Leaky Feeder Equipment 
 
 Andrew Corp. 
 
 10500 West 153d St. 
 
 Orland Park, IL 60462 
 
204 
 
 Winster Engineering Ltd. 
 Manners Ave. 
 Ilkeston, Derbyshire 
 United Kingdom 
 
 Mobile Radio Equipment 
 
 Fairmont Supply Co . 
 
 Box 501 
 
 Washington, PA 15301 
 
 General Electric Co., Mobile 
 
 Radio Dept. 
 P.O. Box 4197 
 Lynchburg, VA 24502 
 
 ComTrol Corp. 
 500 Penna. Ave. 
 Irwin, PA 15642 
 
 FEMCO (See National Mine Service Co.) 
 
 General Electric Co., Mobile Radio Dept, 
 P.O. Box 4197 
 Lynchburg, VA 24502 
 
 General Equipment & Manufacturing 
 
 Co. , Inc. 
 3300 Fern Valley Rd. 
 P.O. Box 13226 
 Louisville, KY 40213 
 
 Motorola Communications & Electronics 
 1301 East Algonquin Rd. 
 Schaumburg, IL 60196 
 
 Mag-Con, Inc. 
 1626 Terrace Dr. 
 St. Paul, MN 55113 
 
 Lee Engineering 
 
 2025 West Wisconsin Ave. 
 
 Milwaukee, WI 53201 
 
 Mine Safety Appliances Co. 
 600 Penn Center Blvd. 
 Pittsburgh, PA 15235 
 
 Phelps Dodge Communication Co. 
 5 Corporate Park Dr. 
 White Plains, NY 10604 
 
 National Mine Service Co. 
 4900/600 Grant St. 
 Pittsburgh, PA 15219 
 
 Winster Engineering Ltd. 
 Manners Ave. 
 Ilkeston, Derbyshire 
 United Kingdom 
 
 Closed Circuit Television 
 
 Midwest Telecommunications Div. , 
 Midwest Corp. 
 300 T First Ave. 
 Nitro, WV 25143 
 
 Winster Engineering Ltd. 
 Manners Ave. 
 Ilkeston, Derbyshire 
 United Kingdom 
 
 Remote Control and Monitor Equipment 
 
 American Mine Research, Inc. 
 P.O. Box 1628 
 Bluefield, WV 24701 
 
 BIF Accutel Inc. 
 1339 Lawrence Dr. 
 Newbury Park, CA 91320 
 
 Notifier of Western Penn. Inc. 
 3460 Babcock Blvd. 
 Pittsburgh, PA 15237 
 
 Pace Transducer Co., Div. of 
 
 C. J. Enterprises 
 P.O. Box 834 
 Tarzana, CA 91356 
 
 Pulsecom Div. 
 Harvey Hubbell, Inc. 
 5714 Columbia Pike 
 Falls Church, VA 22041 
 
 Py o 1 1 -Bonne , Inc . 
 P.O. Box 809 
 Taxewell, VA 24651 
 
 RFL Industries, Inc. 
 Boonton, NJ 07005 
 
 Stevens International Inc. 
 
 P.O. Box 619 
 
 Kennett Square, PA 19348 
 
205 
 
 Winster Engineering Ltd. 
 Manners Ave. 
 Ilkeston, Derbyshire 
 United Kingdom 
 
 Environmental Sensors 
 
 Methane: 
 
 Mine Safety Appliances Co. 
 201 North Braddock. Ave. 
 Pittsburgh, PA 15208 
 
 National Mine Service Co. 
 300 Koppers Bldg. 
 Pittsburgh, PA 15216 
 
 Bacharach Instrument Co. 
 625 Alpha Dr. 
 Pittsburgh, PA 15238 
 
 CSE Mine Service Co. 
 2000 Eldo Rd. 
 Monroeville, PA 15146 
 
 Preiser /Mine CO 
 
 Jones and Oliver Sts. 
 
 St. Albans, WV 25177 
 
 Appalachian Electronics Instruments 
 810 West Monroe Ave. 
 Ronceverte, VA 24970 
 
 American Mine Research, Inc. 
 P.O. Box 1628 
 Bluefield, WV 24701 
 
 Carbon monoxide : 
 
 Mine Safety Appliances 
 201 North Braddock Ave. 
 Pittsburgh, PA 15208 
 
 Edmont -Wilson 
 1300 Walnut St. 
 Coshocton, OH 43812 
 
 Mine Safety Appliances Co. 
 201 North Braddock Ave. 
 Pittsburgh, PA 15208 
 
 Survivair Div. of U.S. Divers 
 3323 West Warner Ave. 
 Santa Ana, CA 91776 
 
 Teledyne Analytical Instruments 
 
 333 West Mission 
 
 San Gabriel, CA 91776 
 
 Oxides of nitrogen: 
 
 Energetics Sciences 
 85 Executive Blvd. 
 Elmsford, NY 10523 
 
 Air flow sensors: 
 
 Alnor Instrument Co. 
 7301 North Caldwell Ave. 
 Niles, IL 60648 
 
 J-Tec Associates, Inc. 
 317 Seventh Ave. SE 
 Cedar Rapids, lA 52401 
 
 Taylor Instrument 
 Consumer Products Div. 
 Arden, NC 28704 
 
 Atmospheric pressure: 
 
 Pace Transducer Co., Div. of 
 
 C. J. Enterprises 
 P.O. Box 834 
 Tarzana, CA 91356 
 
 Energetics Sciences 
 85 Executive Blvd. 
 Elmsford, NY 10523 
 
 Leeds and Northrup Co. 
 
 Dept. MD337 
 
 North Wales, PA 19454 
 
 Oxygen : 
 
 Beckman Instruments Inc. 
 
 3900 River Rd. 
 
 Schiller Park, IL 60176 
 
206 
 
 Seismic Equipment 
 
 Pace Transducer Co., Dlv. of 
 
 C. J. Enterprises 
 P. 0. Box 834 
 Tarzana, CA 91356 
 
 Consultants 
 
 Arthur D. Little, Inc. 
 25 Acorn Park 
 Cambridge, MA 02140 
 
 Advance Mining Services 
 
 616 Beatty RD. Industrial Court 
 
 Monroeville, PA 15146 
 
 Pyott -Bonne, Inc. 
 P.O. Box 809 
 Tazewell, VA 24651 
 
 Corma Resources 
 2857 Mount Vernon SE 
 Cedar Rapids, lA 52403 
 
 U.S. Bureau of Mines 
 4800 Forbes Avenue 
 Pittsburgh, PA 15213 
 
 Winster Engineering Ltd. 
 Manners Avenue 
 Ilkeston, Derbyshire 
 United Kingdom 
 
 ComTrol Corp. 
 500 Penna. Ave. 
 Irwin, PA 15642 
 
 CSE Mine Service Co. 
 600 Seco Rd. 
 Monroeville, PA 15146 
 
 Fairmont Supply Co. 
 
 Box 501 
 
 Washington, PA 15301 
 
 General Electric Co., Mobile Radio Dept. 
 P.O. Box 4197 
 Lynchburg, VA 24502 
 
 Midwest Telecommunications Dlv. , 
 
 Midwest Corp. 
 300 T First Ave. 
 Nitro, WV 25143 
 
 Mineral Services Inc. 
 1276 West Third St. 
 Cleveland, OH 44113 
 
 National Coal Board 
 
 Mining Research and Development 
 
 Establishment 
 Stanhope Bretby 
 Burton Upon Trent DEISOQD 
 United Kingdom 
 
 Fire Detection Devices 
 
 ADT Co. , Inc. 
 
 155 Sixth Ave. 
 
 New York, NY 10013 
 
 The Ansul Co. 
 One Stanton St. 
 Marinette, WI 54143 
 
 B. & B. Electric Manufacturing Co. 
 Seward, PA 15954 
 
 Gammaflex Corp. 
 
 821 Michael Faraday Dr. 
 
 Res ton, VA 22070 
 
 JABCO 
 
 Schroeder Brothers Corp. 
 
 P.O. Box 72 
 
 Nlchol Ave. 
 
 McKees Rocks, PA 15136 
 
 McJunkin Corp. 
 P.O. Boc 2473 
 1400 Hansford St. 
 Charleston, WV 25311 
 
 Mine Safety Appliances Co. 
 201 North Braddock Ave. 
 Pittsburgh, PA 15203 
 
 National Mine Service Co. 
 4900/600 Grant St. 
 Pittsburgh, PA 15219 
 
 National Mine Service Co. 
 3000 Koppers Bldg. 
 436 Seventh Ave. 
 Pittsburgh, PA 15219 
 
207 
 
 Notifier of Western Pennsylvania 
 3283 Babcock Blvd. 
 Pittsburgh, PA 15237 
 
 Prieser 
 
 Jones and Oliver Sts. 
 
 St. Albans, WV 25177 
 
 Pyott-Boone, Inc. 
 P.O. Box 809 
 Tazewell, VA 24651 
 
 Southern Engineering and Equipment Co. 
 P.O. Drawer 329 
 95 Third St. , NE 
 Graysville, AL 35073 
 
 General Cable Corp. 
 600 Reed Rd. 
 Broomall, PA 19008 
 
 Industrial Component Inc. 
 
 342 Madison Ave. 
 
 Suite 702 
 
 New York, NY 10017 
 
 Okonite Co. 
 100 Hilltop Rd. 
 Ramsey, NJ 07446 
 
 Figure-8 Communication Cable 
 
 Delphi Wire & Cable 
 700 Carpenters Crossing 
 Folcroft, PA 19032 
 
208 
 
 APPENDIX D.— GLOSSARY OF TERMS 
 
 Analog 
 
 A method of generating or transmitting information that is repre- 
 sented by a continuous (as opposed to digital) voltage or current 
 that is proportional to the information. 
 
 Angstrom 
 
 A unit of length. Usually used to measure the wavelength of light 
 or other radiation. One angstrom is equal to one hundred-millionth 
 of a centimeter. 
 
 AM 
 
 Abbreviation for "amplitude modulation. " Modulation in which the 
 amplitude of the information waveform modulates the amplitude of a 
 carrier wave. 
 
 Attenuation 
 
 Balance point 
 
 Bandwidth 
 
 The decrease in signal strength during its transmission from one 
 point to another. Attenuation is usually expressed in decibels. 
 
 In an electronic bridge circuit, the point at which the electrical 
 resistances in both branches of the network are the same. 
 
 The difference (in cycles per second) between the highest and lowest 
 frequency components required for the adequate transmission of 
 information. 
 
 Baseband 
 
 The original frequency band (before modulation) of a signal. Usu- 
 ally refers to the baseband of an audio or voice signal, which is 
 approximately 300 to 5,000 Hz. 
 
 Binary 
 
 A digital numbering system with the base 2. In a binary system 
 there are only two possibilities for each digit, selection, choice, 
 or condition. For example, a simple switch is a binary device since 
 it is either open or closed. 
 
 Bridge 
 
 An electrical bridge circuit is a network arranged so that voltage 
 or current in one branch of the circuit may be measured by adjusting 
 components in another branch of the circuit. 
 
 CATV 
 
 Abbreviation for "community antenna television,' 
 cable television. 
 
 commonly known as 
 
 Characteristic 
 impedance 
 
 Pertaining to transmission lines. For a uniform and infinitely long 
 line, it is the ratio of applied voltage to current induced at a 
 given frequency. It is measured in ohms and usually designated as 
 Zo. For maximum signal transfer, the Zo of a line should equal the 
 Zo of a source and load. 
 
 CO Abbreviation for "central office." Refers to the telephone com- 
 
 pany's central office. 
 
 Cross talk Cross-coupling or interference between speech channels or wire 
 pairs. 
 
 dB 
 
 Abbreviation of "decibel," a unit that represents the ratio between 
 two amounts of power on a logarithmic scale. A value of +3 dB in- 
 dicates a doubling of power, while -3 dB is a halving of power. 
 
209 
 
 dBm 
 
 The normal signal level in a pager phone is about 1 milliwatt 
 (1 mW). The designation dBm is used to indicate this 1-mW refer- 
 ence level. Thus, +3 dBm is 3 dB above the reference (2 mW) and 
 -3 dBm is 3 dB below the reference (0.5 mW). 
 
 Demodulation 
 
 DTMF 
 
 A device that receives a carrier wave and recovers or "reconstructs" 
 the original voice or information signal from the carrier wave. 
 
 Abbreviation for "dual-tone multif requency ." A phone signaling 
 method in which each digit dialed is converted to a dual-tone signal 
 that will be recognized by the telephone office or PABX switching 
 equipment. These control tones can be heard in the earpiece when 
 dialing on many pushbutton phones. 
 
 Electromagnetic 
 Encoder 
 
 Having both electric and magnetic properties. 
 
 A unit that produces coded output combinations depending upon the 
 specific input selected. 
 
 FDM 
 
 Abbreviation for "frequency-division multiplexing." A process in 
 which two or more signals are sent over a common path by sending 
 each one in a different frequency band. 
 
 Feedback 
 
 In a transmission system, or electrical device, the returning of a 
 fraction of the output signal to the input. 
 
 FM 
 
 Abbreviation for "frequency modulation." Modulation in which the 
 amplitude of the information waveform modulates the frequency of a 
 carrier signal. 
 
 FSK 
 
 Abbreviation for "frequency-shift keying." A form of FM in which a 
 binary code is transmitted by switching a carrier signal between two 
 different frequencies. 
 
 Ge op hone 
 
 A device used to detect seismic vibrations or Shockwaves in the 
 earth. 
 
 Hall effect In a conductor located in a magnetic field that is perpendicular to 
 the direction of current, the production of a voltage perpendicular 
 to both the current and the magnetic field. 
 
 Handset 
 Headset 
 
 A receiver-transmitter held by hand. 
 
 A receiver-transmitter that can be attached to the person to allow 
 "hands-free" operation. 
 
 Hybrid 
 
 A circuit or communications system that is made up of two or more 
 dissimilar systems. 
 
 Hz 
 
 Abbreviation for Hertz . A unit of frequency equal to 1 cycle per 
 second. 
 
 Impedance The total opposition (reactance plus resistance) that a circuit or 
 transmission line offers to the flow of electrical current. 
 
210 
 
 Inductively 
 coupled 
 
 Joule heating 
 Leaky feeder 
 
 Method of inducing a signal into one conductor or wire from another 
 conductor even though there may be no mechanical connection between 
 the two conductors. (The magnetic field set up in the space around 
 a conductor carrying alternating current will induce a signal in 
 other nearby conductors.) 
 
 In an electrical circuit, the heat produced by the flow of current 
 in the circuit. 
 
 A specially designed coaxial cable that allows radio signals to leak 
 into or out of the cable so that they may be picked up by radio 
 transceivers. 
 
 LED 
 
 Magnetic field 
 
 Magneto 
 
 Milliammeter 
 
 Modem 
 
 Modulator 
 Monochromatic 
 Multiplexed 
 PABX 
 
 PAM 
 
 Parasitic 
 coupling 
 
 PBX 
 
 PCM 
 
 Abbreviation for "light-emitting diode." A solid state electronic 
 device that emits light when a current flows through it. 
 
 The region surrounding a magnet or a conductor through which current 
 is flowing. 
 
 An ac generator for producing ringing signals. 
 
 An electric current meter calibrated in milliamperes. 
 
 A device that is both a modulator and a demodulator. A modem is a 
 two-way device that both modulates (transmits) and receives (demodu- 
 lates) a signal. 
 
 A device that modulates a voice or information signal and transmits 
 the resulting carrier wave. 
 
 A signal or beam of light consisting of a single wavelength or of a 
 very small range of wavelengths. 
 
 The simultaneous transmission of two or more signals using a single 
 transmission path or wire. 
 
 Abbreviation for "private automatic branch exchange." A private 
 branch exchange in which automatically controlled switches make con- 
 nections between the phones in the system. 
 
 Abbreviation for "pulse amplitude modulation." Modulation in which 
 the value or amplitude of each sample of the information waveform 
 modulates the amplitude of a pulse carrier. 
 
 The coupling of radio waves or electrical signals from one wire or 
 medium to another with the result that the signal strength in the 
 first conductor is decreased. 
 
 Abbreviation for "private branch exchange. " A private manual tele- 
 phone exchange requiring an operator at a switchboard to make con- 
 nections between the phones. 
 
 Abbreviation for "pulse coded modulation." Modulation in which the 
 value or amplitude of each sample of the information waveform is 
 quantitized and transmitted as a digital binary code. 
 
211 
 
 PDM 
 
 Abbreviation for "pulse duration modulation." Modulation in which 
 the value or amplitude of each sample of the information waveform 
 modulates the duration, or "width," of a pulse. 
 
 Piezoelectric 
 
 The property of certain crystals or materials that produce a voltage 
 when subjected to mechanical stress. 
 
 Potentiometer 
 
 An electromechanical device with a sliding contact on a resistor. 
 Movement of the sliding contact changes the electrical resistance of 
 the circuit and allows the electronics to sense the position of the 
 sliding contact. 
 
 PPM 
 
 Abbreviation for "pulse position modulation." Modulation in which 
 the value or amplitude of each sample of the information waveform 
 modulates the position in time of a pulse. 
 
 Propagation The travel of electromagnetic (radio) or sound waves through a 
 medium. 
 
 PSK 
 
 Abbreviation for "phase shift keying." A form of FM in which a 
 binary code is transmitted by shifting the phase of a carrier 
 signal. 
 
 Q The "Q" of an ac circuit is the ratio of its reactance to its re- 
 sistance. The voltage developed across the reactance is usable sig- 
 nal, but the voltage developed across the resistance subtracts 
 from the signal. Thus, a high Q indicates an efficient, low-loss 
 ac circuit. 
 
 Reactance The opposition to the flow of alternating current (ac) . Capacitive 
 reactance (Xq) is the opposition offered by capacitors, and induc- 
 tive reactance (Xl) is the opposition offered by a coil or other 
 inductance. 
 
 rf 
 
 Abbreviation for radiof requency. Any frequency at which electromag- 
 netic radiation of energy (radio waves) is possible. 
 
 RFI Radio frequency interference. 
 
 Reluctance The resistance of a magnetic path to the flow of magnetic line of 
 force. Aluminum has a high reluctance; iron has a low reluctance. 
 
 Repeater 
 
 A device that detects or receives a signal and rebroadcasts the same 
 signal. 
 
 Resistance The opposition to the flow of direct current (dc) . The unit of re- 
 sistance is the ohm. 
 
 Resonate 
 Simplex 
 
 To bring to resonance; to tone. 
 
 A communication system, or other device, that operates in only one 
 direction (either transmit or receive) at a time. 
 
212 
 
 Sine wave 
 
 SWR 
 
 Synchronize 
 TDM 
 
 Transducer 
 
 Transceiver 
 
 Tuned voltmeter 
 
 UHF 
 
 Ultrasonic 
 
 vf 
 
 VHF 
 Vortex 
 
 Waveguide 
 
 The wave form corresponding to a pure, single-frequency 
 oscillation. 
 
 Abbreviation for "standing wave ratio." On a transmission line or 
 antenna element the current and voltage set up by waves traveling in 
 the opposite direction are characterized by the presence of a number 
 of stationary maximum and minimum points in the distribution curve. 
 SWR is the ratio of the maximum to minimum current or voltage of 
 these stationary waves. 
 
 To maintain one operation (or signal) in step with another. 
 
 Abbreviation for "time-division multiplexing." A process by which 
 two or more channels of information are transmitted over the same 
 link by allocating a different time interval for the transmission of 
 each channel. 
 
 A device that converts energy from one form to another. A seismic 
 transducer, for instance, converts seismic shock waves into elec- 
 trical signals. 
 
 A device that is both a transmitter and a receiver. A two-way CB 
 radio is a transceiver. 
 
 A voltmeter that has been tuned to detect voltage levels or signal 
 strengths at specific frequencies. 
 
 Ultra high frequency, 300 to 3,000 MHz 
 
 Having a frequency above that of audible sound. 
 
 Abbreviation for voice frequency (same as audio frequency). The 
 frequencies corresponding to speech or other audible sound wave. 
 
 Very high frequency, 30 to 300 MHz. 
 
 A whirlpool or eddy caused by a fluid or gas moving past an 
 obstruction. 
 
 A hollow, round or rectangular pipe (or tunnel), used as a trans- 
 mission line for signaling. 
 
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