For the fiscal years 1930 to 1943, inclusive (14 years), electricity was the greatest single source of coal-mine fires, and the greatest number of mine-fire fatalities resulted from fires of electrical origin."1 Table 1 gives the number of fatal and nonfatal injuries from electricity, including those from mine explosions of electrical origin, for the years 1930 to 1942, inclusive. Surface injuries include those that occurred in bituminous-coal stripping operations. Total fatal and nonfatal injuries from all causes are shown for comparative purposes. Figure 1 illustrates graphically the number of fatal and nonfatal injuries from electricity at the bituminous-coal mines of the United States for the years 1930 to 1942, inclusive. Both surface and underground injuries are shown; surface injuries include strip mining. TABLE 1.-Fatal and nonfatal injuries from electricity and from all causes of accidents1 at the bituminous-coal mines of the United States from 1930 to 1942, inclusive 1 Adams, W. W., and Geyer, L. E., Coal-Mine Accidents in the United States, 1942: Bureau of Mines Bull. 462, 1944, 140 pp., and similar bulletins for other years. 2 Surface accidents include strip mining. SURFACE PROTECTION HIGH AND LOW-POTENTIAL CIRCUITS Low-potential circuits should be isolated from high-potential circuits. Power wires shall not be placed on the same cross arm with telegraph, telephone, or other signal wires. When power circuits and signal circuits are carried on the same poles, signal circuits shall be placed below the power circuits. The distance between the cross arms carrying the two classes of circuits shall not be less than 4 feet, and not less than 6 feet if the power voltage exceeds 7,500; provided, that where the practice has been established of using spacing of 2 feet or more for circuits of not more than 300 volts, such spacing may be continued until the poles are replaced. Pole pins on such jointly used poles shall be at least 24 inches apart; if the voltage of any circuit exceeds 300 volts, at least 30 inches apart." 11 See footnote 9. 12 Bureau of Mines, Safety Rules for Installing and Using Electrical Equipment in Coal Mines, Sponsored by U. S. Bureau of Mines and American Mining Congress: Tech. Paper 402, 1926, 21 pp. Overhead high-potential power lines should be placed at least 15 feet above the ground and 20 feet above driveways and should be supported and guarded adequately to prevent contact with other circuits. Vehicles of usual height and persons passing beneath such power lines should be protected from contact with the circuits. Overhead power circuits and surface electrical equipment should be protected adequately against lightning or voltage surge; devices for such protection should be installed in incoming primary circuits and placed near the equipment. It is considered good practice to provide protection against lightning for direct-current transmission lines that extend underground if they are exposed to lightning. Lightning arresters should be installed on the insulated side of a circuit near the point at which it enters a mine, and a low-resistance circuit should be provided from the arrester to ground if the arrester is designed to provide a shunt path to ground. Arresters are considered necessary on transmission lines that extend underground because of the possibility of lightning entering the mine. MOTORS The following safety rules apply to the installation of motors and generators: 18 13 Electrical equipment above ground shall be installed in accordance with the rules of the National Electrical Code and the National Electrical Safety Code and * * * Electrical equipment with any existing State or municipal regulations. for use in damp places shall have approved moisture-resisting insulation. PROTECTION OF MOTORS (a) Every stationary motor, together with its starting device, shall be protected as follows: (b) Direct-current motors.-On two-wire ungrounded circuits each wire shall be protected by a circuit breaker or by a fuse and switch. On two-wire grounded circuits the ungrounded wire shall be protected by a switch and When the circuit breaker trips free from either a fuse or circuit breaker. On three-wire circuits each the closing handle, the switch may be omitted. outside wire shall be protected by a fuse or circuit breaker, but no fuse or circuit breaker shall be used in the neutral wire. A triple-pole switch shall be used In case circuit to isolate the fuses or circuit breakers from the live source. breakers are used, they shall be so arranged that the opening of the circuit breaker in one wire will cause the other circuit breaker to trip. (c) Alternating-current motors.-On 3-phase delta or Y (star) connected circuits each ungrounded wire shall be provided with a fuse or automatic circuit breaker. When circuit breakers are used, two overload trip coils shall be used for ungrounded neutral systems and three overload trip coils for grounded neutral systems. In either case the automatic circuit breakers shall be so arranged that the opening of one phase will open the other phases. Switches shall be provided for isolating the fuses or circuit breakers from the live source. When air-break circuit breakers which trip free from the handle are used, the switch may be omitted. (d) Overload release devices.-Overload release devices on starting rheostats and compensators shall not be considered as taking the place of circuit breakers if such devices are inoperative during the starting of the motor. (e) The circuit-breaker settings and the fuse size and ratings shall be in compliance with the rules of the National Electrical Code, CONTROL DEVICES FOR MOTORS (a) Insofar as practicable, motor control shall be installed in a convenient position with respect to the motor and the equipment it operates. The controlling 13 See footnote 12. appliances of stationary motors shall be adequately mounted remote from combustible material. Resistors may be mounted upon a separate metallic framework. (b) Motors that operate cars or coal-handling equipment on tipples, at landings, and similar places shall be provided at a point near such equipment with a switch or other means of disconnecting the motor from the power supply. (c) Starting devices shall be enclosed in a fireproof enclosure, or mounted upon a metallic framework, and shall be clear of all combustible material. (d) All wiring between motors and their controlling appliances shall be insulated. Insulating material used as a part of starting rheostats shall be noncombustible. This includes the insulation of wire used for the internal wiring of rheostats. (e) Switches shall be so installed that they cannot close by gravity. TRANSFORMERS Unfenced surface transformers should be supported substantially on poles or platforms at least 8 feet above the surface of the ground or should be surrounded by a suitable enclosure. If the enclosure is of metal, it should be grounded effectively. The gate or door to a transformer enclosure should be kept locked except when the transformer is attended by authorized persons. The purpose of these recommendations is to prevent contact of unauthorized persons, children, or animals with the transformers and electrical conductors and consequent serious or fatal shock. Unauthorized persons would be required to undergo some physical discomfort to come in contact with transformers and their electrical circuits so protected. "Danger-High Voltage" signs should be placed on all transformer enclosures to warn persons of the hazard. If surface transformers containing flammable oil are installed where they present a fire hazard (near mine openings and in or near combustible buildings), means should be provided to drain or confine the oil, in the event of rupture of the transformer casing, so that it will not leak or pour from the transformer and enter a mine opening or, if ignited, start a fire that might spread to the mine or combustible structures. Casings of all transformers should be grounded unless the transformers are isolated and do not present a contact hazard. Grounding is necessary if there is any possibility that the transformer casings might become charged with electricity, with consequent shock hazard to attendants or persons required to work near the transformer installation. SWITCHBOARDS Switchboards should be protected and guarded to minimize the shock hazard to authorized attendants. "Danger-High Voltage" signs should be posted in conspicuous places on high-potential switchboards. Ample working space should be provided around and behind switchboards, and it should not be used for storage of material but should be kept free from rubbish to reduce fire and contact hazards. If a switchboard consists of more than one panel it should have an entrance at each end to permit authorized inspection, adjustment, or repair of apparatus back of it. If it consists of only one panel accessible from only one end, one entrance is sufficient. The rear entrances of switchboards should be closed with locked gates constructed of steel wire mesh or other equally sturdy, noncombustible material. Keys to the gates should be in the possession of authorized persons only; however, the locks should be such that they can be opened from the inside without a key in case an attendant entering the rear should neglect to remove the key and lock himself in; he might be trapped for hours before someone released him. A person locked behind a switchboard might become panicky, or he might fall asleep while waiting to be released and touch live conductors, with resultant electrical shock. A switchboard and the floor in front and behind it should be of incombustible material. Dry wooden platforms, rubber mats, or other electrically nonconducting material should be kept in place at all switchboards and stationary machinery where shock hazards exist. Such protection is necessary at a switchboard to avoid shock to the operator while manipulating switches, rheostats, circuit breakers, or the instruments mounted on a switchboard. Switchboards should be well-lighted so that their devices can be operated or adjusted quickly, properly, and safely. Adequate lighting at the rear of switchboards is necessary to facilitate repair work and reduce contact hazards. Fixed lighting should be provided at the front of switchboards and at the rear where possible. Portable lighting should be provided to aid in repair work. Shut-down controls should be readily accessible so that they can be operated quickly in the event of an emergency. A disconnecting switch on the incoming circuit should be at or near the rear entrance to the switchboard so that the entire board can be deenergized when repairs or alterations are made to the switchboard. A means of locking open the incoming circuit switch is advisable to prevent the switch from being closed either accidentally or through misunderstanding when work on the switchboard is being done. SWITCHES AND CIRCUIT BREAKERS Switches and circuit breakers should be readily accessible and operated without danger of contact with moving or live parts. They should be mounted so that when they are open they cannot be closed accidentally "by gravity." Switches and starting boxes used to control electric circuits should be of safe design. An enclosed switch that can be operated by a mechanism extending to the exterior of the enclosure is usually safe. All principal switches should be so marked that they can be found readily in emergencies. All high-potential power lines should be protected by properly adjusted circuit breakers or fuses of suitable capacity. "High potential" refers to circuits with voltages exceeding 650. Circuit breakers are automatic current-interrupting devices that can be adjusted to open electric circuits at predetermined current excesses. They should. be set to prevent overloading of power circuits and consequent damage to electrical equipment. Heat from excessive current may damage insulation or may cause fires, often with disastrous results. Circuit breakers, proper types of fuses, or other suitable automatic current interrupters protect against short circuits and overloads and the hazards incident thereto. The capacity of circuit wires should be ample to carry the current to prevent overloading and overheating; therefore, the size of wire is of prime importance. RHEOSTATS Rheostats, including electric heaters, should be so installed as to prevent fire, electric shock, or injury to persons by burning. Rheostats or any electrical resistors become heated during use; they sometimes become "red hot" if they are not large enough to perform the duty required. All rheostats should be protected from mechanical injury and should be enclosed to prevent ignition of combustible material or injury to persons by burning. If electrical resistors are used for heating, they should be guarded to prevent fires and injuries to persons. ELECTRIC WIRING The electric wiring in all surface buildings should be so installed as to minimize fire and contact hazards. The National Electric Code (1940) 14 is the standard of the National Board of Fire Underwriters and is recommended by the National Fire Protection Association. Installation of wires in surface structures according to this code will reduce electrical fire and shock hazards. All wires should be of ample size to conduct the maximum current required and to prevent destruction of the insulation by heat. Where possible, all wiring should be enclosed in conduit; otherwise, it should be well-installed on insulators. All wiring should be insulated as specified by the National Electric Code, and each circuit should be protected by suitable fuses or other satisfactory protective devices. A serious fault often observed at coal-mine surface plants is the poor condition of wiring in buildings, especially in wooden structures. Wires frequently are nailed to floor joists or ceiling beams, and a break in the insulation might produce a short circuit, which likely would cause a fire. Extension-cord wiring should not become part of a permanent wiring installation. The use of extension cords for lights and portable tools should be kept to a minimum, and the cords should be disconnected from the power supply when they are not in use. Makeshift repairs in wiring with substandard insulation and installation should not be tolerated; such temporary arrangements often continue in use until a shock injury or a fire focuses attention upon them. The best plan is to prohibit substandard wiring and thus avoid the necessity for "locking the stable after the horse has gone." Extension cords for hand-held electric tools should contain a separate conductor used as a frame ground for protection against shock. The ground conductor should connect to a terminal (or shell) in the extension-cord plug to correspond with a grounding socket (or shell) in the plug receptacle. Fuses or other suitable protection for wiring are necessary to prevent overloads and fires. They are an economic necessity to prevent destruction of the wiring. TELEPHONE CIRCUITS Telephone circuits that are exposed to lightning should be protected by lightning arresters. A protector block or station protector is a device designed to protect telephone circuits from lightning and electricity of high potential from other sources. Fuses and an air-gap 14 National Board of Fire Underwriters, National Electric Code: New York, 1940, 360 pp.; Supplement to 1940 Code, Apr. 1, 1943, 49 pp. |