ELECTRIC SHOCK PREVENTION 

John L. Bennett 

Gordon R. Sima 

Senior Engineers 

The Bendix Corporation 

Environmental and Process Instruments Division 

Baltimore, Maryland 2 1204 

Roger L. King 

Industrial Hazards and Communications 

Pittsburgh Mining and Safety Research Center 

Bureau of Mines 

U. S. Department of the Interior 

Pittsburgh, PA 

ABSTRACT 

Electric shocks are a serious problem in the mining environment. Because of extremely wide ranging use of electrically powered equipment in the 

mining industry, the hazard to personnel due to electrical 'shock and related injuries are not restricted to any single area but cover the entire spectrum of 

mining activity. Electrical accidents, resulting in personal injury, due to improper or complete lack of grounding of electrically operated mining machines, 

form a significant percentage of the overall electrical accident picture. 

The object of this paper is to describe methods of detection of unsafe 

electrical conditions with respect to the grounding of electrically operated 

mining equipment. It also includes a discussion of prototype instruments 

which effectively monitor the condition of the grounding safety devices currently in use on mining equipment and warn of dangerous conditions as soon as they 

appear. 

The paper is based on work accomplished under USBM Contract Number 

50357115. The contract was initiated under the Pittsburgh Mining and Safety 

Research Center Program. It was administered under the technical direction 

of PM and SRC with Mr. Roger L. King as the Technical Project Officer. Mr. 

Daniel B. Dawkins was the Contract Administrator for the Bureau of Mines. 
PERSONNEL HAZARD DUE TO ELECTRIC SHOCK 

Much work has been done in the area of human response to electricity. 

The following values are given as a general indication of typical threshold 

values and serve only to define areas of interest for instruments being developed. 

Threshold of perception to 60 Hz AC between major extremities - 0.5 

mA . 

Muscle disfunction "let go current1' - 9 mA. 

Respiratory paralysis - 30 mA . 

Heart disfunction - 75 mA. 

Cardiac arrest - 4 amps. 

Hazardous potential threshold, skin contact - 40 V. 

Skin contact resistance - 1K-5K depending upon conditions. 

Body resistance between major extremities - 500 ohms (exclusive of 

skin). 

Skin effects disappear as voltage increases, higher potentials being 

able to pierce the skin's protective layer. 

The object of this paper is to discuss methods of detection of unsafe 

electrical conditions with respect to the grounding of electrically operated 

mining equipment. It also includes design information of prototype instruments which effectively monitor the condition of the grounding safety devices 

currently in use on mining equipment and warn of dangerous conditions as soon 

as they appear. 

A device of interest is a voltmeter that is intended to be permanently 

mounted on the mining machine and continuous$ monitor the electrical potential of the frame of the mining machine relative to the earth. The device also 

contains a presettable threshold level which is capable of giving a warning or 

opening a relay contact whenever the threshold potential is exceeded. 

Another is a device intended for use on DC operated machinery employing safety grounding diodes . The diode monitoring function is physically included in the machine mounted instrument housing with the single resulting 

package serving the dual purpose of frame potential measurement and safety 

diode condition monitoring. 
GROUNDING DIODE CHECKING 

Present Electrical Equipment 

The vast majority of electrically operated mining equipment in the 

field today is still manufactured on a unit by unit basis. While basic form, 

fit and function of individual machines remain fixed for relatively long periods 

of time, the individual electrical components included in each machine produced are subject to slight but constant variation. This is true of virtually 

all manufacturers including even the larger volume equipment producers. 

The variations in machine components are often necessitated by the manufacturer's responsibility to delivery large expensive equipment on a regular 

schedule regardless of random inventory shortages of specific component 

types. "Equivalent" substitutions are made whenever necessary to expedite 

production. In addition, the extremely large volume of repaired and rebuilt 

equipment makes the exact parts complement or electrical schematic of any 

operating piece of heavy mining equipment difficult to predict. Any equipment 

intended for installation in existing mining equipment must be universal in 

application and easily adaptable to changing conditions. 

Electrical Control Boxes - 

A large number of mining equipment manufacturers purchase entire 

control boxes, as a complete unit, from an independent supplier. These boxes 

are treated as a component part with the internal design particulars left to the 

box supplier. Here, also, actual numbers of particular internal components 

vary from unit to unit. However, as far as safety grounding diodes are concerned, virtually all diodes are functionally equivalent. 

Grounding Diodes 

Safety grounding diodes are used in DC powered mining equipment such 

as shuttle cards, to prevent voltage from appearing on the machine frame. 

They are connected between the frame and the DC return line so that if the hot 

side of the DC supply comes in contact with the frame, the diode will go into 

heavy conduction and shunt the power to ground via the safety ground lead and/ 

or the DC return until the breaker at the load center trips. 

Although many different individual grounding diode part numbers are 

in current use, the basic electrical characteristics of all units are virtually 

identical. 

Differences in part numbers are due to individual manufacturer's number systems, physical packaging differences between various units produced 

by the same manufacturer and difference in maximum voltage and current 

ratings of units installed in various machines. A typical practice followed by 
one control box manufacturer is to supply a single grounding diode on machines 

up to 50 hp, two diodes in parallel for 50-75 hp machines, and three in parallel for machines up to 100 hp. 

1. FailureModesofGroundingDiodes 

Silicon rectifiers of the type used as grounding diodes are 

generally fabricated by either compression bonding techniques or brazed construction. Compression bonding involves mounting the actual semiconductor 

inside a welded case such that a least one contact to the semiconductor wafer 

is made through a compression type contact. Brazed construction involves 

use of hard soldering techniques. Field experience seems to indicate little 

difference between construction techniques with ultimate device failures generally being characterized by either a shorted diode junction or a device whose 

pigtail lead has burned open due to excessive current under fault condition. 

Little field information is presently available regarding partial failures of 

grounding diodes on mining equipment except for some cases involving the detection of excessive reverse leakage prior to ultimate device failure. 

Failures may be classified as either electrical or mechanical 

nature. 

a. Electrical Failure 

(1) Overvoltage 

Overvoltage failure of silicon power rectifiers results from the application of reverse voltages exceeding the avalanche breakdown rating of the diode. In safety ground applications, this most often results from the extremely high level short duration voltage transients so common in current mine power systems. Once the reverse blocking voltage of a 

device has been exceeded, current begins to flow across the semiconductor 

junction. The resulting high level localized power dissipation results in a 

burn usually near the passivated edge of the element. Continued or repeated 

application of excessive voltage spreads the burn area, finally resulting in 

complete device failure. External indications of overvoltage damage prior to 

complete device failure are a reduction in reverse voltage blocking capability 

and an increase in reverse leakage current as the junction deteriorates. 

(2) Overcurrent 

In the mine usage, overcurrent failure results from 

electrical fault currents flowing through the diode. Failure due to overcurrent 

results from excessive heat concentrated in the junction region of the semiconductor element resulting in a general degradation passivation. This damage can result from either large continuous currents or high surge currents for 
relatively short periods of time. The currents required to cause overcurrent 

failure are orders of magnitude larger than those involved in failure due to 

overvoltage. This is because of the low forward voltage drop of the diodes; 

however, the ultimate effect is very similar. Damage due to overcurrent 

usually encompasses the major portion of the junction area and is manifested, 

prior to total device failure, by a reduction in the reverse blocking voltage of 

the device and an increase in reverse leakage current. Very high surge currents result in bulk burns of the semiconductor material and produce almost 

instantaneous failure. 

b. Mechanical Failure 

(1) bss of Hermeticity 

This type of failure generally results from physical 

abuse or improper mounting, destroying the integrity of the package seal. 

Failure is generally very slow to appear as the effect of the broken seal is 

usually not immediately apparent. The broken seal allows internal contact 

materials to oxidize resulting in higher flow conditions. This results in device failure due to excessive temperature. This type of failure should be rare 

in safety grounding applications as the diodes involved are not intended for and 

do not generally carry large continuous forward currents. They are, however, 

often subjected to physical abuse. In safety ground applications, a possible 

result of contact oxidation progressing for an extended period of time would 

be a gradual increase in forward voltage drop of the diode, due to increasing 

internal resistances; followed by final device failure in the open state, due to 

complete loss of internal contact. 

(2) Fractured Element 

Excessive mounting torque or poor mounting surface 

condition can deform the device package and physically fracture the semiconductor element. This results in a sudden voltage type failure at the facture 

point. Excessive physical stresses present at the time of mounting may not 

immediately fracture the element but cause device failure at some later date 

from a sudden temperature change in the semiconductor element such as that 

normally induced by relatively mild current surges. 

(3) Insufficient Mounting Torque, Poor Mounting Surface 

Insufficient mounting torque or load force results in 

high temperature due to lack of heat transfer between the diode case and its 

heat sink. 

This type of failure should be rare in safety ground 

usage because of the general absence of large continuous currents in these 
devices. Excessive temperature operation from any cause is detectable by a 

marked increase in reverse leakage as junction temperature rises. Leakage 

currents double for every 10°C increase in junction temperature. Leakage 

currents reduce as the junction temperature decreases provided maximum 

operating junction temperatures (approximately 190 OC) are not exceeded. 

VOLTAGE MEASUREMENTS 

Earth Contact Resistance 

A study was made to determine the earth contact resistance to be expected from earth electrodes of various types. This was done to gain insight 

into the magnitude of the problem associated with establishing a local earth 

reference in an operating mine from a moving piece of equipment. Measured 

values were obtained for four separate electrode configurations at two different locations in the Federal #2 Mine of the Eastern Associated Coal Company 

near Fairview, West Virginia. The electrodes tested included a metal whip 

antenna, a heavy chain dragging along the fllor, a conductive wheel, and a 

hoe or plowing type electrode. With the exception of the chain, separate 

measurements were made with the electrodes contacting the roof as well as 

the floor of the mine at each location. The two locations includes a recently 

mined area within 800 feet of the active face of the mine, and a second site 

near the base of the main shaft which had not been the scene of active mining 

for several years. A summary of the data is found in Table A. 

Table A. Ranges of Earth Contact Resistances for Various Electrodes 

Electrode Range of Earth Contact Resistances 

Floor Roof 

Whip 3K - 1000 M 3K - 2000 M 

Chain 500K - 2000 M - 

Wheel 4K - 500 M 40K - 2000 M 

Hoe 400K - 1000 M 400K - 250 M 

In addition to the measured electrode resistance value, it was noted in 

the report that the earth potential within several feet of a line machine frame 

is significantly altered such that the immediately adjacent earth assumes the 

potential of the live frame. 

This information was verified by other researches at West Virginia 

University during independent study work in the area of ground potential 

gradients near haulage rails in coal mines, and is attributed to the relatively 

high resistivity of the earth in and around the coal. 
This finding is significant in that it definitely indicates that any frame 

potential measurement instrument intended for mine usage and using the earth 

as a reference must not obtain its earth reference point from directly under 

the machine but at some remote location. From present data, it appears that 

an earth conduction distance of 10-15 ft. from the frame is sufficient, and 

further testing may show the mine roof above the machine to be the most convenient location, even though contact resistance data indicates the roof contact in general to be of higher resistance than floor contact. 

Machine Ground Contact Resistances ----------- 

One parameter affecting the performance of any instrument monitoring 

frame and/or ground circuit potentials in mine applications is the contact resistance of the machine frame-earth interface. This is of interest because it 

provides a second ground contact path from the machine frame which does not 

include the cable ground conductor. 

Investigations into ground contact resistances of various large open pit 

shovels and drag lines on soils of various resistivity gave measured ground 

contact resistances of from 100 ohms to less than one ohm. Estimates of the 

contact resistance of smaller, crawler mounted machines such as bulldozers, 

miners and loaders, having smaller total weight and corresponding smaller 

contact area, indicate expected values of ground contact resistance to be in the 

range of one ohm to 10,000 ohms. 

Instruments designed to monitor frame potential and/or leakage currents must not be adversely affected by frame-earth contact resistance in this 

range, and may, in fact, be capable of using it to some advantage. 

OPEN PIT GROUND CONDUCTOR VERIFICATION 

It became apparent during the Phase I and Phase I1 investigations of 

our contract that an instrument designed to easily monitor the condition of 

long, open pit mining machine grounding conductors would be of interest to 

the Bureau of Mines. It also appeared that the sensors and techniques developed during this program may have application in this area. 

Looking at the machine frame grounding system from the machine end 

of the trailing cable gives two electrical paths to earth. The first is through 
the cable ground conductor and the second is a parallel path from the frame 

to earth through direct frame-earth contact. It was desirable to design an instrument which can measure the cable ground path resistance regardless of 

the value of the parallel frame-earth path. Bendix did this through the use of 

a technique similar to that used to measure ground contact resistance of 

driven rods. Figure 3 shows a simplified schematic of the system. 

If a small potential relative to earth is impressed on the frame of a 

piece of equipment in direct contact with the earth, a small current will flow. 

The ratio of the impressed frame potential to the resulting current flow would 

result in a measure of its earth contact resistance. If the earth contact is 

made by two separate paths, one directly to earth and one via the cable ground 

conductor, the ratio of the impressed frame potential to the current flow in 

only the cable ground conductor will result in a measure of only the cable 

associated earth contact resistance, including the ground conductor and ground 

bed resistances. 

Measurements made in open pit metal mines by Pennsylvania State 

University indicate residual currents flowing in machine safety ground conductors due to induced volt ages, capacitance effects, and ground potential 

gradients are generally less than 10 amperes, peak value. Since ground 

check measurements using this instrument will be made without the machine 

in use, values of residual currents flowing in the ground conductor at the time 

of measurement are expected to be quite low, with phase current being essentially zero. For this reason, the full-scale sensitivity of the cable current 

sensor may be kept quite low, allowing for accurate detection of the instrument-induced current variations which are expected to be in the range of 0.2 

to 1 ampere. 

The small suitcase sized prototype instrument is designed to measure the safety ground return resistance level, as seen from the machine end of 

the cable. The instrument is intended for open-pit mine usage and the resultant ground return resistance measurment would include any and all paths to 

earth, as seen from the machine frame, including the total trailing cable, the 

power center safety ground bed resistance, and the effects of any additional 

equipment tied to the same safety ground bed. The measurement does not include the earth contact effects of the machine frame at the measurement location. Use of the instrument requires the installation of a pass-through current 

sensor around the safety ground conductor of the trailing cable and requires 

the availability of some good quality earth contact within a reasonable distance 

from the machine location. 

A battery powered current source is connected between the machine 

frame and the current probe. The impressed current is limited to less than 

1 ampere. If a good quality current probe electrode, such as a buried metallic 

pipe or heavy piece of fuel-powered equipment (bulldozer) is unavailable within 
OPEN PIT GROUND CONDUCTOR VERl FlCATlON 

S IMPLlFlED SYSTEM SCHEMAT lC 

RB = Trailing Cable and Safety Ground 

Bed Resistance 

RC = Machine to Ground Resistance 

RD = Current Ground Rod Resistance 

Let RA = RB~~RC 

Let R= RA + RD + 100 

Current from Inverter = 94 + 

Frame Voltage Current X RA 

RB = Frame Voltage i Current 
a distance of several hundred feet, a ground rod will have to be driven for use 

as the current probe. The current impressed between the current probe 

(earth) and the machine frame will divide and return to earth via the trailing 

cable ground conductor and the direct frarne/earth contact. Figure 4 shows a 

simplified system block diagram. 

The current flowing in the trailing cable ground conductor will be monitored using a pass-through current sensor. The electrical potential of the 

frame relative to an earth potential probe driven near the machine site is 

monitored employing a very high input resistance voltmeter (10 Meg). 

The instrument current source is a standard 400 Hz inverter with a 

100VA rating. A 100 ohm resistor has been placed in series with the convert 

output to limit the current to less than 1 ampere. Filtering is used to insure 

the system will measure only the 400 Hz components and not be influenced by 

60 Hz stray currents. 

CONC LUSIONS 

Detection and prevention of electrical shock hazards is an ongoing program of great importance. The equipment and techniques described briefly 

in this paper are but one small effort. 

The current field tests of the diode monitoring equipment will yield 

information as to actual operating environment characteristics of the system. 

It is apparent that effort is needed to arrive at a viable way of establishing a local ground potential reference point in the mines to determine possible shock hazard conditions on the machines. 

With respect to the Open Pit Ground Conductor Verification System, 

field tests are needed to prove out the equipment and techniques. It may be 

possible to use similar equipment to rapidly assess the condition of safety 

ground beds. 

# 

Continuing effort between the Bureau, industry, and the mining community will make the mines a safer and more productive operation. 

ACKNOWLEDGEMENT 

The information presented in this paper is based on work carried out 

under Bureau of Mines Contract 50357115 with Mr. Roger King as the Technical project Officer. 

REFERENCES 

1) Final Report on Contract No. 50357115 

Feasibility Study of Electric Shock Prevention 
$p EARTH *LNT 

POTENTIAL ELECTRODE 

ELECTRODE 

OPEN PIT GROUND CONDUCTOR VERIFICATION, BLOCK DIAGRAM