Even when no part of the current flows through the brain there may be severe nervous shock from nerve stimuli in the parts of the body in contact with the current, which are transmitted to the brain.

Burns of the skin and flesh, with resultant complications, usually are deeper than other burns and often difficult to heal.

RELATIVE HAZARDS OF VARIOUS VOLTAGES

The following conclusions regarding the hazards of various voltages and currents are drawn from the reports of studies by the Underwriters' Laboratories and research electrical engineers.

67

The amount of current and the length of time it flows through the body determine the damage that may be done; the voltage of the circuit is important in overcoming resistance to passage of the current. Although the sensitivity of persons to electricity varies, alternating currents of about 0.010 ampere usually cause contraction of the muscles, and those of 0.015 to 0.020 ampere make the victim unable to release himself from the contact. Because the time of exposure is an important factor, any current that will prevent the person from letting go of the conductor is dangerous. The minimum current with which ventricular fibrillation may occur is estimated to be about 0.1 ampere for a duration of 3 seconds or longer. Experiments to determine the maximum voltage with which contact might be made safely if the skin is wet revealed that, in some instances, alternating current at 12 volts caused loss of muscular control and physical injury; at 20 volts most individuals are seriously affected. Under dry conditions higher voltages can be withstood for a short time.

Direct current is said to be less dangerous than alternating current at voltages under 1,000 volts. This is based largely on the fact that few fatal accidents have resulted from contact with circuits below 150 volts direct current. 8 It should be realized, however, that direct voltages of 150 or under are not used to any great extent in industry, and therefore the exposure is at most only a fraction of that at comparable alternating voltages. Anyone who has ever made accidental contact with a 250-volt mine-trolley system and survived can testify to the lethal quality of such a voltage. There is some basis for the belief that direct current at comparable voltages is less dangerous than alternating current because the let-go current for direct current is 7 times larger than for alternating current, and the current for ventricular fibrillation is 5 times larger for direct current than for alternating current. In general, safety demands that contact with any voltage above 30 should be studiously avoided.

9

Although heating rather than muscular contraction is the effect of steady direct current, sudden changes of the current produce muscular contraction, particularly when the conductor is touched or released. The maximum direct current that may be endured is approximately 0.08 ampere.

When contact is made with a high-voltage circuit the contraction of the muscles may be so violent that the victim is thrown clear. This

Dalziel, C. F., and Lagen, J. B., Effects of Electric Shock on Man: Elec. Eng., February 1941, p. 63.

7 Kouwenhoven, W. B., Effects of Electricity on the Human Body: Elec. Eng., March 1949, pp. 199–203.

8 Work cited in footnote 7.

Adolphe, M. H., Safety in Aircraft Electric Systems: Elec. Eng., March 1949, 227-228.

pp.

fact and the much greater exposure to lower voltages probably account for the opinion held by some workmen that low voltages are more dangerous to human life.

Most authorities agree that personal hazards could be greatly minimized if equipment could be operated at voltages of 150 or less. This would be impractical, however, because of the high power losses involved with comparable conductor size or the prohibitive cost of larger conductors.

RESCUE AND RESUSCITATION

In electrical accidents death is often only apparent, and there is no way by which the layman can determine whether the heart is affected so that recovery may not be expected. Therefore artificial respiration should be applied immediately and maintained continuously without interruption until natural respiration is restored or until rigor mortis has set in. The most vital points in the successful treatment of elecric shock victims are prompt removal from contact with live conductors or energized frames and application of artificial respiration at the earliest possible moment.10

In attempting to rescue a victim from contact with a live conductor or frame the rescuer himself may suffer severe and possible fatal shock because of failure to understand or practice the precautions required in such action. These accidents to well-intentioned rescuers happen more often on the surface than underground, probably because more electrical accidents on the surface involve alternating current and high voltages. When the victim of electric shock remains in contact with the conductor the flow of current can be stopped by breaking the circuit or by forcibly breaking the contact. When a switch is close at hand the current can be cut off, or in some instances connection can be broken by other means. When this cannot be done without delay the person can be safely removed from contact with the conductor if the rescuer stands on a dry board or other insulating substance, wraps his hands with dry, nonconducting material, uses a dry stick to pry or push the person free from the conductor, or pulls the victim away with a loop of dry cloth or nonconducting rope.

a

Persons working about electrical equipment should be trained in the most efficient methods of giving artificial respiration and the proper treatment of electric shock. Workmen responsible for installing and maintaining electric transmission lines on the surface should also be trained in methods of pole-top resuscitation.11 12 Posters showing how to give artificial respiration should be suitably displayed at mine shops and underground stations where electric shock hazards exist.

ELECTRICAL HAZARDS

Electricity may cause personal injury through such abnormal conditions of electrical equipment as short circuits, accidental grounds, overloads, or poor contacts; however, every year numerous fatal electrical accidents are due to persons touching "live wires," which are normally exposed. These contacts usually occur through careless

10 Bureau of Mines, First Aid, A Bureau of Mines Instruction Manual: 1953, pp. 31, 32. 11 Gordon, A. S., Frye, Charles, and Sadove, M. S., Comparative Studies of New PushPull Methods for Pole-Top Resuscitation: Elec. Eng., February 1953, pp. 132-136. 12 Oesterreich, E. W., Discussion of article, "Comparative Studies of New Push-Pull Methods for Pole-Top Resuscitation" (see footnote 11): Elec. Eng., February 1953, pp.

137-140.

movements by persons in the vicinity of wires in places not normally reached or by accidental contact through metal objects being handled. Dangerous conditions such as these can be prevented by selecting proper equipment; by standard installation; by enclosing or elevating current-carrying parts; by providing ample working space around equipment; by adequate insulation; by locking switches in the open position under certain circumstances; by using identifying and warning devices; by protective grounding; by using insulating flooring, platforms, or rubber gloves; and by regular inspection and main

tenance.

Electricians are exposed to more electrical hazards than other workmen about a mine or plant, yet most of the victims of electrical accidents are those who use the equipment or work in its vicinity. This would seem to indicate that education in the proper handling and operation of electrical equipment would help to reduce electrical accidents. Certainly many such accidents may be avoided by proper training.

Although trained electricians occasionally have accidents while working with electrical equipment, any person will at least be better prepared to avoid careless or unwitting contact with energized conductors or equipment frames if he is properly trained before being permitted to work where he may come in contact with electric conductors or machines. He should be taught not to rely on the effectiveness of the insulation or grounding; to determine definitely that power is cut off a circuit before making contact with it; and to know the location of disconnecting and control switches, as well as safe methods of operating them.

Most electrical codes give tables showing the normal currentcarrying capacity of different-size conductors based on safe heating for single conductors in air. The insulation on such conductors is designed to withstand the heat produced by the flow of normal rated current under conditions of proper ventilation. The conductors of cables supplying power to mining machines used underground are subject to abnormal heating for several reasons. Cable sizes are held to a minimum to facilitate handling, and in addition they are often wound on reels or in coils where the ventilation is greatly restricted. Mining machinery is frequently subjected to severe overloading. All of these tend to increase the normal heating of the power conductors. Under severe heating the insulation of cables often generates gases and becomes viscous, making it much more vulnerable to ignition from arcs produced by short circuits or load interruptions.

Continued overloading, with daily cycles of heating and cooling, induces rapid deterioration of the insulation of cables and equipment wiring, thus greatly increasing fire and shock hazards normal to the operation.

Largely because of the explosion hazards incident to coal mining, totally enclosed motors and controls have been developed for coalmining machinery. These motors are designed to operate at normal rated output under the severe working conditions to be found in coal mining. In metal mines and quarries the explosion hazard is of course not a factor in the selection of electrical equipment, but the totally enclosed type of equipment has been found desirable because of its ability to give more reliable service than open-type equipment under severe dust and moisture conditions.

Rapid, dependable, overcurrent and short-circuit protection should be provided on all power cables to minimize fire hazards. Groundfault protection should be provided to limit the voltage on the frames of energized equipment in case of insulation failure. Proper methods of splicing should be taught, and the value of adequate conductivity and insulation on splices should be continuously inculcated.

In a State that has varied industries the electrical injuries to workmen, including miners, were summarized as follows for 1940–44.

TABLE 4.-Electrical accidents in one State, 1940-44

Type of accident:

Low-voltage circuits and equipment (under 600 volts).-High-voltage circuits and equipment, including injuries to linemen and electricians_.

Contacting overhead high-voltage lines with crane booms, mobile equipment, steel cables, etc..

Total...

CONTACT WITH CONDUCTORS

BARE WIRES AND CABLES

In underground mines the trolley wire is usually installed where men can come in contact with it. When trolley wire or uninsulated feeder cables cannot be isolated from contact by reason of location they should be properly guarded at locations where men work in proximity to the wires or cross under them. Trolley and bare feeder lines should be deenergized, if possible, whenever it is necessary to work on them. Where it is necessary to work on "live" trolley or feeder lines highvoltage-tested lineman's rubber gloves should be used and every precaution taken to prevent accidental contact. An installation of properly guarded trolley wire at an underground car-dumping station is shown in figure 3.

Some serious or fatal accidents from contacts with bare wires are taken from the records of recent years:

A shop helper was painting the ceiling from the top of a crane when his wrist touched a bare trolley wire. He was thrown 18 feet to the floor, breaking both legs and 1 arm and dislocating 1 ankle.

When men are working in proximity to bare conductors, the conductors should be deenergized or appropriately guarded.

* * *

A pipefitter was electrocuted when a 24-inch Stilson wrench he was carrying on his shoulder touched a 250-volt, direct-current trolley wire as he was getting out of a steel mine car which blocked the passageway in a restricted drift. Artificial respiration was started several minutes after the accident and was continued for about 3 hours, at which time the man was pronounced dead by a doctor.

Proper clearance should have been provided on the side opposite the trolley wire. Men should never climb out of a mine car on the trolley side, and tools should not be carried on the shoulder, under, or in proximity to, trolley wires or feeder cables.

* * *

A miner coming down a raise to a haulage level backed into the unguarded Another miner who trolley wire as he left the ladder at the foot of the raise. had been drilling in a raise came down at lunchtime and on reaching a pony His set over the haulage drift descended a ladder on the trolley-wire side. neck came in contact with the trolley wire at the same time his hand was touching a steel mine car. In both instances artificial respiration was started within a few minutes after the accident happened, but the victims were not resuscitated.

The trolley wire should have been guarded in both instances, but ladders should be brought down on the side opposite the trolley wire, or if such ladders must be placed on the trolley side a recess should be cut in the side of the drift to provide adequate clearance for persons using the ladder; and the trolley wire should be properly guarded where the men must pass under it, unless it is placed high enough to be isolated from

contact.

* * *

Numerous fatal accidents have been reported because of electric blasting near powerlines. A seismograph crew fired a charge in a 70-foot hole less than 30 feet from a 4,200-volt powerline. The wires from the electric detonators were blown across the powerline, electrocuting the shooter and injuring the recorder through telephone wires attached to a headphone.

The company rules specified that holes blasted in this manner should be not less than 200 feet from high-tension powerlines.

* * *

A repairman at a small opencut mine was removing a pump and pipe from a well. He was raising a 35-foot section of pipe from the hole when it touched high-tension wires overhead, giving him a fatal shock. Two men were drilling a test hole with a long auger bit under a 20,000-volt powerline, and as they pulled out the auger it came in contact with the wires overhead. One man

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