u.S. Bureau of Mines/nioSH Mining Electrical Safety research: 

a Legacy of Protection against Shock, fires, and Explosions 

Michael r. yenchek 

NIOSH, Pittsburgh, Pennsylvania, United States 

Gerald t. Homce 

NIOSH, Pittsburgh, Pennsylvania, United States 

aBStraCt: This paper reviews the 100-year history of federal electrical safety research in the U.S mining industry, originally by the US Bureau of Mines, and as carried on today by NIOSH. First, there is a brief 

historical review of the use of electrical power in mining and an examination of the associated shock, fire, and 

explosion hazards. These hazards were the impetus for government intervention to improve the safety of U.S 

mineworkers. The evolution of electrical power systems and equipment in mining then serves as a backdrop 

for a review of continued electrical safety research. The paper concludes with a recap of the contributions made 

to mine worker electrical safety by federal research efforts, and thoughts on where the future will lead such 

research under NIOSH. 

introDuCtion 

This paper reviews the 100-year history of federal 

electrical safety research in the U.S mining industry, 

originally by the US Bureau of Mines, and as carried 

on today by NIOSH. First, there is a brief historical review of the use of electrical power in mining 

and an examination of the associated shock, fire, and 

explosion hazards. These hazards were the impetus 

for government intervention to improve the safety of 

U.S mineworkers. The evolution of electrical power 

systems and equipment in mining then serves as a 

backdrop for a review of continued electrical safety 

research. The paper concludes with a recap of the 

contributions made to mine worker electrical safety 

by federal research efforts, and thoughts on where 

the future will lead such research under NIOSH. 

Early in its history, the lack of federal safety 

regulations forced the Bureau into an advisory role 

for the mining industry. Since the Bureau had no regulatory powers, it relied principally on publications, 

such as technical papers and bulletins, to enlighten 

the industry about potential hazards. These suggestions were to be the precursors for the mandatory 

rules and regulations that later governed the use of 

electricity in mining. Many of the “good practices.” 

were the result of surveys of operators and manufacturers as was the case for the installation of cables 

in shafts and boreholes (Ilsley and Gleim 1932). At 

that time, several states had enacted laws governing 

mining equipment based upon the recommendations 

of the Bureau of Mines. It was only later, in 1946, 

that a federal safety code for bituminous coal and 

lignite mines was enacted. Ultimately, the Federal 

Coal Mine Safety Act of 1952 provided for annual 

inspections and gave the Bureau limited enforcement authority. However, it was the passage of the 

Coal Mine Safety and Health Act of 1969 that greatly 

influenced and expanded the Bureaus research, with 

much work being accomplished through grants and 

contracts. In 1973, the Bureau of Mines electrical 

safety functions associated with equipment approvals transferred to the Mining Enforcement and Safety 

Administration (MESA, later MSHA). 

The Bureau of Mines was actively involved in 

electrical safety research since its inception in 1910. 

An electrical section was organized in 1909 under 

the technologic branch of the U.S. Geological Survey 

(Ilsley 1923) and continued under the Bureau. The 

act establishing the Bureau detailed its scope, which 

included in part, investigating the methods of mining especially in relation to the use of explosives and 

electricity (Powell 1922). Technical Paper 4 (Clark 

1911), describing the purpose and equipment of the 

Bureau’s electrical section stated that, “There is a 

distinct branch of electrical engineering in connection with the equipment of mines. The conditions in 

mines are very different from those on the surface. 

Not only is it more difficult to install and properly 

maintain electrical equipment underground, but, 

unless suitable precautions are observed, the presence of such equipment in mines adds danger to a 

calling already hazardous.” The paper also acknowledged the cooperation of various state inspection 

departments and equipment manufacturers in safeguarding life and property underground. 

By 1910, electricity was already being used 

underground for haulage, lighting, driving pumps, 

fans, drills, coal cutting machines, and hoists, for 
detonating explosives, and for signaling. Both direct 

and alternating current were used, the former much 

more extensively. Direct current (DC) was distributed at up to 600 volts, while alternating current 

(AC) at over 2,000 volts was carried a short distance 

underground to feed motor-generator sets or rotary 

converters. Explosives could be detonated by batteries, magneto generators, or power circuits. Signals, 

which may include lights, bells, and telephones, 

were operated principally from batteries. 

Technical Paper 4 (Clark 1911) recognized the 

temporary nature of electrical service at many points 

and the fact that many workers did not appreciate its 

inherent danger. These dangers from electricity were 

classified as shock, fire, and explosions. Shock hazards included the trolley wire and other bare conductors along with ungrounded equipment. Fires could 

result from grounds to coal, flashing of motors, and 

short circuits. Explosions could originate from arcing and sparking that could ignite methane or dust 

accumulations. 

The electrical section of the Bureau was originally a part of the experimental station in Pittsburgh, 

PA and was initially staffed by an electrical engineer, an assistant electrical engineer, a junior electrical engineer, and an electrical engineering aid 

(Powell 1922). The section’s purpose (Clark 1911) 

was to “attempt to discover the causes of accidents 

from the use of electricity in mines and to suggest means for the prevention of such accidents; 

to make tests of the safety of electrical equipment 

under conditions most conducive to disaster; and to 

make such tests of a general nature as bear upon the 

safety of electricity in mines, accidents attributed 

to electrical causes, etc.” The laboratory, initially 

at the Government Arsenal at Fortieth and Butler 

Streets in Pittsburgh, was primarily for testing mining equipment. It featured direct current capability 

up to 750 volts and alternating current up to 2000 

volts for power, and 30,000 volts for high potential 

tests. Two galleries were used for testing electrical 

equipment in the presence of explosive gas. The 

smaller consisted of a boiler-iron box with connections for gas and air, heavy plate-glass observation 

windows, and pressure-relief vents. Incandescent 

lamps and other small items were evaluated in this 

chamber. The larger was a tube designed to represent 

a short section of a mine entry, thirty feet long and 

10 feet in diameter, and was used for tests of larger 

apparatus such as motors and switches (Clark 1911). 

This facility can be contrasted with the present-day 

Mine Electrical Laboratory, which is located at the 

NIOSH Pittsburgh Research Laboratory. Established 

in the early 1980s, it can support full-scale testing of 

mine electrical power system components, and over 

the years has been used for projects examining mining safety issues such as fuse performance, electric 

motor failure, trolley system faults, electric motor 

circuit protection, electrical cable performance, and 

overhead power line hazards. It is equipped with 

electrical power supplies capable of delivering voltage up to 100 kV AC or DC, and current up to 50 

kA AC, as well as a secure control room from which 

tests and experiments can be controlled and observed 

safely. It has also been used to support Mine Safety 

and Health Administration investigations. A small 

rail haulageway at Lake Lynn was used for trolley 

system research. 

This research history is organized by major 

types of electrical apparatus, i.e., explosion-proof 

enclosures, motors, trailing cables, trolley wires, etc. 

Within each category, the work is listed chronologically along with mine technological developments. 

Space limitations precluded inclusion of some 

related areas such as communications, lasers, and 

instrumentation as well as research that was applicable mainly to non-mining industries. 

PErMiSSiBLE EQuiPMEnt 

Electrical equipment used in potentially methaneladen atmospheres must be housed in explosionproof (X/P) enclosures or be of intrinsically-safe 

design, so as to have insufficient energy, under 

normal or faulted conditions, to ignite a methaneair mixture. The earliest electrical safety investigations involved the explosion hazards associated with 

various types of electrical apparatus already in use 

in gaseous underground coal mines. These included 

portable electric mine lamps, motors, storage-battery locomotives, mine lamp cords, gas detectors, 

danger signals, and coal-cutting apparatus. A series 

of schedules, published by the Bureau of Mines, 

specified the evaluation fees and requirements for 

the use of such equipment in gaseous mines and 

further stated that the Bureau would give its seal of 

approval to acceptable apparatus as being permissible. Schedule 2, approved by the Secretary of the 

Interior, October 26, 1911 was the first schedule 

issued and covered testing of motors. The Bureau 

published revised requirements (Schedule 2A) for 

test and approval of explosion-proof electric motors 

in 1915 (Clark 1915). This included tests where the 

motor casing was filled and surrounded with the 

“the most explosive mixture of Pittsburgh natural 

gas and air.” Subsequent tests would prove that the 

results obtained with natural gas and pure methane 

were essentially the same (Leitch, Hooker, and Yent 

1925). The motor would be operated at rated speed 

and the gas mixture ignited by a spark inside the 
 

casing. Gas mixtures would be varied and coal dust 

added in over fifty tests prescribed to determine the 

greatest internal enclosure pressure. 

Occasionally excessive pressures, due to “pressure heaping.” would be recorded especially where 

two internal compartments were joined via a small 

interconnecting passage (Gleim 1929). The conditions leading to these excessive pressures were better defined in a subsequent study (Gleim and Marcy 

1952). A metallic foam material, investigated for 

potential use in large volume enclosures as a vent 

and flame arrestor, performed satisfactorily for methane-air mixtures (Scott and Hudson 1992). 

Tests were also conducted on broken incandescent lamps to relate newly introduced tungsten 

filament temperatures to the “fire damp.” (methaneair) gas ignition threshold (Clark 1912) and to show 

that broken lamps can ignite methane-air mixtures 

(Clark and Ilsley 1913). These tests also showed that 

higher wattage carbon filament lamps were necessary to ignite a given methane-air mixture compared 

with tungsten filaments. They were followed up by 

tests of electric lamps for miners for permissibility 

according to Schedule 5, a key criterion of which 

was that (Clark, 1914) “under no circumstances can 

the bulb of a completely assembled lamp be broken 

while the lamp filament is glowing at a temperature 

sufficient to ignite explosive mixtures of mine gas 

and air.” Electric cap lamps were introduced and 

approved under Schedule 6. Yet in 1935, more than 

half of the Nation’s miners still used open carbide 

lights for illumination (Zellers and Hooker 1935). 

The first electrically powered coal mining 

machine, the coal cutter, was seeing widespread use 

by the 1920s (Morley 1990). Tests to determine the 

permissibility of cutting machine components were 

conducted in a new gallery in Pittsburgh (at that 

time located on Forbes Avenue adjacent to Carnegie 

Mellon University) which was designed jointly by 

the Bureau and the Jeffrey Manufacturing Company 

(Ilsley and Gleim 1920). This chamber shown in 

Figure 1 (Ilsley, Gleim, and Brunot 1941) consisted 

of a cylindrical steel tank with an internal diameter 

of 10 feet and a height of 41⁄2 feet. Four observation 

windows were spaced equally around the chamber. 

figure 1. Explosion-proof motor testing gallery, 

Pittsburgh (ilsley and Gleim 1920) 

The Bureau recognized the potential fire and 

explosion hazards from batteries used in mine locomotives. Investigations indicated that the safety of 

the locomotive would depend largely on the proper 

care of the batteries. Therefore, mechanical endurance tests were devised to prove their adequacy and 

later were incorporated into Schedule 15 (Ilsley and 

Brunot 1922). The first storage-battery approval was 

issued in 1921 (Brunot and Freeman 1923). 

Permissible equipment and explosives lists 

were published annually as Technical Papers. 

Periodically guides, based upon the latest approval 

schedules, were issued to facilitate the field inspection of permissible equipment (Gleim 1932; Gleim 

1954) and, for machine designers (Gleim, James, and 

Brunot 1955). By 1923, a number of electric apparatus had been listed as approved including: 5 electric 

cap lamps, 20 coal-cutting machines, 4 electric drills 

and 5 battery locomotives (Ilsley 1923). Despite 

the publication of approval requirements, the widespread use of unapproved equipment in underground 

mines persisted into the 1920s (Ilsley 1922). From 

1910 to 1924, there were 499 fatalities in disasters 

and fires caused by ignitions from electrical apparatus and circuits (Ilsley 1924). However, by the end 

of the decade there was a noticeable increase in the 

use of permissible equipment, even though maintaining them in permissible condition was still a problem (Gleim and Freeman 1932). By the mid 1930s 

rock-dust distributors, air-compressors, pumps, and 

room hoists were added to the classes of permissible 

equipment. 

In the early 1980s, a guide was prepared for 

the design of explosion-proof electrical enclosures. 

It addressed those aspects of design that affected 

enclosure strength and ruggedness. Materials such 

as polycarbonates, as well as adhesives and sealants, 

which may be degraded by the mine environment, 

were also analyzed (Staff 1982). Research into the 
by-products of electrical arcing in X/P enclosures 

found that hydrogen and acetylene can be formed as 

certain organic insulators decompose. Research produced acceptance criterion for adhesives and sealants 

and a design guide for explosion-proof enclosures 

(Cox and Schick 1985). Performance-based design 

and test recommendations were published for enclosures containing high-voltage components (Berry 

and Gillenwater 1986). These recommendations 

were referenced by MSHA to help formulate new 

regulations governing the use of high-voltage longwall machines in coal mines (Federal Register 2002). 

Intrinsic safety research concentrated on basic 

studies into ignition mechanisms. Factors influencing the minimum igniting current level were identified. Early work focused on capacitive discharges 

(Lipman and Guest 1959). Later, spark ignition probabilities were defined for various electrode materials 

and for resistive, inductive, and capacitive circuits 

under varying temperature, humidity, and atmospheric conditions (Cawley 1988; Peterson 1992). 

traiLinG CaBLES 

Initially most of the inspection, testing, and approval 

of electrical equipment concentrated on the explosion-proof integrity of enclosures. However, trailing cables had been employed since shortly after the 

turn of the century and the Bureau of Mines generally accepted the cables that were specified by the 

manufacturer for a particular machine. One of their 

first uses was in connection with cable-reeled, railmounted locomotives. These cables, featuring a 

weatherproof braid, had a short life and were largely 

superseded by rubber-sheathed cables in the 1920s 

when three types of cables were in general use: a 

concentric 2-conductor, a flat or twin duplex, and a 

round triplex (Ilsley, Hooker and Coggeshall 1932). 

By 1930, a new emphasis was being placed upon 

the dangers from wiring and connections outside the 

enclosures (Ilsley and Brunot 1931). Field inspections had shown that the workmanship of splices in 

concentric cables was typically poor and that they 

were seldom vulcanized to exclude moisture (Ilsley 

and Hooker 1929). Schedule 2C, published in 1930, 

specified a performance test for two-conductor trailing cables subjected to abuse due to being run over 

by rail-mounted cars and locomotives (Ilsley 1930). 

The performance of twin cables was found in tests 

to be far superior to the concentric design. The overheating of rubber-insulated reeled cables was also 

investigated by electrical loading and measurement 

of resultant temperature rise. Revised ampacities 

were established for various cable sizes (Ilsley and 

Hooker 1931). 

The standard procedure for locating faults in 

trailing cables into the 1970s was to apply a high 

level of electric power to blow a hole in the cable 

at the fault site, a crude and unsafe procedure. The 

Bureau developed safe, portable devices that locate 

faults accurately and rapidly based upon infrared, 

electromagnetic, and time domain reflectometer 

(TDR) techniques. Three manufacturers marketed 

electromagnetic devices based upon the original 

Bureau design (Bureau of Mines 1981). 

Industry cable splicing practices were studied 

in a 1960 survey of 440-V and 4,160-V cables at an 

Indiana surface mine (Douglas 1960). A follow-up 

survey of industry practices uncovered deficiencies 

in cable repair and care (Williams and Devett 1962). 

In the late 1970s, cable-related research concentrated on improved splicing procedures for trailing 

cables (Staff 1984). Programs were initiated to redesign splice kits to better protect spliced cables from 

short circuits and moisture intrusion and to increase 

mechanical strength. Improved crimp connectors and 

joining sleeves were designed. This work culminated 

in splicing workshops conducted for industry personnel and the effort saw 12 major kit manufacturers improve their products based upon Bureau work. 

The operating characteristics of repaired (spliced) 

portable power cables were later compared with 

undamaged cables (Yenchek, Schuster, and Hudson 

1995) and deficiencies in the thermal rating of splice 

kit insulating tapes were noted. 

In the 1980s, in cooperation with MSHA, 

the American Mining Congress and the Insulated 

Cable Engineers Association, the Bureau initiated 

an extensive investigation into the operational characteristics of trailing cables powering coal mining 

machinery. Initially a relationship was established 

between electric current and resulting temperature 

rise of the cable conductors (Yenchek and Kovalchik 

1989; Yenchek and Kovalchik 1991). These extensive laboratory tests were the basis for the subsequent development of a computerized thermal model 

for cables operating in open air (Yenchek and Cole 

1997). Machine designers could apply this interactive program to determine anticipated cable temperatures given the electrical load requirements of new 

machines. Accelerated life tests of cable insulation 

and jacket materials were conducted to determine the 

long-term effects of elevated temperatures (Yenchek 

and Kovalchik 1989). Arrhenius models were constructed that related cable thermal life to operating 

temperature for the cable materials. A correspondence was then established between current load and 

useful thermal life (Yenchek and Kovalchik 1993). 

Using this methodology and knowing the electrical 

characteristics of a particular machine, a designer 
 

could theoretically predict and compare the available service lives of different cable sizes. Finally, 

research was conducted to measure the temperature 

of cables wrapped on shuttle car reels as shown in 

Figure 2 (Kovalchik, Scott, Dubaniewicz, and Duda 

1999). 

figure 2. Cable reel thermal test in the Mine 

Electrical Laboratory 

A novel method was devised to measure 

internal cable temperatures with an imbedded optical 

fiber. This work allowed for a more accurate calculation of ampacity derating factors for reeled cables, 

especially under cyclic loads. 

ELECtriCaL PoWEr SyStEMS 

In 1916 in cooperation with professional societies, 

operators, and equipment manufacturers, the Bureau 

of Mines published a suggested guide for the installation and use of underground and surface electrical equipment in bituminous coal mines (Clark and 

Means 1916). In 1923 a suggested guide for electrical safety inspection was published (Ilsley 1923) 

and in 1926 one for the design of accessories used 

in permissible equipment (Ilsley and Gleim 1926). 

A subsequent publication covered surface as well 

as underground installations and recommended the 

National Electric Code as a suitable reference (Gleim 

1936). Ultimately an electrical safety standard, cosponsored by the American Mining Congress and 

the Bureau, was published as a supplement to federal 

and state regulations (American Standard 1952) and 

was periodically revised. 

Periodically the Bureau would publish warnings to the industry about dangerous electrical system practices. In 1929 a Bureau publication pointed 

out a hazard associated with equipment use, that of 

cables and equipment overheating due to low voltage on direct current systems as a result of insufficient copper and poor rail bonding (Ilsley 1929). 

Incredibly, voltages measured at equipment in some 

mines were as low as 40 to 80 volts on 250 DC systems! Yet, equipment operating at voltages greater 

than 250 volts was viewed as potentially more hazardous and difficult to maintain as permissible (Ilsley 

1932). Although most mines in the U.S. were still 

using DC to power machinery in 1960, the use of 

alternating current was growing due to concerns 

with fires and voltage regulation on DC systems, 

concurrent with the lifting of AC distribution voltage limits underground by individual states (Morley 

1990). Also current interruption was far easier with 

AC. The Bureau, recognizing this trend, published 

recommendations for the use of AC, and endorsed 

the wye-connected, resistance-grounded neutral 

transformer secondary configuration due to its inherent safety advantages (Staff 1960). In the 1970s a 

series of Application Notes (Figure 3) on a variety 

of aspects of mine power system operation were produced for distribution to the mining community. 

figure 3. Loose-leaf power system handbook 

In 

1990, a comprehensive engineering reference was 

published and was used to instruct university students on the design of mine power systems (Morley 

1990). More recently, NIOSH, in an analysis of 

underground explosions, showed that many were 

caused by electrical equipment in intake air entries. 

This led to recommendations to apply the hazardous 

location provisions of the National Electrical Code 

in these areas (Dubaniewicz 2007). 

GrounDinG 

A good grounding system is paramount for the protection of personnel in a mine or mineral processing 

facility. Prompted by unwillingness on the part of 

the mining industry to provide adequate grounding 

of equipment enclosures in the 1920s, a compilation of best practices was published to promote the 
 

importance of grounding mine electrical systems 

to minimize shock hazards (Ilsley 1930). Bureau 

of Mines engineers characterized the effectiveness of ground rods, machines in earth contact, and 

rail through resistance measurements (Griffith and 

Gleim 1943). The effectiveness of drill-hole casings 

as a grounding medium was confirmed through field 

measurements (Griffith and Gleim 1944). A study, 

conducted out of the Bureau’s Birmingham, Alabama 

office, showed how the impedance offered by moist 

earth could increase over time when conducting 

ground fault currents (McCall and Harrison 1952). 

To assist in mines in complying with the requirements for a low-resistance grounding medium, the 

Bureau undertook research to systematize the design 

of ground beds based upon soil and other environmental conditions. This work led to publication of 

a guide that greatly simplified the design of drivenrod ground beds (King, Hill, Bafana, and Cooley 

1979; Staff 1979) along with workshops (Figure 4) 

for industry personnel. 

figure 4. Ground bed design workshop at 

Pittsburgh research Laboratory, 1980 

Other ground bed subjects 

investigated were ground bed measurement, corrosion of ground rods, and composite bed materials 

(Staff 1985). 

The 1969 Act required a device to monitor 

ground conductor continuity to prevent accidental electrocution resulting from high frame potentials. Bureau research produced such a device, 

which served as an industry prototype (Staff 1979). 

Contractual research in the 1970s developed a method 

to monitor safety grounding diodes on underground 

DC circuits, quantified earth contact resistance for 

various electrodes, and devised a prototype device to 

measure ground return resistance for open-pit mine 

machines (Bennett, Sima, and King 1978). 

ELECtriCaL MaintEnanCE 

Accidents associated with the maintenance of electrical equipment, especially troubleshooting and repair, 

have, over the years, been a major cause of injuries 

in mines. Design practices, including dead-front construction, voltage segregation, and interlocks, were 

identified as means to minimize shock during control box maintenance (Staff 1982). Beginning in the 

1970s and continuing well into the 1990s the Bureau 

of Mines sponsored and conducted research into the 

early prediction of insulation failure in electrical 

components, focusing primarily on electric motors. 

Research efforts concentrated on sophisticated 

computer-based analysis of motor input power measurements during normal operation, and included 

contract research (Kohler and Trutt 1987), in-house 

work at the Pittsburgh Research Laboratory’s (PRL) 

Mine Electrical Laboratory (Homce 1989), cooperative work with the Navy, and extensive field data 

collection (Homce and Thalimer 1996). The serious 

problem of electrical burns due to arc flash incidents 

in mining was addressed by identifying and promoting prevention methods used successfully in other 

industries (Figure 5) (Homce and Cawley 2007). As 

part of this work, a safety training package, titled 

“Arc Flash Awareness.” was released on DVD. 

Thousands of copies were distributed directly, and 

it had extensive promotion, distribution, and use by 

other agencies and organizations, including translation into Spanish by the Electrical Safety Foundation 

International (Kowalski-Trakofler, Barrett, Urban, 

and Homce 2007). 

ProtECtion DEviCES 

Devices such as fuses, circuit breakers, vacuum 

interrupters, and relays are use to protect against fire 

and shock. A 1951 study out of the Bureau of Mine’s 

Birmingham, Alabama office showed that a fuse in 

combination with a short-circuiting contactor could 

provide effectively the same protection against short 

circuits as a more expensive circuit breaker (Harrison 

1951). A resettable current-limiting prototype was 

built and tested to minimize arcing during interruption of DC currents (Hamilton and Strangas 1978). 

A portable calibrator was developed for on-line use 

with DC circuit breakers (Bureau of Mines 1981). 

In the 1980s, in efforts to improve personnel shock 

protection, the Bureau investigated the application of 

sensitive ground fault relays (GFR’s) to mine power 

circuits (Trutt, Kohler, Rotithor, Morley, and Novak 

1988). Mine-worthy prototype devices with sensitivities in the milliampere range were designed and 
 

 

constructed for both AC and DC circuits (Figure 6) 

(Yenchek and Hudson 1990). NIOSH investigated 

how the starting of large AC induction motors might 

cause nuisance tripping of short-circuit protection on 

coal mine power systems. A method was devised to 

provide short-circuit protection without intentional 

time delays to account for motor starts (Yenchek, 

Cawley, Brautigam, and Peterson 2002). A recent 

study concluded that the significant capacitance of 

shielded cables on mine distribution systems can 

have a detrimental effect on high voltage ground 

fault relay selectivity (Sottile, Gnapragasam, Novak, 

and Kohler 2006). 

figure 5. appropriate arc flash personal 

protective equipment demonstrated by nioSH 

researcher 

figure 6. Sensitive aC Gfr’s installed in Safety research Coal Mine 

troLLEy WirES 

The fire and shock dangers associated with the use of 

bare trolley wires for rail coal haulage had long been 

recognized by the Bureau of Mines. A 1932 Bureau 

study showed that of 71 electrical fatalities in one 

state during a 5-1⁄2 year period, 55 were from contact 

with trolley wires operating at 250 volts or less. The 

hazards associated with the use of bare trolley wires 

were tied to poor installation, lack of guarding, inadequate maintenance, and back-poling (Gleim 1932). 

During a 19-month period, in 1943–44, 101 mining 

fatalities resulted from four fires that originated on 

the trolley system, as the coal industry attempted to 

meet the demands of war production. A contemporary 
Bureau publication suggested the elimination of bare, 

single-wire trolley by the application of two-wire 

overhead systems (eliminating the track as a return), 

circuit breakers that would discriminate between 

heavy fault and load currents, and diesel or batterypowered locomotives (Griffith, Gleim, Artz, and 

Harrington 1944). In the early 1980s the Pittsburgh 

Research Center demonstrated technology that could 

detect such faults (Staff 1982). Since this approach 

required additional wiring along the length of the 

haulage way, it was never adopted by the industry. A 

final attempt to develop a ‘smart’ circuit breaker for 

trolley haulage involved tests at cooperating mines 

to develop a neural-network detection algorithm 

based upon the rectifier current signature (Figure 7) 

(Peterson and Cole 1997). 

figure 7. tests on DC haulageway at Lake Lynn Laboratory 

A simple, low-cost means 

to detect hot trolley insulators was prototyped 

(Yenchek and Hudson 2000). Although successful, 

the smart circuit breaker and hot insulator detector 

were never deployed due to the industry-wide transition from rail to belt for coal haulage. 

SurfaCE MininG 

The electrification of shovels and draglines began 

before 1920 (Morley 1990). As horsepower requirements increased, so did distribution and utilization 

voltages. An area of interest to the Bureau of Mines 

and later, NIOSH, was the electrical shock hazard from accidental contact of overhead electrical 

power lines, both at surface mines and the surface 

facilities of underground mines. Such accidents usually involve high reaching mobile equipment like 

cranes or other equipment such as scaffolds and ladders and are a leading cause of electrical fatalities at 

mine sites. Early contract work thoroughly analyzed 

the problem, and looked at available warning-device 

solutions (Hipp, Henson, Martin, and Phillips 1982) 

(Morley, Trutt, and Homce 1982). NIOSH developed 

and patented (Sacks, Cawley, Homce, and Yenchek 

1999) a practical low-cost concept to detect line 

contact and emit a warning (Sacks, Cawley, Homce, 

and Yenchek 2001). More recent research has evaluated currently available warning devices (Figure 8) 

(Homce, Cawley, and Yenchek 2008). 

aCCiDEnt anaLySES 

Periodically, analyses of accidents were conducted 

which showed a continued need for research aimed 

at improving electrical safety in mines. A 1936 analysis of electrical accidents in coal mines showed 

that electricity ranked fourth as a cause of coal mine 

fatalities, excluding deaths from fires and explosions 

of electrical origin (Harrington, Owings, and Maize 

1936). Ninety percent of electrocutions were caused 

by contact with the bare trolley wire. Forty-one percent of the explosions for the period 1928–1935, 

resulting in 476 deaths, were caused by the electrical ignition of gas or coal dust. These were mainly 

due to the use of open-type equipment or poorly 

maintained permissible equipment. From 1930–35, 
 

fires of electrical origin were the cause in 29% of the 

fatalities from all mine fires (Harrington, Owings, 

and Maize 1936). An analysis of electrical accidents in the late 1970s and Bureau research yielded 

a cost-benefit analysis for electrical safety projects 

(Tenney, Cooley, and Elrazaz 1982). Recent NIOSH 

studies of occupational electrical injuries have indicated that, despite dramatic improvements over the 

years, mining has an electrical injury rate that is still 

much higher than many other industries. These studies, in addition to highlighting the need for continued improvements in mining electrical safety, were 

very influential in the broader electrical safety community. The first (Cawley and Homce 2003) was 

described as a “landmark research study.” and “one 

of the most, if not the most, comprehensive analyses 

of occupational electrical injuries ever undertaken.” 

in the April 2004 IEEE Industry Applications magazine. The second was hailed as “seminal research.” 

in the May/June 2007 edition of the same publication (Cawley and Homce, 2006). Within mining 

(Figure 9), electricity continued to rank fourth as a 

cause of death (Cawley 2003). Burns were found to 

be the leading type of injury, but were rarely fatal. 

Electrical shock was the overwhelming cause of 

electrical fatalities. 

figure 8. full scale power line proximity alarm 

test at Pittsburgh research Laboratory 

figure 9. Electrical accident data analysis 

SuMMary anD futurE DirECtion 

The Bureau of Mines/NIOSH electrical safety 

research program has focused on a wide variety of 

topics over its 100-year history. Research has been 

conducted to address safety issues associated with 

advancing technology as well as to develop new 

solutions prompted by regulatory mandates. Future 

priorities will be driven by surveillance, customer/ 

stakeholder input, and risk analyses and should 

include overhead power line, electrical maintenance, 

and lightning hazards. The technical feasibility of an 

improved power line proximity warning system that 

does not depend on electric field detection should be 

studied. There will be a continuing need to provide 

effective training for both experienced and younger, 

inexperienced maintenance personnel to increase 

awareness of electrical shock and burn hazards and 

the need for, and type of, protections available. The 

procedures for electrical maintenance can be systematized by developing job hazard methodologies for 

critical tasks. Finally, further investigation is needed 

in the future to achieve a better understanding of 

lightning and develop practical guidelines and recommendations that can help mine operators reduce 

the chance of ignitions and protect workers from the 

resulting hazards. 

rEfErEnCES 

American Standard M2.1. 1952. American Standard 

Safety Code for Installing and Using Electrical 

Equipment in and about Coal Mines. Bureau 

of Mines Bulletin 514. Washington, DC: U.S. 

Government Printing Office. 

Bennett, J.L., G.R. Sima, and R.L. King. 1978. 

Electric Shock Prevention. Proceedings of 

the Fourth WVU Conference on Coal Mine 

Electrotechnology, Morgantown, WV, Aug 2–4, 

1978. 
Berry, D.R. and B. Gillenwater. 1986. Development 

of a High-Voltage Permissible Load Center 

(contract H0308093, Foster Miller, Inc.) 

BuMines OFR 58-86, 136 pp.: NTIS PB 

86-215803. 

Brunot, H.B and H.B. Freeman. 1923. Permissible 

Electric Drills. Bureau of Mines Reports 

of Investigations 2434. Washington, DC: 

Government Printing Office. 

Bureau of Mines. 1981. Technical Highlights: 

Health and Safety Research, 1970–1980. Supt. 

of Docs. No. I 28.151. 

Cawley, J.C. 1988. The Probability of Spark Ignition 

in Intrinsically Safe Circuits. Proceedings of 

the Ninth WVU Mining Electrotechnology 

Conference. Morgantown, WV, July 26–29. 

p. 128. 

Cawley, J.C. 2003. Electrical Accidents in the Mining 

Industry, 1990–99. IEEE Trans Ind Appl Nov/ 

Dec 39(6): 1,570–1,577. 

Cawley, J.C. and G.T. Homce. 2003. Occupational 

Electrical Injuries in the U.S., 1992–1998, and 

Recommendations for Safety Research. Journal 

of Safety Research, Vol. 34 pages 241–248. 

Cawley, J.C. and G.T. Homce. 2008. Trends 

in Electrical Injury, 1992–2002. IEEE 

Transactions on Industry Applications, Vol. 44, 

No. 4. 

Clark, H.H. 1911. The Electrical Section of the 

Bureau of Mines, Its Purpose and Equipment. 

Bureau of Mines Technical Paper 4. Washington, 

DC: Government Printing Office. 

Clark, H.H. 1912. Ignition of Gas by Miniature 

Electric Lamps with Tungsten Filaments. Bureau 

of Mines Technical Paper 23. Washington, DC: 

Government Printing Office. 

Clark, H.H. 1914. Permissible Electric Lamps for 

Miners. Bureau of Mines Technical Paper 75. 

Washington, DC: Government Printing Office. 

Clark, H.H. 1915. Permissible Explosion-Proof 

Electric Motors for Mines; Conditions and 

Requirements for Test and Approval. Bureau of 

Mines Technical Paper 101. Washington, DC: 

Government Printing Office. 

Clark, H.H., and L.C. Ilsley. 1913. Ignition of Mine 

Gases by the Filaments of Incandescent Lamps. 

Bureau of Mines Bulletin 52. Washington, DC: 

Government Printing Office. 

Clark, H.H., and C.M. Means. 1916. Suggested 

Safety Rules for Installing and Using Electrical 

Equipment in Bituminous Coal Mines. Bureau 

of Mines Technical Paper 138. Washington, 

DC: Government Printing Office. 

Cox, P.A. and W.R. Schick. 1985. A Design Guide 

for Explosion-Proof Electrical Enclosures. Part 

3-Final Technical Report. (Contract H0387009, 

SW res. Inst.). BuMines OFR 47C-86, 353 pp.: 

NTIS PB 86-209525. 

Douglas, S.J. 1960. Testing and Splicing Electric 

Cables and Frame-Grounding Pit Equipment. 

Bureau of Mines Information Circular 7995. 

Washington, DC: Government Printing Office. 

Dubaniewicz, T.H. 2007. The Brookwood Disaster 

and Electrical Requirements for Hazardous 

(Classified) Locations. Proceedings of the IEEE 

Industry Applications Society Annual Meeting, 

N. Orleans, LA: 1,353–1,359. 

Federal Register. 2002. Electric Motor-Driven 

Mine Equipment and Accessories and HighVoltage Longwall Equipment Standards for 

Underground Coal Mines. Vol. 67, No. 47, 

pp. 10,972–11,005. 

Gleim, E.J. 1929. Abnormal Pressures in ExplosionProof Compartments of Electrical Mining 

Machines. Reports of Investigations 2974. 

Washington, DC: Government Printing Office. 

Gleim, E.J. 1932. Guarding Trolley Wires in Mines. 

Bureau of Mines Information Circular 6577. 

Washington, DC: Government Printing Office. 

Gleim, E.J. 1932. Notes Pertaining to Safety 

Inspections of Permissible Electric Mine 

Equipment. Bureau of Mines Information 

Circular 6584. Washington, DC: Government 

Printing Office. 

Gleim, E.J. 1936. Electrical Viewpoint in a Complete 

Safety Survey of a Coal Mine. Bureau of Mines 

Information Circular 6894. Washington, DC: 

Government Printing Office. 

Gleim, E.J. 1954. United States Bureau of Mines 

Standard for Inspection and Test of ExplosionProof Enclosures for Mining Equipment. 

Bureau of Mines Report of Investigations 5057. 

Washington, DC: U.S. Government Printing 

Office. 

Gleim, E.J. and H.B Freeman. 1932. Maintenance 

of Electrical Mine Equipment from the 

Viewpoint of the Safety Inspector. Bureau of 

Mines Technical Paper 537. Washington, DC: 

Government Printing Office. 

Gleim, E.J., R.S. James, and H.B. Brunot. 1955. 

Explosion-Proof Design and Wiring for 

Permissible Mining Equipment. Bureau of 

Mines Bulletin 541. Washington, DC: U.S. 

Government Printing Office. 

Griffith, F.E. and E.J. Gleim. 1943. Grounding 

Electrical Equipment in and about Coal Mines. 

Bureau of Mines Report of Investigation 3734. 

Washington, DC: Government Printing Office. 
Griffith, F.E. and E.J. Gleim. 1944. GroundResistance Measurements of Drill-Hole Casings. 

Bureau of Mines Report of Investigations 3756. 

Washington, DC: Government Printing Office. 

Griffith, F.E., E.J. Gleim, R.T. Artz, and D. 

Harrington. 1944. Prevention of Fires Caused 

by Electric Arcs and Sparks from Trolley Wires. 

Bureau of Mines Information Circular 7302. 

Washington, DC: Government Printing Office. 

Hamilton, H.B. and E. Strangas. 1978. Implementation 

of a High-Speed, Heavy Current, DC Switching 

System. BuMines OFR 56-78. 77 pp. NTIS: PB 

283 388/AS. 

Harrington, D., C.W. Owings, and E.R. Maize. 

1936. Some Suggestions on the Prevention of 

Electrical Accidents in Coal Mines. Bureau of 

Mines Information Circular 6919. Washington, 

DC: Government Printing Office. 

Harrison, L.H. 1951. The Short-Circuiting 

Contactor as an Electrical Protective Device 

for Coal Mine Service. Bureau of Mines 

Report of Investigation 4759. Washington, DC: 

Government Printing Office. 

Hipp, J.E., F.D. Henson, P.E. Martin, and G.N. 

Phillips, G.N. 1982. Evaluation of Proximity 

Warning Devices. Bureau of Mines Final 

Report, Contract JO188082. Washington, DC: 

Superintendent of Documents. 

Homce, G.T. 1989. Predicting the Failure of Electric 

Motors. Bureau of Mines Information Circular 

9211. Washington, DC: U.S. Government 

Printing Office. 

Homce, G.T. and J.C. Cawley. 2007. Understanding 

and Quantifying Arc Flash Hazards in the 

Mining Industry. Proceedings for the 2007 

Institute of Electrical and Electronic Engineers 

Industry Applications Society conference, New 

Orleans, LA. 

Homce, G.T. and J.R. Thalimer. 1996. Reducing 

Unscheduled Plant Maintenance Delays— 

Field Test of a New Method to Predict Electric 

Motor Failure. IEEE Transactions on Industry 

Applications, Vol. 32, No. 3, pp. 689–694. 

Homce, G.T., J.C. Cawley, and M.R. Yenchek. 2008. 

A Performance Evaluation of Overhead Power 

Line Proximity Warning Devices. NIOSH 

Information Circular 9510. Pittsburgh, PA: 

DHHS (NIOSH) Publication No. 2009-110. 

Ilsley, L.C. 1922. Ten Reasons Why Permissible 

Equipments Approved under Schedules 2B and 

15 are Safer than Unapproved Types. Bureau 

of Mines Reports of Investigations 2398. 

Washington, DC: Government Printing Office. 

Ilsley, L.C. 1923. Electrical Safety Inspection: 

Suggestions for Mine-Safety Engineers. Bureau 

of Mines Reports of Investigations 2541. 

Washington, DC: Government Printing Office. 

Ilsley, L.C. 1924. Hazards of Electric Sparks and 

Arcs in Coal Mines. Bureau of Mines Reports 

of Investigations 2626. Washington, DC: 

Government Printing Office. 

Ilsley, L.C. 1929. Hazards from Low or Under 

Voltage. Bureau of Mines Information Circular 

6201. Washington, DC: Government Printing 

Office. 

Ilsley, L.C. 1930. Performance Tests for Trailing 

Cables. Bureau of Mines Information Circular 

6263. Washington, DC: Government Printing 

Office. 

Ilsley, L.C. 1930. The Grounding of Electric 

Systems in and around Mines. Bureau of Mines 

Information Circular 6318. Washington, DC: 

Government Printing Office. 

Ilsley, L.C. 1932. 250 versus 500 Volts or More 

for Circuits in Gassy Mines. Bureau of Mines 

Information Circular 6556. Washington, DC: 

Government Printing Office. 

Ilsley, L.C. and H.B. Brunot. 1922. Endurance Tests 

of Storage Batteries for Use in Permissible 

Mine Locomotives. Bureau of Mines Reports 

of Investigations 2358. Washington, DC: 

Government Printing Office. 

Ilsley, L.C. and H.B. Brunot. 1931. Suggestions 

for Wiring Permissible Equipment. Bureau of 

Mines Information Circular 6463. Washington, 

DC: Government Printing Office. 

Ilsley, L.C. and E.J. Gleim. 1920. Approved 

Explosion-Proof Coal-Cutting Equipment. 

Bureau of Mines Bulletin 78. Washington DC: 

Government Printing Office. 

Ilsley L.C. and E.J. Gleim. 1926. Suggestions for the 

Design of Electrical Accessories for Permissible 

Mining Equipment. Bureau of Mines Bulletin 

258. Washington, DC: Government Printing 

Office. 

Ilsley, L.C. and E.J. Gleim. 1932. Data in Reference 

to Installation of Cables in Shafts and Boreholes. 

Bureau of Mines Information Circular 6595. 

Washington, DC: Government Printing Office. 

Ilsley, L.C., E.J. Gleim, and H.B. Brunot. 1941. 

Inspection and Testing of Mine-Type Electrical 

Equipment for Permissibility. Bureau of Mines 

Information Circular 7185. Washington, DC: 

Government Printing Office. 
Ilsley, L.C. and A.B. Hooker. 1929. A Preliminary 

Investigation of Rubber Sheathed ConcentricType Trailing Cables for Mining Machines. 

Bureau of Mines Reports of Investigations 

2909. Washington, DC: Government Printing 

Office. 

Ilsley, L.C. and A.B. Hooker. 1931. The Overheating 

of Rubber-Sheathed Trailing Cables. Bureau 

of Mines Report of Investigations 3104. 

Washington, DC: Government Printing Office. 

Ilsley, L.C., A.B. Hooker and E.J. Coggeshall. 1932. 

Rubber-Sheathed Trailing Cables. Bureau 

of Mines Bulletin 358. Washington, DC: 

Government Printing Office. 

James, R.S. 1950. Effect of Temperature on FlameArresting Properties of Flat Joints in ExplosionProof Mine Equipment. Bureau of Mines 

Report of Investigations 4639. Washington, 

DC: Government Printing Office. 

King, R.L., H.W. Hill, Jr., R.R. Bafana, and W.L. 

Cooley. 1979. Guide for the Construction of 

Driven-Rod Ground Beds. Bureau of Mines 

Information Circular 8767, 26 pp. 

Kohler, J.L. and F.C. Trutt. 1987. Prediction of 

Incipient Electrical-Component Failures. 

Bureau of Mines Final Report, Contract 

JO338028. Washington, DC: Superintendent of 

Documents. 

Kovalchik, P.G, L.W. Scott, T.H. Dubaniewicz, 

and F.T. Duda. 1999. Dynamic Temperature 

Measurement of Overheated Shuttle Car 

Trailing Cables in Underground Coal Mines. 

Trans. Soc. Min. Metal Explor. 306: 1–4. 

Kowalski-Trakofler, K., E. Barrett, C. Urban, and 

G.T., Homce. 2007. Arc Flash Awareness. DVD 

training package. DHHS (NIOSH) Publication 

No. 2007-116D 

Leitch, R.D., A.B. Hooker, and W.P. Yent. The 

Ignition of Firedamp by Exposed Filaments 

of Electric Mine Cap-Lamp Bulbs. Bureau 

of Mines Reports of Investigations 2674. 

Washington, DC: Government Printing Office. 

Lipman, K. and P.G. Guest. 1959. Measurement of 

Electrical Waveforms of Low-Energy Spark 

Discharges. Bureau of Mines Report of 

Investigations 5460. Washington, DC: U.S. 

Government Printing Office. 

McCall, M.C. and L.H. Harrison. 1952. Some 

Characteristics of the Earth as a Conductor 

of Electric Current. Bureau of Mines Report 

of Investigations 4903. Washington, DC: U.S. 

Government Printing Office. 

Morley, L.A. 1990. Mine Power Systems. Bureau of 

Mines Information Circular 9258. Washington, 

DC: U.S. Government Printing Office. 

Morley, L.A., F.C. Trutt, G.T. Homce. 1982. Electrical 

Shock Prevention, Volume IV—Overhead-Line 

Contact Fatalities. Bureau of Mines Final 

Report, Contract JO113009. Washington, DC: 

Superintendent of Documents. 

Peterson, J.S. 1992. Influence of Electrode Material 

on Spark Ignition Probability. Bureau of Mines 

Report of Investigation 9416. Washington, DC: 

U.S. Government Printing Office. 

Peterson, J.S. and G.P. Cole. 1997. Detection on 

Downed Trolley Lines Using Arc Signature 

Analysis. NIOSH Report of Investigations 9639. 

Pittsburgh, PA: DHHS (NIOSH) Publication 

No. 97-123. 

Powell, F.W. 1922. The Bureau of Mines, Its History, 

Activities, and Organization. The Institute for 

Government Research Monograph No. 3. New 

York and London: D. Appleton and Co. 

Sacks, H.K., J.C. Cawley, G.T. Homce, and M.R. 

Yenchek. 1999. Alarm System for Detecting 

Hazards Due to Power Line Transmission 

Lines. U.S. Patent No. 6,600,426. 

Sacks, H.K., J.C. Cawley, G.T. Homce, and M.R. 

Yenchek. 2001. Feasibility Study to Reduce 

Injuries and Fatalities Caused by Contact of 

Cranes, Drill Rigs, and Haul Trucks with High 

Tension Lines. IEEE Trans Ind Appl, 37(3): 

914–919. 

Scott, L.W. and A.J. Hudson. 1992. Performance of 

RETIMET Metal Foam Vents on ExplosionProof Enclosures. Bureau of Mines Report 

of Investigations 9410. Washington, DC: 

Superintendent of Documents. 

Sottile, J., S.J. Gnapragasam, T. Novak, and 

J.L. Kohler. 2006. Detrimental Effects of 

Capacitance on High-Resistance-Grounded 

Mine Distribution Systems. IEEE Trans Ind 

Appl Sep-Oct; 42(5): 1,333–1,339. 

Staff, Bureau of Mines. 1982. Underground Coal 

Mine Power Systems, Proceedings: Bureau 

of Mines Technology Transfer Seminar, 

Pittsburgh, Pa. Bureau of Mines Information 

Circular 8893. Sup. of Docs, 88 pp. 

Staff, Bureau of Mines. 1984. Splicing Mine Cables. 

Bureau of Mines Handbook 1-84, 92 pp. 

Staff, Bureau of Mines. 1985. Earth Grounding BedsDesign and Evaluation. Proceedings: Bureau 

of Mines Technology Transfers, Pittsburgh, PA 

and Reno, NV. Bureau of Mines Information 

Circular 9049, 24 pp. 

Staff, Health and Safety. 1960. Recommended 

Standards for Alternating Current in Coal 

Mines. Bureau of Mines Information Circular 

7962. Washington, DC: U.S. Government 

Printing Office. 
Staff, Mining Research. 1979. Mine Power Systems 

Research (In Four Parts). 2. Grounding 

Research. Bureau of Mines Information 

Circular 8800, 78 pp. 

Staff, Mining Research. 1979. Mine Power Systems 

Research (In Four Parts). 3. Circuit Protection. 

Bureau of Mines Information Circular 8801, 

148 pp. 

Tenney, B.S., W.L. Cooley, and Z. Elrazaz. 1982. 

Coal Mine Electrical Accident Reduction. 

Proceedings of the WVU Conf. on Coal Mine 

Electrotechnology, Morgantown, WV; July 

28–30, 1982, p. 137. 

Trutt, F.C., J.L. Kohler, H. Rotithor, L.A. Morley, 

and T. Novak. 1988. Sensitive Ground Fault 

Protection for Mines. Phase III: CoordinationFree GFR’s for AC Distribution (contract 

J0134025, PA State Univ.). 95 pp. 

Williams, F.A. and W.R. Devett. 1962. Prevailing 

Practices in Splicing Electric Trailing Cables 

in Coal Mines. Bureau of Mines Information 

Circular 8090. Washington, DC: Government 

Printing Office. 

Yenchek, M.R., J.C. Cawley, A.L. Brautigam, and 

J.S. Peterson. 2002. Distinguishing Motor 

Starts from Short Circuits through Phase Angle 

Measurements. IEEE IAS Transactions, Vol. 38, 

No. 1. 

Yenchek, M.R. and G.P. Cole. 1997. Thermal modeling of portable power cables. IEEE Transactions 

on Industry Applications 33(1):72–79. 

Yenchek, M.R. and A.J. Hudson. 2000. A Passive 

Means to Detect Hot Trolley Insulators. Min 

Eng 2000 Jan; 52(1): 43–47. 

Yenchek, M.R. and A.J. Hudson. 1990. The Bureau 

of Mines Ground-Fault Protection Research 

Program-A Summary. Bureau of Mines 

Information Circular 9260, 23 pp. 

Yenchek, M.R. and P.G. Kovalchik. 1989. Thermal 

characteristics of energized coal mine trailing cables. IEEE Transactions on Industry 

Applications 25(5):824–829. 

Yenchek, M.R. and P.G. Kovalchik. 1989. 

Mechanical performance of thermally-aged 

trailing cable insulation. IEEE Transactions on 

Industry Applications 25(6):1,000–1,005. 

Yenchek, M.R. and P.G. Kovalchik. 1991. Thermal 

characteristics of energized shielded and 

an unshielded reeled trailing cable. IEEE 

Transactions on Industry Applications. 

27(4):791–796. 

Yenchek, M.R. and P.G. Kovalchik. 1993. The 

impact of current load on mine trailing cable 

thermal life. IEEE Transactions on Industry 

Applications 29(4):762–767. 

Yenchek, M.R., K.C. Schuster, and A.J. Hudson. 

1995. Operational characteristics of trailing cable splices. Bureau of Mines, Report 

of Investigations 9540. Pittsburgh, PA: U.S. 

Department of the Interior 

Zellers, D.H. and A.B. Hooker. 1935. Maintaining 

the Permissibility of Electric Cap Lamps. 

Bureau of Mines Information Circular 6832. 

Washington, DC: Government Printing Office.