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died as the result of painful burns and shock; the other man survived but received severe burns of hands and feet.

These are examples of failure to realize a hazard that should be apparent to anyone. Intelligent supervision and continued safety education will be helpful in reducing the number of such accidents.

A dragline unit used in an opencut excavation was carelessly placed under a 23,000-volt powerline, which was 26 feet above the ground. The 60-foot boom of the excavator swung against the powerline while 2 laborers were filling the fuel tank with gasoline. The two men were killed instantly, and the arc formed by the contact ignited the gasoline vapors. The dragline operator suffered no harm from the accident.

An important point in connection with protection from shock hazard is illustrated here. When the boom came in contact with the powerline an electric current was caused to flow through the frame of the execavator into the ground. The operator of the dragline felt no shock because he was sitting on the equipment, all parts of which were at approximately the same potential. The men who were pouring the gasoline were standing on the ground, and the upper parts of their bodies were in contact with the frame of the dragline. There is usually considerable resistance in the contact between the tracks or base of such a machine and the ground on which it rests. As an example, this resistance might be 6 ohms; it could be much higher. Under this condition, if a current of 100 omperes flowed over the dragline frame into the ground the voltage drop across this frame-to-earth resistance would be, by Ohm's law, 6X100-600 volts, and this voltage would be impressed across the bodies of the victims.

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Dragline or shovel equipment should never be operated in the vicinity of an energized powerline. If for any reason such operation is necessary all possible precautions should be taken to avoid contact. All persons working in the vicinity should avoid simultaneous contact with the equipment and the earth, or objects in contact with the earth.

Similar hazards are presented in the handling of wires, tapes, pipes, or other long, metallic objects in places where it is possible for them to touch energized conductors. The safe procedure is to have the power cut off the line or to provide suitable guards to prevent contact.

DEFECTIVE INSULATION

Insulation on mine cables that are installed where they are subject to contact by personnel or with other conducting surfaces should have a high factor of safety. Insulation failures may be caused by moisture or overheating of the cables. Persons handling insulated cables are prone to depend on the insulation for protection against shock, and fatal accidents frequently happen as a result of careless handling of such cables.

The foreman of a gold mine was making a tour of the mine with the superintendent when he slipped as they were passing the top of an open winze. To save himself from falling into the winze he grasped a lead-sheathed power cable and was electrocuted. The cable was part of a 3-phasť, 440-volt, alternating-current circuit. The insulation of one conductor had broken down, and the conductor was in contact with the lead sheath. Artificial respiration was begun after a delay of about 3 minutes and continued for almost 3 hours without effect.

All metallic sheaths and armors of power cables should be effectively grounded and be electrically continuous throughout their length. Figures 4 and 5 show typical installations of high- and low-voltage cables underground.

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A miner removed a 220-volt extension light from a working place where he and his partner were preparing to blast. When he had gone back about 35 feet he was heard to cry out and was seen lying in a pool of water with the light cord beneath him smoking. Another miner immediately pulled the switch while others hurried to the victim. He was removed to a dry location and given artificial respiration for one-half hour but failed to respond. A medical doctor after examination announced that death had been almost instantaneous.

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FIGURE 4.-Power Cables Installed in a Metal-Mine Drift.

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FIGURE 5.-Electrical Installation at a Shaft Station.

It was found that the extension cord-a rubber-covered, 2-conductor, No. 12 cable— was in good condition except for a bare spot on 1 wire about 30 feet from the end. Evidently the victim had touched this bare spot when coiling the lamp cord and was electrocuted by the current flowing through his body to ground. Recommendations were that these cords be replaced by others of a type having a ground sheath, if obtainable, and that 3-ampere circuit breakers be installed on each lamp circuit.

A cable of this size with a ground sheath would be somewhat special, would cost considerably more, and unless adequately grounded at all times could increase rather than decrease the hazard. It is generally recognized that 0.1 ampere is enough to kill a man.

A current of 1 ampere is more than enough to prove fatal; therefore the suggestion that a 3-ampere circuit breaker would have in any way protected the victim of this accident is not valid.

The voltage of circuits for lighting, especially where extension cords are used, should not be higher than 115 volts. Men handling electric lighting and electrically driven tools in damp places should be encouraged to wear rubber boots or shoes. Such lighting circuits should be taken from isolating transformers near the working area and the circuits isolated from ground, or the secondary winding of the transformer should be center-tapped and grounded to reduce the voltage from either line to ground to half of the line voltage.

In shafts or manways where powerlines are installed some form of guard should be provided to protect men who use the manway from contact with the wires or cables and to protect the conductors from falling objects. Special care is necessary to insure that signal wires and telephone lines cannot accidentally come in contact with powerlines or trolley circuits. Junction boxes, properly installed in protected positions on each level where leads are taken from the power cables, will prevent poor splices in exposed

locations.

CONTACT WITH CHARGED EQUIPMENT

PORTABLE TOOLS AND HEAVY EQUIPMENT

Portable electrical tools and other light, movable equipment are subject to insulation breakdown, resulting in dangerous voltages energizing the frames of such equipment. These are particularly lethal because they are usually held in the miner's hands. The frames of heavy mobile machines receiving power through trailing cables also may become dangerously energized because of insulation failures or displacement of "live" parts. This hazard is greater with rubbertired equipment, such as shuttle cars; however, the possibility of shock hazard from the frames of tractor-mounted equipment is very definite and should be protected against.

A welder was making repairs beneath a pan conveyor; to reach his work he had to crawl through a small opening in the supports. On entering he removed a light bulb from a socket on a 220-volt lighting system that was suspended over the opening. When he came out after finishing the work his back touched the light socket, and he received a fatal shock.

The protective insulation of the socket was broken, exposing part of the energized metal screw socket, and no attempt had been made to replace or repair the socket. accident the 220-volt line was removed to a safe distance.

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In a metal mine the first-aid equipment was stored in a metal box. A 300watt incandescent bulb in a weatherproof socket was installed in a box to keep the equipment dry and connected to the 275-volt direct-current mine power circuit. A 1-inch-pipe waterline and a 4-inch-pipe air line were near the box. It was the miners' habit to sit on this box and eat their lunch. On the day of the accident two miners sat on the box as usual and proceeded to eat their lunches. In some way the box suddenly became electrically energized, and both men jumped quickly away from it. One of them had been in contact with the air line and probably received a more severe shock than his partner. He had moved a few feet away from the charged box when he collapsed and, although he was given artificial respiration immediately, failed to recover.

Investigation revealed that the positive side of the line had come in contact with the metal box near the socket and the wire had fused to the side of the box. Frame grounding had not been provided.

Voltage above 115 should not be employed for lighting or heating when lamps and sockets are used, and metal objects enclosing or supporting electrical circuits should be properly frame-grounded.

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A laborer in an opencut copper mine was electrocuted when he stepped onto the tractor tread of an electric shovel. The power cable had been crushed against the tractor treads by a falling boulder, and an arcing ground fault was established. The shovel was not equipped with a ground-fault tripping system, and the current to ground was not adequate to operate the overcurrent devices and clear the fault. The shovel was operated on a 4,000-volt alternating-current power system, and the laborer was electrocuted by the voltage developed across the contact resistance between the shovel treads and the earth.

In an open-pit mine an electrician's helper was electrocuted when he attempted to loosen a clamp on a ground wire while standing on the metal-lined sled runners of a mobile switch house. On the preceding day arcing had been detected between a tow cable attached to the sled runners and a grounding conductor for a set of lightning arresters mounted on the switch house. The arcing was thought to have been caused by the breakdown of one of the lightning arresters; the line was therefore deenergized and the arresters disconnected. On the day of the accident the helper intended to remove the arresters and replace the defective ones. Upon grasping the clamp with his pliers he received a fatal shock. Artificial respiration was administered without results for about 45 minutes, when the helper was pronounced dead by a medical doctor.

Upon investigation it was discovered that the insulation on a current transformer inside the switch house had broken down, electrically charging the metal rack on which the transformer was mounted. This rack had been grounded by means of a ground wire attached to the lightning-arrester ground outside the switch house, which in turn was connected to a 34- by 16-inch galvanized bolt driven into the ground and serving as the grounding electrode. Current from the faulted transformer thus flowed over the grounding circuit and into the earth by way of the grounding electrode. The resistance-toground of the electrode was so high that a lethal voltage developed across this resistance. When the helper touched the lamp on the ground wire while standing on the metal sled runners his body provided another path to ground for the fault current flowing in the grounding circuit.

Installation of a ground-fault protective system probably would have saved the lives of both men. Individual iron or copper rods driven into the earth cannot be depended on for adequate protection on high-voltage circuits.

FRAMES OF STATIONARY MACHINES

Electrically driven equipment, such as hoists, pumps, and conveyors, that is more or less permanently located is less vulnerable to insulation breakdown than mobile units. In general, the frame grounding of such equipment is much simpler and easier to maintain. Nevertheless, from time to time electrocutions occur with this type of equipment, and proper precautions should be taken to guard against it.

A miner received a slight shock while operating a small supply hoist in a raise. He shut off the controller and started down to the level to inform the shift boss of the trouble. As he opened the trapdoor over the manway he grasped the %-inch steel hoisting cable and received a severe shock, which caused him to fall into the open timber compartment. The fall resulted in serious injuries, which left him with a partial permanent disability. Investigation disclosed that the insulation of a cable had been abraded by contact and vibration until the conductor had come in contact with the hoist frame. The hoist was not frame-grounded.

A miner operating a conveyor in a nonmetal mine was electrocuted when the starting box became charged because of an insulation failure on one of the leads; the starting box was not frame-grounded. Artificial respiration was given for a short time but was discontinued when the victim failed to respond. A pumpman was electrocuted while operating a pump at a surface stripping operation. The pump was driven by a 3-phase, 25-horsepower, 2,200-volt alternating-current motor controlled by an oil circuit breaker having time-delay overload trips and undervoltage release. Power for this pump motor was furnished from a circuit which also supplied power for several electric shovels operating in the vicinity. The accident resulted from an insulation failure in the oil cir

cuit breaker and a similar insulation failure on a shovel cable about 900 feet away. These insulation breakdowns occurring on different phases of the 3-phase system resulted in a voltage of 2,200 being impressed across the ground between the 2 faults. The voltage was dissipated in 2 parts, 1 near each fault location, and each part would be inversely proportional to the resistance of the fault. This condition is not uncommon with normally ungrounded, three-phase systems. Adequate ground-fault protective methods should be provided on all high-voltage systems; but where it is not provided, ground detectors should be installed, and a single fault should not be allowed to exist for any appreciable length of time. Further explanation of this condition will be found under the following section, Grounding Electrical Installations.

A frequent cause of electrical accidents is lack of knowledge or understanding of fundamental electrical circuits and the hazards that may exist under certain abnormal operating conditions. Operators and repairmen should be thoroughly instructed in the proper procedures for safe handling and maintenance of all electrical equipment under their charge. Proper precautions should always be taken to determine whether a circuit is deenergized before handling.

SWITCHBOARDS AND CONTROL UNITS

Transformers, resistance boxes, and other control equipment on which there are exposed live connections should be provided with a screen enclosure or guard railing when in a place accessible to persons other than their regular attendants. Enclosures about live electrical units, such as switchboards, busbars, resistance grids, etc. should be locked to prevent the entrance of unauthorized personnel. Switches and cutouts should be arranged for remote operation from outside the enclosure and should be mounted high enough to prevent accidental contact by persons entering the enclosure. All switchboard framework, grills, or metal enclosing cases should be connected to a common ground. Telephones should be grounded, and telephones as well as other signaling devices may sometimes need to be enclosed in heated compartments to prevent an accumulation of moisture that might cause electrical failures.

GROUNDING ELECTRICAL INSTALLATIONS

SYSTEM GROUNDING

The word "ground," as applied to electrical systems, has a broad coverage.13 Much confusion and misunderstanding have resulted in the mining industry and elsewhere because of the failure of writers and instructors to distinguish between system grounding and frame grounding. The primary purpose of frame grounding is protection against shock hazard, whereas safety is merely one of the benefits incidental to system grounding. The primary purpose of system grounding is improved service reliability through better overcurrent protection and improved protection against damage from lightning. Statements of fact with regard to system grounding do not necessarily hold true for frame grounding and vice versa.

When the neutral of a system is not grounded destructive transient overvoltages of several times normal can appear from line to ground during normal switching of a circuit having a line-to-ground fault. Experience has proved that these overvoltages may cause failure of insulation at other locations and thereby result in interruptions to service.

13 Dorey, F. M., System Grounding in Industrial Plants: AIEE paper, Elec. Eng., vol. 22, December 1953, pp. 1098-1103.

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