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Bureau of Mines Information Circular/1985 
 
 An Overview of Research 
 on Explosionproof Enclosures 
 
 By L. W. Scott and MR. Yenchek 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
 T5 
 
 ^INES 75TH AV^ 
 
jw^^>2) 
 
 Information Circular 9051 
 
 An Overview of Research 
 on Explosionproof Enclosures 
 
 By L. W. Scott and MR. Yenchek 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
 Donald Paul Hodel, Secretary 
 
 BUREAU OF MINES 
 Robert C. Horton, Director 
 

 Library of Congress Cataloging in Publication Data: 
 
 Scott, Lawrence W 
 
 An overview of research on explosionproof enclosures. 
 
 (Information circular ; 9051) 
 
 Bibliography: p. 17-18. 
 
 Supt. of Docs, no.: I 28.27: 9051. 
 
 1. Mine explosions— Safety measures. 2. Coal mines and mining- 
 Safety measures. I. Yenchek, M. R. (Michael R.). II. Title. III. Se- 
 ries: Information circular (United States. Bureau of Mines) ; 9051. 
 
 TN295.U4 [TN313] 622s [622'. 334] 85-600164 
 
^ CONTENTS 
 
 \y~ Page 
 
 Abstract 
 
 Introduction 
 
 Electrical clearances 
 
 Reliability of enclosures with windows 
 
 Glass 
 
 Plastic 
 
 Safety factors in X-P enclosures 
 
 Potting materials used in X-P enclosures 
 
 ^ 
 
 ^ 
 
 
 1 
 
 2 
 
 2 
 
 3 
 
 4 
 
 4 
 
 4 
 
 6 
 
 6 
 
 6 
 
 12 
 
 13 
 
 14 
 
 14 
 
 16 
 
 17 
 
 17 
 
 Performance tests for X-P enclosures 
 
 Structural performance 
 
 Ruggedness 
 
 Effects of high-voltage on explosion-protection techniques 
 Innovative X-P devices 
 
 Pressure vent 
 
 Elastomeric grommet cable entry 
 
 Summary 
 
 References 
 
 ILLUSTRATIONS 
 
 1. Minimum arc voltage versus air-gap spacings of electrodes 3 
 
 2. Hydrostatic test apparatus 10 
 
 3. Recommended sealant pattern 11 
 
 4. Kinetic energy produced by roof falls at top of continuous-mining machine.. 12 
 
 5. Pressure vent 14 
 
 6. Pressure buildup in vented enclosures 16 
 
 7. Suggested guidelines for number of screens and allowable flange gaps for 
 
 vents and explosionproof electrical enclosures 16 
 
 8. Trailing-cable entry assembly 17 
 
 TABLES 
 
 1 . Summary of safety factors for four enclosures 5 
 
 2 . Arc-decomposition products 7 
 

 UNIT OF MEASURE 
 
 ABBREVIATIONS USED 
 
 IN THIS REPORT 
 
 °c 
 
 degree Celsius 
 
 
 
 J/c 
 
 joule per cycle 
 
 °F 
 
 degree Fahrenheit 
 
 
 
 kV 
 
 kilovolt 
 
 ft 
 
 foot 
 
 
 
 KE 
 
 kinetic energy 
 
 ft 3 
 
 cubic foot 
 
 
 
 lb 
 
 pound 
 
 ft-lb 
 
 foot pound 
 
 
 
 lb 2 
 
 square pound 
 
 ft-lb/ft 2 
 
 foot pound per square 
 
 foot 
 
 mm 
 
 millimeter 
 
 gal 
 
 gallon 
 
 
 
 mph 
 
 mile per hour 
 
 in 
 
 inch 
 
 
 
 mV 
 
 millivolt 
 
 in 2 
 
 square inch 
 
 
 
 psig 
 
 pound per square inch, gauge 
 
 in 3 
 
 cubic inch 
 
 
 
 V 
 
 volt 
 
 in/ft 
 
 inch per foot 
 
 
 
 V ac 
 
 volt, alternating current 
 
 in 2 /ft 3 
 
 square inch per cubic 
 
 foot 
 
 V dc 
 
 volt, direct current 
 
 J 
 
 joule 
 
 
 
 
 
AN OVERVIEW OF RESEARCH ON EXPLOSIONPROOF ENCLOSURES 
 
 By L. W. Scott ' and M. R. Yenchek 1 
 
 ABSTRACT 
 
 This report presents an overview of explosionproof (X-P) enclosure 
 research being conducted by the Bureau of Mines. This report empha- 
 sizes the increasing importance of research related to the safety con- 
 siderations of X-P enclosures used in underground coal mines. Selected 
 topics are included that summarize results of research in the areas of 
 electrical clearances, reliability of enclosures with windows, safety 
 factors in X-P enclosures, potting materials, performance tests for X-P 
 enclosures, protection against high-voltage explosions, and innovative 
 X-P devices. 
 
 Electrical engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA, 
 
INTRODUCTION 
 
 For several years, the Bureau of Mines, 
 through in-house and contract research, 
 analyzed several of the critical factors 
 involved in the design of X-P enclosures 
 (_1_).2 The study of X-P enclosures is an 
 area requiring investigation regarding 
 materials, methods of construction, and 
 acceptance and testing criteria. The X-P 
 enclosures used in underground mines are 
 constructed according to rigid design re- 
 quirements that contribute to the dif- 
 ficulties in maintaining the enclosures 
 in a permissible condition. If designers 
 were permitted more freedom, it would be 
 possible to construct enclosures that are 
 
 easier to maintain in a permissible con- 
 dition. These new enclosures would be 
 performance tested to ensure that they 
 would be as safe as the enclosures being 
 constructed to the design requirements. 
 
 The objective of this research is to 
 determine the safety factors in the pres- 
 ent design requirements and to develop 
 and demonstrate the feasibility of per- 
 formance tests. The progress presented 
 here should be of considerable inter- 
 est to the mining industry in general 
 and to designers of X-P enclosures in 
 particular. 
 
 
 ELECTRICAL CLEARANCES 
 
 It has been known for many years that 
 the flame front of a methane-air explo- 
 sion produces free ions that can cause a 
 current to flow when the front bridges 
 two conductors of opposite polarity. If 
 the voltage is high enough, this current 
 can increase to a value that would be 
 self-sustaining, and a dangerous arc dis- 
 charge can occur. Arc discharges pro- 
 duced in this way could cause premature 
 failure of electrical equipment, produce 
 excessive heating, or burn through an en- 
 closure wall. This study was initiated 
 to obtain experimental results that would 
 be useful in determining safe levels of 
 voltages and clearances required to pro- 
 tect X-P enclosures from failures due to 
 arc discharges. 
 
 The critical arcing voltages (single 
 phase, alternating current) for vari- 
 ous air gaps are shown in figure 1 (2). 
 Curve A shows the minimum arcing voltage 
 versus electrode spacing in air. This 
 curve was obtained by selecting the spac- 
 ing and increasing the voltage until arc- 
 ing occurred. Curve B shows the minimum 
 arcing voltage versus electrode spacing 
 in a 9.8% methane-air mixture. Because 
 it was necessary that an explosion occur 
 for each measurement, it was not possi- 
 ble to conduct the experiments as simply 
 
 5 
 
 ^Underlined numbers in parentheses re- 
 fer to items in the list of references at 
 the end of this report. 
 
 as those on arcing-in-air. For curve B, 
 again a spacing was selected and the 
 voltage was varied until arcing occurred. 
 However, at this point, the voltage was 
 decreased by 100 V, and the explosion 
 test repeated. If arcing occurred, the 
 voltage was decreased again by 100 V. 
 This procedure was continued until no 
 arcing was observed. At the level of 
 voltage where no arcing occurred, 10 
 tests were conducted. If no arcing was 
 observed, the previous level of voltage 
 was considered to be the minimum arcing 
 voltage. 
 
 The minimum arcing voltage was found to 
 increase with the distance between the 
 electrodes. It follows that the danger 
 of arcing can be decreased in X-P enclo- 
 sures by using larger air-gap spacing be- 
 tween live conductors of opposite polar- 
 ity and between live conductors and 
 earth. 
 
 Figure 1 is a plot of the actual mini- 
 mum arcing voltage versus air-gap spac- 
 ing. However, since these experiments 
 were conducted for design purposes, and 
 because of limited space for X-P enclo- 
 sures, it was decided that a factor of 
 1.5 should be applied to the spacing ob- 
 tained in figure 1, in order to provide 
 a reasonable margin of safety. For exam- 
 ple, curve B indicates that arcing will 
 occur at a spacing of 1.2 in (30 mm) if 
 5,000 V are impressed across the elec- 
 trodes in a 9.8% methane-air mixture 
 
ID 
 g 
 
 > 
 
 uj 
 
 o 
 > 
 
 - 
 _l 
 CL 
 Q. 
 < 
 
 Curve B 
 
 : 
 
 10 
 
 20 
 
 30 
 
 80 
 
 90 
 
 40 50 60 70 
 
 SPACING, mm 
 
 FIGURE 1. - Minimum arc voltage versus air-gap spacings of electrodes. 
 
 100 
 
 no 
 
 (compared to a spacing of 0.071 in (1.38 
 mm) for normal atmosphere, as shown in 
 curve A). Therefore, a spacing of 1.8 in 
 (45 mm) [0.106 in (2.07 mm) for curve A] 
 should provide adequate clearance for 
 this voltage. 
 
 All clearance data were obtained using 
 0.5-in (12.5-mm) diameter spherical brass 
 
 electrodes at room temperature and atmos- 
 pheric pressure, thus giving optimis- 
 tic values of clearances. The required 
 clearance for pointed electrodes are 
 considerably greater. Pointed termina- 
 tions should be avoided, and every effort 
 should be made to employ rounded termina- 
 tions with smooth surfaces. 
 
 RELIABILITY OF ENCLOSURES WITH WINDOWS 
 
 Windows in X-P enclosures require care- 
 ful design, fabrication, and installa- 
 tion. The designer must be certain that 
 the window or lens is adequate for the 
 design conditions of the enclosure, par- 
 ticularly the dynamic pressure, tem- 
 perature, point impact, thermal shock, 
 and corrosive effect of the underground 
 environment. 
 
 Windows and lenses should be fabricated 
 only from materials suited for the opera- 
 tional environment encountered in mines. 
 The suitability of a material is based 
 either on documented extensive past ex- 
 perience or on exhaustive evaluation by a 
 materials testing laboratory in simulated 
 mine environments. At present, glass and 
 polycarbonate plastic are considered the 
 
only practical materials for fabrication 
 of windows and lenses (3). 
 
 Glass 
 
 The chemical composition, casting pro- 
 cess, and thermal treatment determine the 
 physical, chemical, optical, and electri- 
 cal properties of glass. Because of a 
 very complex relationship among these 
 variables, no single glass composition, 
 casting process, or thermal treatment is 
 considered superior to others. Thus, the 
 designer is free to select the combi- 
 nation of fabrication parameters that 
 best matches a specific set of product 
 requirements. 
 
 The primary advantages of glass are its 
 ability to retain its physical and opti- 
 cal properties under high ambient temper- 
 ature, ultraviolet radiation, and humid- 
 ity for a long period of time; to resist 
 surface abrasion by rock particles; and 
 to tolerate immersion in aqueous and 
 organic solvents without initiation of 
 stress cracking or corrosion. Glass win- 
 dows can tolerate 100% relative humidity, 
 temperature of 400° F (204° C), intensive 
 ultraviolet radiation, and continuous or 
 intermittent immersion in basic or acidic 
 water or organic solvents for indefinite 
 periods. 
 
 The primary shortcomings of glass are 
 its brittleness and low tensile strength. 
 To compensate for these shortcomings, the 
 design of the window seat assembly must 
 provide, whenever feasible, protection 
 against point contact with the metallic 
 components of the enclosure and impact by 
 rock fragments capable of fracturing the 
 window. The protection against fracture 
 initiated by point contact is usually ac- 
 complished by inserting gaskets between 
 the glass and metallic components of the 
 
 seat assembly. Protection against break- 
 age by impact is generally provided by an 
 external shield in the form of a cage or 
 plastic envelope, or by precompressing 
 the glass window surfaces with thermal 
 tempering or chemical ion exchange. 
 
 Because of their history of success- 
 ful use in enclosures, the follow- 
 ing are practical for use as windows 
 in enclosures: (1) borosilicate glasses, 
 (2) soda lime glasses, and (3) silica 
 glasses. 
 
 Plastic 
 
 The high temperature, humidity, inten- 
 sive ultraviolet radiation, and presence 
 of vapors from petroleum-base oils tend 
 to rapidly degrade the mechanical proper- 
 ties of plastic windows and lenses in X-P 
 enclosures. Some plastics deteriorate in 
 lamp enclosure service faster than oth- 
 ers, but even the most resistant ones age 
 sufficiently to mandate their removal 
 from service in less than 10 years. For 
 this reason, a thorough engineering eval- 
 uation of plastic material must be con- 
 ducted prior to its selection for ser- 
 vice as a window in an X-P enclosure. At 
 present, polycarbonate plastic is consid- 
 ered practical for fabrication of windows 
 and lenses for enclosures; however, the 
 Mine Safety and Health Administration 
 (MSHA) , U.S. Department of Labor, policy 
 limits the design temperature to 240° F 
 (115° C). Industrial experience has 
 shown that three other restrictions 
 should be noted: 
 
 1. Contact with hydraulic oil and pe- 
 troleum-based lubricants are prohibited. 
 
 2. Low ultraviolet (UV) environment. 
 
 3. Service life should be limited to 
 4 years. 
 
 SAFETY FACTORS IN X-P ENCLOSURES 
 
 The design requirements for X-P enclo- 
 sures used in underground coal mines are 
 contained in Part 18 of Code of Federal 
 Regulations Title 30 (30 CFR 18) ( h) . 
 However, these requirements do not allow 
 for much deviation and the approval pro- 
 cess is based primarily on enclosures 
 
 constructed to these requirements. Any 
 innovative design attempts by enclosure 
 manufacturers would require substantial 
 effort to prove that the enclosure is 
 as safe as an enclosure built to the re- 
 quirements of 30 CFR 18. 
 
One phase of Bureau research has con- 
 sisted of determinating the charac- 
 teristics of materials commonly used 
 in enclosures. Such materials include 
 steel, aluminum, polycarbonates, and var- 
 ious sealants. Emphasis is placed on 
 determining (1) how these materials sat- 
 isfy design requirements and (2) the min- 
 imum safety factors contained in the 
 requirements . 
 
 A finite-element computer model was 
 used to ascertain the structural integ- 
 rity of four existing X-P enclosures (1) . 
 A finite-element model consists of many 
 small elements, each of which has stress 
 and deflection characteristics defined 
 by classical theory. By proper element 
 selection and by writing any necessary 
 constraint equations at the corners or 
 "nodes" of the elements, a good estimate 
 can be obtained of the stresses generated 
 
 in an enclosure by an internal loading 
 function. 
 
 The computer code ANSYS selected to 
 perform the finite-element analysis is 
 commercially available and contains all 
 the capabilities necessary for analyz- 
 ing X-P enclosures. It has a library of 
 finite elements that includes general 
 shells, three-dimensional beams and sol- 
 ids , and gap elements . It has the capa- 
 bility of performing elastic, elastic- 
 plastic, or thermal stress analysis, and 
 static and dynamic loading. 
 
 In determining the yielding of the en- 
 closures, the von Mises yield criterion 
 was used (it assumes that the structure 
 will yield when the distortion energy 
 equals the distortion energy in simple 
 tension). A combined stress, called the 
 von Mises stress, is defined as 
 
 VM = 0.707 /(a, - a 2 ) 2 + (o 2 - a 3 ) 2 + (a 3 - a,) 2 ' 
 
 where O), 02, 03 are principal stresses. 
 The von Mises criterion expressed in 
 terms of stresses states that yielding 
 will occur when 
 
 °VM 
 
 where a is the yield stress in simple 
 tension. 
 
 In determining the safety factors for 
 the four enclosures analyzed, a design 
 pressure of 150 psig (as specified by 
 MSHA) was used. A safety factor for each 
 component of the enclosure was determined 
 by 
 
 Safety factor = P V M/Pdesign» 
 
 where P VM is the pressure that will give 
 the von Mises stress. 
 
 Safety factors for the four enclosures 
 analyzed are summarized in table 1. For 
 each enclosure, multiple factors of safe- 
 ty were calculated for different compo- 
 nents and with different finite-element 
 models. Table 1 gives the minimum safety 
 factor for each enclosure, which is the 
 factor that governs the failure mode of 
 the enclosure; other factors were cal- 
 culated for evaluation and comparison 
 only. 
 
 As table 1 shows , there is a wide vari- 
 ation in the strength of X-P enclosures. 
 Although enclosure 1 showed a safety 
 
 TABLE 1. - Summary of safety factors for four enclosures 
 
 
 Enclosure 
 
 Enclosure 
 
 Minimum strength 
 
 Safety 
 
 Enclosure 
 
 description 
 
 volume , 
 in 3 
 
 component 
 
 factor 
 
 1 
 
 Steel rectangular box, 
 aluminum cover. 
 
 926 
 
 
 0.73 
 
 2 
 
 Steel rectangular box, 
 small steel cover. 
 
 234 
 
 
 1.60 
 
 
 
 
 3 
 
 Rectangular luminaire, 
 aluminum casting. 
 
 140 
 
 Bottom plate, 
 aluminum casting. 
 
 1.20 
 
 4 
 
 Cylindrical junction 
 box, steel casting. 
 
 142 
 
 Cover, steel casting. 
 
 4.83 
 
factor of less than 1, all of these en- 
 closures have been certified as explo- 
 sionproof. However, it should be remem- 
 bered that the design criterion of 150 
 
 psig used to compute the safety factors 
 is a much more severe load than that pre- 
 sented by the internal methane-air explo- 
 sion specified by MSHA. 
 
 POTTING MATERIALS USED IN X-P ENCLOSURES 
 
 Potting materials are frequently used 
 in X-P enclosures to reduce hazards by 
 separating potential ignition sources 
 from flammable environments. They may 
 serve as heat sinks to quench sparks as 
 well as heat from zones where abnormal 
 operating conditions arise. Under normal 
 conditions, materials serve as an elec- 
 trical insulator, whether the material is 
 a solid, liquid, or gas. Besides the 
 usual mechanical and electrical proper- 
 ties exhibited by a potting material, two 
 additional factors are important in eval- 
 uating materials. One factor concerns 
 the flammability of the material. The 
 second factor relates to the flammability 
 of the products produced when the mate- 
 rial is decomposed by an electric arc 
 caused by failure of the potted electri- 
 cal equipment. 
 
 Organic potting materials will, in gen- 
 eral, undergo decomposition, carboniza- 
 tion, and burn in an oxidizing atmos- 
 phere. In an inert atmosphere, pyrolysis 
 is likely to occur. However, no general 
 guideline exists for establishing an ac- 
 ceptance criterion for potting materials 
 for use in X-P enclosures used in under- 
 ground mines. 
 
 The purpose of this phase of Bureau 
 research was to develop a testing 
 
 methodology where a rank ordering of can- 
 didate materials may be established based 
 on arc decomposition products. A list of 
 the arc decomposition products of seven 
 potting compounds evaluated in this study 
 is presented in table 2. A prescribed 
 number of arcing cycles were produced by 
 a cycle timer (cycles per second) that 
 when multiplied by the average energy per 
 cycle gave the energy dissipated in the 
 arc. Results show clearly the potential 
 for addition of more sensitive hydrocar- 
 bons and hydrogen (_5) . 
 
 Results obtained indicate that a rank- 
 ing order could be based on the least 
 amount of flammable gas (hydrogen) 
 evolved. This is evidenced by results 
 obtained during arcing tests conducted 
 at 43 J/c for 30 cycles. Therefore, the 
 materials tested should be ranked in 
 the following order, (from best to 
 worst) : 
 
 ° Eccosil 3 (solid, liquid, gas). 
 
 ° DPR242H (solid). 
 
 ° Eccothane (gas). 
 
 o DPR242 (solid). 
 
 o Epoxy (solid). 
 
 ° Dow Silicone Oil (liquid). 
 
 o RS#3 (asphaltic solid). 
 
 PERFORMANCE TESTS FOR X-P ENCLOSURES 
 
 Since 1977, the Bureau has conducted 
 research to develop performance standards 
 for X-P enclosures. These new standards 
 are intended to provide a method of ap- 
 proving enclosures for use in underground 
 mines based solely on the performance of 
 the enclosure during specified tests. 
 When enclosures are evaluated in this 
 way, rather than by specifying certain 
 dimensions as in 30 CFR 18, the enclosure 
 designers will have more freedom in their 
 design approach. 
 
 Two performance tests have been 
 developed to date: (1) a structural 
 
 performance (hydrostatic pressure) test 
 and (2) a ruggedness test. 
 
 STRUCTURAL PERFORMANCE 
 
 The purpose of the structural perform- 
 ance test is to verify that X-P enclo- 
 sures are designed for a minimum static 
 pressure of 150 psig. 
 
 ■^Reference to specific products does 
 not imply endorsement by the Bureau of 
 Mines . 
 
TABLE 2. - Arc-decomposition products 
 
 Material 
 
 Environment 
 
 Cycles 
 
 Average energy 
 per cycle, J 
 
 Decomposition 
 
 products 
 
 Total combus- 
 
 
 Component ' 
 
 ppm 
 
 tibles, % 
 
 
 Nitrogen. . . 
 
 60 
 
 43 
 
 H 2 
 CH 4 
 
 22,712 
 943 
 
 71.5 
 
 
 2.9 
 
 
 
 
 
 C 2 H 2 
 
 5,217 
 
 16.4 
 
 
 
 
 
 C 2 H 4 
 
 2,609 
 
 8.2 
 
 
 
 
 
 C 3 H 6 
 
 26 
 
 .1 
 
 
 
 
 
 C3H4 
 
 48 
 
 .2 
 
 
 
 
 
 C 6 H 6 
 
 230 
 
 .7 
 
 
 
 15 
 
 43 
 
 H 2 
 CH 4 
 C 2 H 2 
 C 2 H 4 
 
 1,458 
 
 87 
 
 109 
 
 16 
 
 87.3 
 5.2 
 6.5 
 1.0 
 
 
 
 15 
 
 43 
 
 CH 4 
 
 27 
 
 84.3 
 
 
 
 60 
 
 43 
 
 CH 4 
 
 5 
 
 100 
 
 
 
 30 
 
 43 
 
 CH 4 
 
 <5 
 
 NC 
 
 
 Nitrogen. . . 
 
 30 
 
 43 
 
 H 2 
 
 11,288 
 
 94.5 
 
 
 
 30 
 
 21.5 
 
 CH 4 
 
 10 
 
 50 
 
 
 Nitrogen. . . 
 
 30 
 
 21.5 
 
 H 2 
 
 4,101 
 
 91 
 
 
 
 60 
 
 43 
 
 H 2 
 
 CH 4 
 
 C 2 H 2 
 
 C 2 H 4 
 
 C 3 H 6 
 C3H4 
 
 C 6 H 6 
 
 27,979 
 
 4,496 
 
 4,783 
 
 3,913 
 
 661 
 
 430 
 
 361 
 
 67 
 8.4 
 
 11.5 
 
 9.4 
 
 1.6 
 
 1.1 
 
 .9 
 
 
 
 15 
 
 43 
 
 H 2 
 
 CH 4 
 
 C 2 H 2 
 
 C 2 H 4 
 
 C 3 H 8 
 
 8,305 
 435 
 739 
 163 
 
 22 
 
 85.9 
 
 4.5 
 
 7.6 
 
 1.7 
 
 .3 
 
 
 
 15 
 
 43 
 
 CH 4 
 C 2 H 2 
 
 32 
 22 
 
 59.3 
 40.7 
 
 
 
 60 
 
 43 
 
 H 2 
 CH 4 
 C 2 H 2 
 C 2 H 4 
 
 339 
 
 109 
 
 152 
 
 98 
 
 48.6 
 15.6 
 21.8 
 14.0 
 
 
 
 30 
 
 43 
 
 CH 4 
 
 C 2 H 2 
 
 C 2 H 4 
 
 <5 
 22 
 <5 
 
 15.6 
 68.8 
 15.6 
 
 
 Nitrogen. . . 
 
 30 
 
 43 
 
 H 2 
 
 CH 4 
 
 2^2 
 C 2 H 4 
 
 C 3 H 6 
 
 39,424 
 
 739 
 
 2,935 
 
 1,033 
 
 98 
 
 89.1 
 
 1.7 
 
 6.6 
 
 2.3 
 
 .3 
 
 
 
 30 
 
 21.5 
 
 CH 4 
 
 C 2^ 2 
 
 C 2 H 4 
 
 13 
 15 
 10 
 
 34.2 
 39.5 
 26.3 
 
 
 Nitrogen. . . 
 
 30 
 
 21.5 
 
 H 2 
 
 CH 4 
 
 C 2 H 6 
 C 2 H 2 
 C 2 H 4 
 
 11,864 
 
 526 
 
 24 
 
 926 
 
 441 
 
 86.1 
 
 3.8 
 
 .2 
 
 6.7 
 
 3.2 
 
 See notes at end of table, 
 
TABLE 2. - Arc-decomposition products — Continued 
 
 Material 
 
 Environment 
 
 Cycles 
 
 Average energy 
 per cycle, J 
 
 Decomposition 
 
 products 
 
 Total combus- 
 
 
 Component 1 
 
 ppm 
 
 tibles, % 
 
 Silicone oil. . 
 
 Nitrogen. . . 
 
 60 
 
 43 
 
 H 2 
 CH 4 
 
 C 2^2 
 C 2 H 4 
 
 5,085 
 35 
 65 
 15 
 
 97.8 
 
 .7 
 
 1.3 
 
 .2 
 
 
 • • « QO • • • •• • 
 
 15 
 
 43 
 
 H 2 
 CH 4 
 
 C 2" 2 
 C 2 H 4 
 
 3,119 
 
 108 
 
 54 
 
 11 
 
 94.7 
 
 3.3 
 
 1.6 
 
 .4 
 
 
 
 15 
 
 43 
 
 CH 4 
 
 5 
 
 100 
 
 
 
 60 
 
 43 
 
 CH 4 
 
 27 
 
 100 
 
 
 
 39 
 
 43 
 
 CH 4 
 
 <5 
 
 NC 
 
 
 Nitrogen. . . 
 
 30 
 
 43 
 
 H 2 
 CH 4 
 C 2 H 2 
 C 2 H 4 
 
 C 3 H 6 
 
 17,797 
 
 217 
 
 283 
 
 54 
 
 <5 
 
 96.9 
 1.2 
 1.6 
 0.3 
 
 
 
 
 30 
 
 21.5 
 
 CH 4 
 C 2 H 2 
 
 5 
 <1 
 
 NC 
 NC 
 
 
 Nitrogen. . . 
 
 30 
 
 21.5 
 
 H 2 
 CH 4 
 C 2 H 2 
 C 2 H 4 
 
 7,458 
 
 245 
 
 106 
 
 17 
 
 95.3 
 
 3.1 
 
 1.4 
 
 .2 
 
 
 
 30 
 
 21.5 
 
 CH 4 
 C 2^2 
 C 2 H 4 
 
 7 
 5 
 3 
 
 46.7 
 33.3 
 20.0 
 
 
 Nitrogen. . . 
 
 30 
 
 21.5 
 
 H 2 
 CH 4 
 
 C 2^2 
 C 2 H 4 
 
 11,186 
 
 580 
 
 1,030 
 
 135 
 
 76.2 
 7.9 
 
 14.0 
 1.9 
 
 DPR 242 
 
 • • *QO« • » t • i 
 
 30 
 
 43 
 
 H 2 
 CO 
 CO 2 
 CH 4 
 
 2 2 
 C 2 H 4 
 
 C 2 H 6 
 
 5,593 
 1,300 
 580 
 565 
 500 
 413 
 13 
 
 79.0 
 
 NC 
 
 NC 
 
 8.0 
 
 7.1 
 
 5.8 
 
 .1 
 
 
 
 30 
 
 43 
 
 CO 
 CO 2 
 CH 4 
 C 2^2 
 C 2 H 6 
 
 210 
 
 4,700 
 
 9 
 
 7 
 
 9 
 
 NC 
 
 NC 
 
 50.0 
 
 38.9 
 
 .2 
 
 
 Nitrogen. . . 
 
 30 
 
 21.5 
 
 H 2 
 CO 2 
 CH 4 
 C 2 H 2 
 C 2 H 4 
 
 2,034 
 
 350 
 
 130 
 
 217 
 
 98 
 
 82.0 
 
 NC 
 
 5.2 
 
 8.8 
 
 4.0 
 
 
 
 30 
 
 21.5 
 
 CO 2 
 CH 4 
 C 2^2 
 
 1,700 
 3 
 3 
 
 NC 
 50.0 
 50.0 
 
TABLE 2. - Arc-decomposition products — Continued 
 
 Material 
 
 Environment 
 
 Cycles 
 
 Average energy 
 per cycle, J 
 
 Decomposition 
 
 products 
 
 Total combus- 
 
 
 Component ' 
 
 ppm 
 
 tibles , % 
 
 
 Nitrogen. . . 
 
 30 
 
 43 
 
 H 2 
 
 CO 2 
 
 CH 4 
 
 C 2 H 2 
 
 C2H4 
 
 C 2 H 6 
 
 3,898 
 
 1,700 
 
 576 
 
 609 
 
 315 
 
 9 
 
 72.1 
 
 NC 
 
 10.6 
 
 11.3 
 
 5.8 
 
 .2 
 
 
 
 30 
 
 43 
 
 CO 
 CO 2 
 CH 4 
 C 2 H 2 
 
 340 
 
 4,800 
 
 14 
 
 11 
 
 NC 
 
 NC 
 
 56.0 
 
 44.0 
 
 
 Nitrogen. . . 
 
 30 
 
 21.5 
 
 H 2 
 CO 
 
 co 2 
 
 CH 4 
 C 2 H 2 
 C 2 H 4 
 C 2 H 6 
 
 3,390 
 480 
 300 
 217 
 250 
 109 
 12 
 
 85.0 
 
 NC 
 
 NC 
 
 5.5 
 
 6.3 
 
 2.7 
 
 .3 
 
 
 
 30 
 
 21.5 
 
 C0 2 
 CH 4 
 
 C 2"2 
 C 2 H 4 
 
 1,600 
 
 14 
 5 
 3 
 
 NC 
 63.6 
 22.7 
 13.7 
 
 
 Nitrogen. . . 
 
 30 
 
 43 
 
 H 2 
 CH 4 
 C 2 H 2 
 C 2 H 4 
 
 1,661 
 
 174 
 
 43 
 
 9 
 
 88.0 
 
 9.2 
 
 2.3 
 
 .5 
 
 
 
 30 
 
 43 
 
 CO 2 
 CH 4 
 
 1,300 
 
 7 
 
 NC 
 100 
 
 
 Nitrogen. . . 
 
 30 
 
 21.5 
 
 H 2 
 CO 
 
 CH4 
 
 2 2 
 C 2 H 6 
 
 2,034 
 
 330 
 
 65 
 
 17 
 
 9 
 
 95.7 
 
 NC 
 
 3.1 
 
 .8 
 
 .4 
 
 
 
 30 
 
 21.5 
 
 CO 2 
 CH4 
 
 C 2 H 2 
 
 1,100 
 
 14 
 
 2 
 
 NC 
 87.5 
 12.5 
 
 
 Nitrogen. . . 
 
 30 
 
 43 
 
 H 2 
 CO 
 CO 2 
 
 CH 4 
 
 C 2 H 2 
 
 C 2 H 4 
 
 5,593 
 
 1,300 
 
 150 
 
 228 
 
 1,011 
 
 217 
 
 79.3 
 
 NC 
 
 NC 
 
 3.2 
 
 14.3 
 
 3.2 
 
 
 
 39 
 
 43 
 
 CH 4 
 
 C 2 H 2 
 
 C 2 H 4 
 
 3 
 6 
 3 
 
 25.0 
 50.0 
 25.0 
 
 
 Nitrogen. . . 
 
 30 
 
 21.5 
 
 H 2 
 
 CH 4 
 
 C 2 H 2 
 
 C 2 H 4 
 
 1,186 
 33 
 
 87 
 
 27 
 
 89.0 
 2.5 
 6.5 
 2.0 
 
 
 Air 
 
 
 
 CO 2 
 CH 4 
 C 2 H 2 
 
 2,200 
 3 
 
 6 
 
 NC 
 33.3 
 66.7 
 
 NC Not considered in the calculation of percentage t 
 H 2 = hydrogen; CH 4 = methane; C 2 H 2 = acetylene; 
 C3H4 = propyne; C3H5 = benzene. 
 
 otal. 
 
 C 2 H 4 = 
 
 ethylene; C3H 6 = propylene; 
 
10 
 
 In order to routinely test X-P enclo- 
 sures for structural performance as spe- 
 cified in 30 CFR 18, the apparatus shown 
 in figure 2 is required. This setup 
 consists primarily of a water reservoir, 
 a regulated nitrogen source, intercon- 
 necting hardware, and pressure-sensing 
 instrumentation. 
 
 The water tank should be at least 50 
 gal in volume and be rated for 300-psig 
 water service. A regulated high-pressure 
 bottle of nitrogen (2,000 psig), or the 
 equivalent, is used to provide the input 
 pressure to the water tank. The geome- 
 try, size, and complexity of the test en- 
 closure will dictate how it is connected 
 to the pressurization apparatus. 
 
 Once a pressure connection is avail- 
 able, the next step is to provide a tight 
 seal between the cover and the enclosure 
 if one has not been provided by the manu- 
 facturer. Two sealing methods have been 
 found to be satisfactory for hydrostatic 
 
 testing. The most suitable method for 
 the enclosure being tested can be used. 
 
 Me thod 1 (suitable for small, regular 
 shaped enclosures). — In this method, a 
 continuous gasket is cut from 1/16-in- 
 thick reinforced neoprene. The gasket 
 should extend outside the bolt circle, 
 but no closer than 3/16 in from the outer 
 edge of the flange or cover. Clean the 
 mating surfaces with acetone or an equiv- 
 alent solvent before placing the gasket 
 and closing the enclosure. No sealants 
 are necessary. Torque bolts to their 
 rated load. 
 
 Method 2 (suitable for enclosures of 
 all sizes) . — This method uses General 
 Electric Silicon Construction Sealant 
 1200 series. Clean the surfaces to be 
 sealed with acetone or an equivalent 
 solvent and apply the sealant in a uni- 
 form bead 1/16 to 1/8 in. in diameter. 
 A zigzag pattern, as shown in figure 3, 
 has been found to work satisfactorily. 
 
 Test cell Control cell 
 
 i l/4-in steel tubing and fittings 
 
 — £>£}— Vent 
 valve 
 
 r\ 
 
 Water 
 tank 
 
 To water .x. 
 supply ~~ ^^ 
 
 Level 
 gage 
 
 Vent 
 ■fcl<j- 4 -conductor 
 
 Test 
 
 item Um 
 
 -O- 
 
 Pressure 
 
 t 
 
 transducer 
 
 "^S: 
 
 T^S, 
 
 Reference 
 dial gage 
 
 ® 
 
 L 
 
 t&\ — Vent 
 valve 
 
 Nitrogen . 
 cutoff X* 
 valve 
 
 Water cutoff 
 valve 
 
 1£] 1 
 
 aU 
 
 Digital voltmeter 
 
 d§ 
 
 i 
 
 i — 
 
 tt 
 
 Power supply 
 
 X\\\\\V 
 
 Regulated 
 
 nitrogen 
 
 bottle 
 
 X\ 
 
 FIGURE 2. - Hydrostatic test apparatus. 
 
11 
 
 Sealant 
 
 FIGURE 3. - Recommended sealant pattern. 
 
 Some sealant will be extruded from the 
 outer edge of the enclosure by this 
 method, but there will also be spaces 
 for a feeler gage. Use heat lamps to 
 raise sealant temperature above 110° F 
 (43.3° C) for a faster cure. The lid can 
 be applied after the sealant "skims" or 
 after it has cured completely. High 
 humidity and high heat reduce the cur- 
 ing time. A faster cure should occur 
 without the cover in place. Torque bolts 
 to their rated loads. 
 
 For both methods, prior to connecting 
 the enclosures to the pressurization sys- 
 tem and bolting the cover, check all sur- 
 faces for flatness so that a pretest 
 baseline can be established. Deforma- 
 tions, if any, due to the hydrostatic 
 test can then be determined. Use a steel 
 straightedge against every flat surface; 
 measure and record any undulations. Mea- 
 sure across the width and length of each 
 side, and, if necessary, across the two 
 diagonals . 
 
 When all of the pretest measurements 
 have been recorded, place the enclosure 
 on the test stand with the sealing sur- 
 face level and at the highest point on 
 the enclosure. Connect the enclosre to 
 
 the pressurization system and completely 
 fill the enclosure with water. Eliminate 
 or minimize air pockets when filling the 
 enclosure. Apply the sealant or gasket 
 and allow it to cure if necessary. Cur- 
 ing time can be accelerated with heat 
 1 amp s . 
 
 The enclosure is now ready to be closed 
 for pressurization. Top the enclosure 
 with water if necessary (water can over- 
 flow) , open the vent valve, install cov- 
 er, and torque bolts to rated load. Us- 
 ing feeler gages, measure and record the 
 gap, if any (between the enclosure and 
 its cover) on all sides of the enclosure, 
 with at least one measurement between 
 each pair of bolts. If the extruded 
 sealant precludes insertion of the feeler 
 gages, it can be easily removed with a 
 fine-bladed knife. 
 
 After an enclosure has been connected 
 to the water tank, the hydrostatic test 
 is conducted as follows: 
 
 1. Open regulator and gradually ap- 
 ply pressure to the system in 30-psig 
 increments . 
 
 2. After each pressure step, close 
 nitrogen cutoff valve and inspect the 
 enclosure for water leaks. Very small 
 leaks can be tolerated, particularly at 
 the higher pressures. 
 
 3. In case of a flowing leak, shut off 
 nitrogen cutoff valve, close the water 
 cutoff valve, and close the regulator. 
 Open vent valve by reference dial gage. 
 Reseal the enclosure before starting test 
 again. 
 
 4. After the test pressure reaches 150 
 psig, close the nitrogen cutoff valve. 
 
 5. Shut off the regulator and open 
 vent valve by reference dial gage. 
 
 6. Close water cutoff valve. 
 
 7. Measure and record the gap between 
 the enclosure and its cover at the same 
 locations surveyed after sealing. 
 
 8. Remove enclosure. 
 
 9. Remeasure and record flatness of 
 each side to determine if any permanent 
 deformations resulted 
 static pressure test. 
 
 The enclosure will 
 structural performance 
 
 from the hydro- 
 
 have passed the 
 test if (1) no 
 
 deformations larger than 0.04 in/ft are 
 
12 
 
 measured on any side, and (2) the gap has 
 not increased more than 0.002 in relative 
 to pretest measurements. 
 
 RUGGEDNESS 
 
 The environment in which X-P enclosures 
 must operate underground can be so severe 
 that loads produced by internal methane- 
 air explosions may not govern the design 
 of some components of the enclosure 
 (i.e., covers and sides). Other loads 
 may be greater. As enclosures evolve, 
 the venting of enclosures through X-P 
 vents may become permissible. If this 
 occurs, the internal pressure loads may 
 be quite low and other design criteria, 
 such as ruggedness, will control. The 
 purpose of this work was to evaluate the 
 environment in the mines and to identify 
 suitable criteria that an enclosure 
 should meet to ensure that it will func- 
 tion properly. 
 
 To estimate the forces that can occur 
 in typical mining operations (forces that 
 can be applied to exposed surfaces of 
 X-P enclosures), operating mines were 
 
 examined for evidence of damage , and 
 forces that might be produced by differ- 
 ent mine accidents were calculated. The 
 forces evaluated were — 
 
 ° Forces required to produce observed 
 damage in a mining machine. 
 
 o Contact forces when a mining machine 
 strikes a rib. 
 
 o Impact forces when a shuttle car 
 rams a mining machine. 
 
 ° Impact forces produced by roof 
 falls. 
 
 Figure 4 gives the kinetic energy (KE) 
 per unit area at the top of a continous- 
 mining machine versus the cumulative per- 
 centage of roof falls (1_) . The height 
 was taken as 6 ft above the mine floor, 
 but no higher than 1 ft below the mine 
 roof. For X-P enclosures located on top 
 of the mining machine, the height above 
 the mine floor was taken as 3 ft. The 
 data are bounded by upper and lower 
 curves , and values are read from the 
 lower bound, which gives the highest KE 
 at any percentage level. For example, 
 60% of all roof falls should produce 
 KE levels equal to or less than 3,200 
 
 100 
 
 o 
 a. 
 
 > 
 
 E 
 
 O 
 CO 
 
 O 
 O 
 
 or 
 
 4 6 8 10 12 14 16 18 
 
 KINETIC ENERGY 3 FT ABOVE MINE FLOOR, I0 3 fflb/ft 2 
 
 FIGURE 4. - Kinetic energy produced by roof falls at top of continuous-mining machine. 
 
13 
 
 ft-lb/ft 2 . Thus, to protect against 60% 
 of all roof fall accidents, X-P enclo- 
 sures should be designed for a KE level 
 of 3,200 ft-lb/ft 2 . To protect against 
 90% of all roof falls, the enclosure 
 should be designed for a KE level of 
 16,000 ft-lb/ft 2 . 
 
 In summary, the following recommenda- 
 tions are made: 
 
 ° Test enclosure surfaces exposed to 
 the roof at a KE level that will protect 
 against 60% of roof falls. 
 
 ° Test enclosure surfaces exposed to 
 side impacts to an energy level that cor- 
 responds to a 60,000-lb shuttle car im- 
 pacting at 2 mph. 
 
 ° Design protected surfaces to a KE 
 level that is 10% of that required for 
 side impact. 
 
 These recommendations correspond to the 
 following energy levels: 
 
 ° Top exposure: Design and test to KE 
 = 3,200 ft-lb/ft 2 . 
 
 ° Side exposure: Design and test to 
 KE = 8,000 ft-lb/ft 2 . 
 
 ° Protected areas 
 operator's canopy): 
 KE = 800 ft-lb/ft 2 . 
 
 Only impact forces produced by roof 
 falls are discussed here. Detailed in- 
 formation on the other forces are con- 
 tained in a Bureau contract report (1). 
 
 (such as under the 
 Design and test to 
 
 EFFECTS OF HIGH-VOLTAGE ON EXPLOSION-PROTECTION TECHNIQUES 
 
 The development of more efficient coal 
 removal equipment for use in underground 
 coal mines has resulted in a demand for 
 more electrical power at the working 
 face. This could be accomplished in sev- 
 eral ways; however, the most cost-effec- 
 tive way appears to be the increase of 
 the supply voltage. The Bureau is re- 
 viewing the effects of increasing the 
 high voltage limit (presently limited to 
 4,160 V by MSHA) to 15,000 V. One aspect 
 of this review is the effect of this new 
 limit on explosion protection techniques. 
 
 The Bureau has conducted research on 
 the three methods of explosion protec- 
 tion: X-P enclosures, pressurization, 
 and potting (_5 ) . The three methods pre- 
 sent varying degrees of difficulty in 
 protecting the mine environments from 
 potential ignition. The effects of high 
 voltage on conductor isolation, insulator 
 selection, and power dissipation must be 
 considered when comparing explosion pro- 
 tection techniques. 
 
 High-voltage conductor isolation must 
 be reliably accomplished within the se- 
 lected enclosure based on the available 
 energy and the potential for damage to 
 enclosure materials during breakdown. 
 The theory of operation for the potted 
 enclosure assumes inherent conductor iso- 
 lation. However, practical application 
 of this method requires definition of 
 various verification and certification 
 tests, the potting compound specifi- 
 cation, and the protective enclosure 
 
 specification. The explosion-proof meth- 
 od provides conductor isolation by sep- 
 aration and insulation. It has been 
 demonstrated that separation is not a re- 
 liable isolation technique at high volt- 
 ages for uninsulated conductors during 
 methane combustion. High-voltage conduc- 
 tors should be insulated to provide re- 
 liable isolation in an X-P enclosure. 
 The pressurized enclosure provides con- 
 ductor isolation by the separation and 
 insulation. However, the inherent lack 
 of methane combustion within the enclo- 
 sure greatly increases the reliability 
 of the isolation method of isolation 
 separation. 
 
 A high-voltage insulator must reliably 
 isolate the conductor under any condi- 
 tions anticipated for the enclosure. The 
 selection of an insulation material is 
 most critical in the X-P enclosure owing 
 to the harsh conditions present during 
 methane combustion. These insulators 
 must not only survive these combustion 
 episodes, but they also must resist the 
 effects of corona discharge. The insula- 
 tors selected for use in pressurized and 
 potted enclosures are not required to 
 survive methane combustion due to the 
 lack of flammable gas concentrations 
 within these enclosures. However, the 
 insulators must be resistant to the ef- 
 fects of corona discharge. 
 
 The power dissipation of a given 
 machine will increase as the supplied 
 power is increased. The potted enclosure 
 
14 
 
 is very sensitive to variations in the 
 power dissipation owing to the natural 
 thermal insulation of the potting com- 
 pound. Small changes in the power dissi- 
 pated within this enclosure can produce 
 large temperature gradients in the pot- 
 ting compound. The resultant tempera- 
 ture gradients are dependent on the heat 
 
 capacity, conductivity, and thickness of 
 the potting compound. The pressurization 
 and X-P enclosures provide air circu- 
 lation around equipment by forced circu- 
 lation or convection. Thus, these two 
 methods are relatively insensitive to 
 changes in the machine power dissipation. 
 
 INNOVATIVE X-P DEVICES 
 
 PRESSURE VENT 
 
 The design requirements for X-P enclo- 
 sures used in gassy areas of underground 
 mines are contained in 30 CFR 18. These 
 regulations define an X-P enclosure as 
 "an enclosure that complies with the ap- 
 plicable design requirements in Subpart B 
 of this part and is so constructed that 
 it will withstand internal explosions of 
 methane-air mixtures: (1) Without damage 
 to or excessive distortion of its walls 
 or cover(s) and (2) without ignition of 
 surrounding methane-air mixtures or dis- 
 charge of flame from inside to outside 
 the enclosure." The intent of this regu- 
 lation is to provide criteria to ensure 
 that the explosive energy will be con- 
 tained within the enclosure. 
 
 Since internal pressure may exceed 100 
 psig, these enclosures are characterized 
 by heavy wall construction, tight flange 
 gaps, and a multitude of cover bolts. If 
 an enclosure could be designed to release 
 or vent the heat and pressure in a safe, 
 controlled manner, the design criteria 
 of 30 CFR 18 could be relaxed to permit 
 lighter enclosures and less stringent 
 flame path requirements. Lightweight 
 construction would facilitate handling of 
 large enclosure covers or complete enclo- 
 sures by one person. In addition, larger 
 allowable flange gaps would negate the 
 present effects that corrosion and dust 
 entrapment have on attempts to comply 
 with 30 CFR 18. 
 
 Under contract H0357107 (7-8), the Bu- 
 reau investigated numerous concepts in- 
 cluding venting, designed to reduce the 
 internal pressure generated in X-P 
 enclosures during internal methane-air 
 explosions. From this study, it was 
 determined that any venting mechanism 
 must — 
 
 Quench both methane gas and coal 
 dust flame fronts. 
 
 o Be relatively permeable to gas flow 
 to minimize vent size. 
 
 ° Have self-cleaning characteristics 
 to reduce the possibilities of clogging 
 during use underground. 
 
 o Have sufficient corrosion and me- 
 chanical shock resistance to be compati- 
 ble with the mine environment. 
 
 In light of these requirements , an 
 open-cell metal foam was judged to offer 
 the best combination of mechanical and 
 flame-arresting properties. For actual 
 hardware designs, Retimet, a stainless 
 steel foam from the United Kingdom, was 
 chosen; it is an excellent flame arrestor 
 and was the only open-cell metal form 
 available. For the prototype vent, a 
 1/2-in-thick foam slab of Retimet was 
 fixed in a metallic frame and mounted in 
 the enclosure wall. A cross-sectional 
 view is shown in figure 5. The foam was 
 protected against mechanical damage by a 
 hinged metal cover that swung open when 
 internal pressure exceeded 2 psig. Nor- 
 mally, this cover was held in place by a 
 small permanent magnet. 
 
 To have a significant impact upon en- 
 closure design, it was determined that a 
 vent must limit internal explosion pres- 
 sure to a 12-psig maximum. To determine 
 the relationship between vent area size, 
 the internal pressure developed, and the 
 enclosure volume, a vent area-to-volume 
 ratio was established through laboratory 
 tests. As illustrated in figure 6, any 
 enclosure with a vent area to volume ra- 
 tio larger than 4 in 2 /ft 3 will meet the 
 12-psig criterion. 
 
 To protect the metal foam from oxida- 
 tion damage due to high temperature, a 
 series of 20-mesh stainless steel wire 
 
15 
 
 Vent body 
 
 Vent cover 
 retainer 
 
 Swinging- door 
 vent cover 
 
 Magnet holds 
 cover closed 
 
 Enclosure cover 
 
 Flame path meets 
 MSHA requirements 
 
 Steel retaining ring 
 vent retainer 
 
 Retimet vent 
 material, 
 foam stainless 
 steel 
 
 Bolt with lock 
 washer 
 
 Bolt with lock 
 washer 
 
 ^jC(j Tapped blind hole 
 
 as required by MSHA 
 
 FIGURE 5. - Pressure vent. 
 
 screens was placed on the inner face of 
 the foam and acted as a thermal barrier 
 to the flame front. The minimum number 
 of screens to be used increases as the 
 vent area-to-enclosure volume ratio de- 
 creases. The number of screens used in 
 the vent design must be more than or 
 equal to the number of screens indicated 
 in figure 7. 
 
 It was determined that flange-gap tem- 
 peratures must not exceed 1,200° F 
 (649° C) peak to preclude ignition of a 
 surrounding external gassy atmosphere. 
 Since one of the goals of utilizing pres- 
 sure vents was to permit the safe use of 
 larger flange gaps, a series of labora- 
 tory tests was conducted to determine 
 allowable gaps for various vent area-to- 
 enclosure volume ratios. The maximum al- 
 lowable gap was found to decrease as the 
 vent area-to-enclosure volume decreases 
 (see figure 7). 
 
 To use figure 7, the designer deter- 
 mines a convenient size for the pressure 
 vent assembly or the enclosure. Using 
 this, one then can determine the number 
 of screens required from the top axis and 
 the maximum flange gap spacing from the 
 
 left axis. The enclosure could then be 
 designed solely on the basis of rugged- 
 ness for the mine environment. 
 
 ELASTOMERIC GROMMET CABLE ENTRY 
 
 Conventional asbestos-packed cable en- 
 tries require considerable skill, ex- 
 perience, and motivation on the part of 
 the mine mechanic to achieve a permis- 
 sible installation. Proper cable entry 
 is time-consuming. It not only entails 
 packing the correct length of asbestos 
 rope in the stuffing box but also tight- 
 ening the gland nut to the correct depth 
 in accordance with 30 CFR 18. 
 
 Under contract H0357107 (7-8), the Bu- 
 reau investigated numerous cable entry 
 concepts in addressing the deficiencies 
 of asbestos entries. A cross section of 
 the optimum design is shown in figure 8. 
 It incorporates a tapered elastomeric 
 grommet compressed against the cable 
 jacket by a clamping nut. The advantages 
 of this arrangement are as follows: 
 
 ° Reduced entry time. 
 
 ° Reusability. 
 
 ° One size fits many cable diameters. 
 
 ° Captive grommet cannot be lost. 
 
 Evaluation of several materials led 
 to the selection of polyurethane as the 
 grommet material. It is highly resistant 
 to wear and abrasion, and impervious to 
 oil, grease, and water. Laboratory tests 
 of the grommet proved its equivalency to 
 conventional asbestos packing in the fol- 
 lowing areas: 
 
 ° Ability to grip the cable. 
 
 ° Gland nut torque required. 
 
 ° Distance in contact with cable. 
 
 o Flammability. 
 
 ° Explosion-proof integrity. 
 
 Both the pressure vent and the grommet 
 cable entry were successfully field 
 tested at an active underground coal 
 mine. Approval for this demonstration 
 was obtained via an experimental permit 
 from MSHA. The innovative devices were 
 installed on the connection box of a 
 Jeffrey 120M continuous-mining machine 
 for 8-1/2 months. Subsequent laboratory 
 tests revealed no measurable deteriora- 
 tion in their condition. 
 
16 
 
 'to 
 
 QL 
 
 Ld 
 or 
 
 Z> 
 CO 
 CO 
 UJ 
 
 or 
 
 0_ 
 
 £0 
 20 
 
 - 
 
 ,,,,,,,, | i | i 
 
 1 
 
 i 
 
 15 
 
 — 
 
 I 12-psig criterion 
 
 
 — 
 
 10 
 5 
 
 
 1 l 1 i 1 . 1 . 1 — I 1- r- 
 
 — J— 
 
 1 1 
 
 
 
 4 8 12 16 20 24 28 
 
 VENT AREA PER ENCLOSURE VOLUME, in 2 /ft 3 
 
 FIGURE 6. - Pressure buildup in vented enclosures. 
 
 32 
 
 o_ 
 < 
 
 CD 
 
 i 
 
 UJ 
 CD c 
 
 5" 
 < CD 
 
 5 CO 
 
 0.05 
 
 .04 
 
 
 
 16 
 
 10 
 
 NUMBER OF SCREENS 
 6 3 
 
 T 
 
 No screens over 28in 2 /ft 
 
 Guideline curve 
 
 8 12 16 20 24 28 32 
 
 VENT AREA PER ENCLOSURE VOLUME RATIO, in 2 /ft 3 
 
 FIGURE 7. - Suggested guidelines for number of screens and allowable flange gaps for vents and 
 explosionproof electrical enclosures. 
 
17 
 
 Enclosure wal 
 
 Retaining clip 
 
 Tapered urethane 
 grommet 
 
 Thrust 
 washer 
 
 Clamping 
 nut 
 
 Cable body 
 entry 
 
 FIGURE 8. - Trai ling-cable entry assembly. 
 
 SUMMARY 
 
 Research by the Bureau has revealed 
 much about the mechanisms of contain- 
 ing a methane-air explosion. Finite- 
 element analysis and hydrostatic pressure 
 tests revealed wide variations in the 
 margin of safety of selected X-P enclo- 
 sures. Results of research on electrical 
 clearances showed that clearances that 
 would be safe in ordinary locations are 
 insufficient when there is a possibility 
 of a methane-air explosion. The use of 
 glass and polycarbonate have proved sat- 
 is factory for window materials in X-P 
 enclosures with certain restrictions. 
 
 when choosing potting materials for 
 use in X-P enclosures, attention must 
 
 be given to the possibility of the accu- 
 mulation of arc-decomposition products, 
 especially when used in high-voltage en- 
 closures. The lack, of definition, speci- 
 fications, and basic research on most 
 potting materials make this method a dif- 
 ficult choice. The X-P enclosure and the 
 pressurized enclosure can be applied di- 
 rectly to high-voltage conditions using 
 the present state of the art. Addition- 
 ally, research results showed that the 
 pressure vent precludes a buildup of high 
 pressure caused by an explosion inside a 
 permissible enclosure, and the elasto- 
 meric grommet simplifies and hastens ca- 
 ble entry while minimizing worker error. 
 
 REFERENCES 
 
 1. Cox, P. A., 0. H. Burnside, E. D. 
 Esparza, F. D. Lin, and R. E. White. A 
 Study of Explosionproof Enclosures (con- 
 tract H0377052, Southwest Res. Inst.). 
 BuMines OFR 96-83, 1982, 426 pp.; NTIS PB 
 83-205450. 
 
 2. Scott, L. W. , and J. G. Doglos. 
 Electrical Arcing at High Voltage During 
 Methane-Air Explosions Inside Explosion- 
 proof Enclosures. BuMines TPR 115, 1982, 
 9 PP. 
 
18 
 
 3. Scott, L. W. Some Design Factors 
 for Windows and Lenses Used in Explosion- 
 proof Enclosures. BuMines IC 8880, 1982, 
 9 pp. 
 
 4. U.S. Code of Federal Regulations. 
 Title 30 — Mineral Resources; Chapter I, 
 Mine Safety and Health Administration. 
 U.S. Department of Labor; Subchapter D — 
 Electrical Equipment, Lamps, Methane 
 Detectors; Tests for Permissibility; 
 Fees; Part 18 — Electrical Motor-Driven 
 Mine Equipment and Accessories; July 1, 
 1984. 
 
 5. Herrera, W. R. , R. E. White, L. M. 
 Adams, and H. S. Silvus. Recommended Ac- 
 ceptance Criteria for Potting Materials 
 Used in Explosionproof Enclosures in Coal 
 Mines (contract J0100041, Southwest Res. 
 Inst.). BuMines OFR 168-83, 1982, 100 
 pp.; NTIS PB 83-254722. 
 
 6. Linley, L. J., and A. B. Luper. 
 Performance Criteria Guideline for Three 
 Methods of Electrical Equipment Rated Up 
 to 15,000 Volts AC (contract J0318081, 
 NASA). BuMines OFR 60-84, 1982, 63 pp.; 
 NTIS PB 84-172303. 
 
 7. Gunderman, R. J. Innovations for 
 Explosionproof Electrical Enclosures 
 (contract H0357107, Dresser Ind., Inc.). 
 BuMines OFR 121-81, 1980, 109 pp.; NTIS 
 PB 82-104936. 
 
 8. . Evaluation and Acceptance 
 
 Criteria for Innovations in Explosion- 
 proof Electrical Enclosures (contract 
 H0357107, Dresser Ind., Inc.). Bu- 
 Mines OFR 127-83, 1982, 141 pp.; NTIS PB 
 233379. 
 
 irU.S. CPO: 1985-505-019/20,111 
 
 IN T.-BU.O F MINES,PGH.,P A. 28121 
 
Ml 1 
 
 5 
 
U.S. Department of the Interior 
 Bureau of Mines— Prod, and Distr. 
 Cochrans Mill Road 
 P.O. Box 18070 
 Pittsburgh. Pa. 15236 
 
 OFFICIALBUSINESS 
 PENALTY FOR PRIVATE USE, «00 
 
 ] Do not wi sh to receive thi s 
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