arrester usually are mounted on the base of the device. The unit may be open or enclosed, but the enclosed type is more satisfactory for mine use. Telephone wires crossing power or trolley wires should be insulated effectively.

Methods of installing protector blocks are given in section 3316 of the National Electrical Code Handbook as follows: 15

Where the distribution system consists of aerial wires, an approved protector shall be placed as near as practicable to point of entrance to the building. The protector shall be mounted on a noncombustible, nonabsorptive insulating base and shall consist of an arrester between each line wire and ground and a fuse in each line wire, the fuses protecting the arrester. The protector terminals shall be plainly marked to indicate "line," "instrument," and "ground."

The protector shall not be placed in the immediate vicinity of easily ignitible material or flammable gases or dust or flyings of combustible material.

Where the entire street circuit is run underground, a protector shall not be required unless the circuit within the block is so placed as to be liable to accidental contact with electric light and power wires operating at a potential exceeding 250 volts.

PROTECTION BY GROUNDING

Guy wires from poles supporting high-potential transmission lines should be grounded unless equipped with insulators. If insulators are used, they should be installed near the poles.

All metallic frames, casings, and coverings of motors, generators, switchboards, hand-held electric tools, and other electrical equipment that can become "alive" through failure of insulation or by contact with energized parts should be grounded. The subject of protection by grounding is discussed in more detail under Underground Electrical Equipment.

UNDERGROUND PROTECTION

The Health and Safety Branch of the Bureau of Mines has developed standards for protection against electrical hazards. These standards, particularly those pertaining to underground operations, were the result of investigations and experiences of coal-mining companies, State agencies, manufacturers of electrical equipment used in coal mines, and the Bureau of Mines.

The principles embodied in Safety Rules for Installing and Using Electrical Equipment in Coal Mines 16 were sponsored jointly by the American Mining Congress and the Bureau of Mines. The fundamental principles are still valid; however, some revision, sponsored by the American Standards Association, has been considered desirable to bring the safety rules up to date, particularly in the light of the rapid growth of mine mechanization and the increase in electrical devices and accessories connected therewith. When this miners' circular was prepared, however, the rules had not been revised.

The formation of rules or standards for establishing safeguards in the use of electricity underground in bituminous-coal mines should be governed by certain basic principles:

1. Remove the contributory causes of accidents or danger.

2. Remove from the vicinity of electrical devices all elements that may affect or may be affected adversely by electricity.

15 Abbott, Arthur L., National Electrical Code Handbook: McGraw-Hill Book Co., New York, 1934, 547 pp.

16 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.

3. Keep the electric current in its proper channel if possible; if not, limit the area of its activity by protective devices.

4. Use a large factor of safety in the selection, installation, and maintenance of equipment.

5. Have full control over the operation of electric machines.

Trolley and other bare power wires have been among the most important sources of electrical-shock injuries. In recent years they have been the outstanding causes of mine fires. Removal of all bare power wires from the bituminous-coal mines of the United States would prevent injuries from exposed conductors of electricity; this, however, would not be economically feasible. Storage-batterypowered equipment to replace machines that operate from external power circuits has limitations that restrict its use. The Bureau of Mines has developed a schedule 17 for testing the permissibility of Diesel mine locomotives, but when this miners' circular was prepared it was not possible to determine the future of such locomotives in underground coal-mine haulage. If Diesel mine locomotives are used extensively, undoubtedly the removal of trolley locomotives and bare wires will reduce substantially the electrical accidents from these

sources.

Except in a few very gassy mines in southern West Vriginia, storage batteries have not been used widely in bituminous-coal mines for supplying power to miscellaneous mining equipment. Storagebattery power has been utilized principally for locomotives, particularly those used for gathering haulage.

In the immediate future no single element is likely to alter materially the use of bare conductors in bituminous-coal mines; a study is therefore being made to determine what can be done to minimize the hazards from this source. A group of coal-mine operators, manufacturers' representatives, members of State mine-inspection departments, and Bureau of Mines engineers are studying the problem of protection against trolley-wire hazards. The studies have been stimulated by some recent disastrous mine fires and explosions caused by falls of roof which brought down trolley wires, with subsequent short-circuiting; the arcing either resulted in a gas or dust explosion or set the coal on fire. As the studies of this group are still in the formative stage, no results are available, but it is hoped that an important contribution to safety in the bituminous-coal-mining industry will evolve so that the dangers from the presence of trolley wires can be reduced to a minimum.

NONPERMISSIBLE EQUIPMENT

Nonpermissible mining equipment, particularly that used in face regions, should be restricted. The recurrence of disastrous mine explosions and fires from nonpermissible electrical equipment, together with the toll of lives and property damage, is mute testimony for the need of such restriction.

If nonpermissible electrical equipment is used in face regions (inby the first open crosscut or in air that has ventilated one or more working places) of gassy mines, it should be operated under the following conditions:

17 Bureau of Mines, Procedure for Testing Diesel Mine Locomotives for Permissibility and Recommendations on the Use of Diesel Locomotives Underground: Sched. 22, 1944,

1. Each working place should be examined carefully for methane by a certified official or other competent person immediately before the equipment is taken into or operated in a working place. Nonpermissible equipment can ignite methane; therefore, it is necessary to determine that no dangerous accumulation is present in a working place.

2. The general air of a working place should not contain more than 0.5 percent of methane as determined by analysis or other recognized means of detecting methane. General air of a working place means air not nearer than 1 foot from the face. Any indication of methane on a flame safety lamp should be regarded as more than 0.5 percent. Collection of samples of air in vacuum bottles and analysis in a laboratory are the most accurate means of determining the methane content, but such procedure takes time. Therefore, a flame safety lamp or an electrically operated methane detector can be used for an "on the ground" gas determination, but no important decision based on the quality of the air in a working place should be made without checking the reading of a portable methane detector against the laboratory analysis of a sample taken at the same time and place.

3. If more than 1 percent of methane can be found in holes in the roof of any working place by analysis or by methane detector (flame safety lamp or electric detector), the equipment should not be taken into or operated in the working place until the pocket of gas has been removed by additional ventilation.

4. Tests for methane should be made at least every 30 minutes by a certified official or other competent person when nonpermissible electrical equipment is used in face regions. Tests for gas should be made more frequently in squeezed areas or in disturbed strata such as pinch-outs or clay veins.

The foregoing conditions indicate that nonpermissible machines, particularly those used in face regions, are potentially dangerous because of the gas-ignition hazard. All so-called nongassy mines are potentially gassy. Moreover, all bituminous-coal mines present a dust hazard, which is increased because of possible dust ignition by nonpermissible equipment. The Bureau recommends that nonpermissible equipment in gassy mines should be replaced by permissible equipment if possible. Where nonpermissible equipment is used in gassy mines, every precaution should be taken.

PERMISSIBLE EQUIPMENT

Soon after its establishment by Congress the Bureau of Mines recognized the need of safeguarding mine-type electrical equipment. The Electrical-Mechanical Section of the Health and Safety Branch has established schedules of permissibility tests for various types of mine equipment, and today hundreds of thousands of pieces of electrically operated equipment bearing "approval" plates are in use in the bituminous-coal mines of the United States. It is interesting to note that the first approval plate was issued by the Bureau about 30 years ago. The Bureau recognized that if catastrophes from electricity were to be avoided at mines protection in the form of proper construction, maintenance, and use was essential.

Electrical equipment operated inby the first open crosscut between entries or rooms or in air that has ventilated one or more working places in mines classed as gassy should be permissible.

The Electrical-Mechanical Section has inspected, tested, and approved several hundred types of mining equipment submitted for testing in accordance with its permissibility schedules. Such approvals have been awarded to cutting machines, loading machines, conveying equipment, shuttle cars, pumps, storage-battery locomotives, drills, rock-dust distributors, room hoists, power trucks, air compressors, and miscellaneous equipment including electric cap, hand, and trip lamps, flashlights, blasting units, methane detectors, junction boxes, and telephones.

No provision has been made in Bureau schedules for the approval of trolley and cable-reel locomotives. Sparking at the trolley wheel or shoe may ignite gas; therefore, the travel of a trolley locomotive in a gassy mine should be restricted to areas ventilated with fresh intake air. Such travel can be confined by erecting trolley wire only in intake airways and designating the places where it should terminate. The travel of a cable-reel locomotive beyond a trolley circuit is limited only by the length of its cable; and the machine may make frequent visits to a face where gas is being liberated. The spark-producing parts of such machines can be housed in explosion-proof enclosures; nevertheless, the trailing cable may be subjected to severe treatment, which may produce arcs and flashes when it is damaged. A locomotive can run over its trailing cable through failure of the reeling mechanism; this cable hazard is the principal reason that the Bureau has withheld approval of a cable-reel locomotive. A cable-reel locomotive can be designated as explosion-tested after all its electrical enclosures have been tested by the Bureau of Mines for explosion-proof qualities; such locomotives are often used for gathering haulage. Bureau engineers recommend that explosion-tested cable-reel locomotives should be equipped with a two-conductor rather than a singleconductor trailing cable; the former requires no rail return for the electric current inby the point of attachment of the cable to the rail.

BUREAU OF MINES APPROVAL SYSTEM

When a manufacturer has built a mine-type electric machine, which he desires to have approved as permissible for use in gassy mines, he files a written application with the Director of the Bureau of Mines, Washington, D. C., together with the payment of fees stipulated by the schedule to cover the cost of inspection and testing. The accessory electrical parts of the machine and detailed drawings covering their construction are sent to the Central Experiment Station, Pittsburgh, Pa., where the suitability of the parts to prevent ignition is determined by inspection and test.

NEED FOR PERMISSIBLE MINE-TYPE ELECTRICAL EQUIPMENT

The recurrence of disastrous mine explosions in the coal-mining industry drew attention long ago to the imperative need for proper safeguarding in the use of electricity if such catastrophes were to be prevented:

18

The present-day methods of mining, which involve concentrations of a number of electrically driven machines at the working face, make it more than ever necessary that suitable precautions be taken to guard against electric sparks

18 Ilsley, L. C., Gleim, E. J., and Brunot, H. B., Inspection and Testing of Mine-Type Electrical Equipment for Permissibility: Bureau of Mines Inf. Circ. 7185, 1941, 21 pp.

or flashes in explosive atmospheres. The importance of adequate safeguards is realized when it is understood that a current of less than 1 ampere at the brushes of a small 250-volt motor is sufficient to ignite an explosive mixture of mine gas and air and that this is only one of the numerous ways in which sparks can be produced.

The effectual safeguarding of electrical equipment depends on application of two of the practices followed in safeguarding mines against general hazards. The more important and more widely adopted of these is ventilation. Ventilation of gassy mines that is sufficiently effective to force enough fresh air into the mine to dilute and sweep out the gas being liberated and prevent its accumulation in explosive proportions is a tremendous factor in preventing explosions of electrical origin. Ventilation may therefore be considered the first line of defense in preventing ignitions of gas by electricity. Another important safeguard is rock dusting, which may be considered the second line of defense. Following these two in importance should come the use of permissible electrical equipment. The first two safeguards, if adequately planned and maintained, will prevent catastrophes; the third will tend to prevent less-widespread explosions at the face. If the first two preventives are neglected, the last or third may prevent disaster; therefore it is very important to have all electrical equipment in gassy sections of the mine properly safeguarded. "Permissible" equipment should never be used in places where gas exists in amounts sufficient to be detected by a flame safety lamp. Such a procedure is unsafe and cannot be too strongly condemned.

CONDITIONS CONDUCIVE TO MINE EXPLOSIONS CAUSED BY ELECTRICITY

The basic conditions that bring about a mine explosion initiated by electrical equipment improperly constructed or maintained are as follows: (1) The presence of gas in the immediate vicinity of the equipment, (2) the penetration of the gas into the equipment by diffusion or other means, (3) the ignition of gas in the equipment by an electric spark or flash, and (4) the failure of the equipment to prevent the spread of the flame resulting from that ignition.

Flashes from short circuits or grounds from trailing cables of portable machines may readily cause ignition of gas. The trailing cable is, in fact, one of the major hazards in the use of electrical equipment at the face. Ignitions are also possible where short circuits or grounds occur on power and lighting circuits in the presence of gas. The problem of devising adequate safeguards for such circuits has not yet been given the same special study that has been devoted to the protection of motors, controllers, and other electrical appliances.

PRESENCE OF GAS IN ELECTRICAL APPARATUS

* * *

Gas is likely to enter the interior of electrical apparatus that has openings large enough to permit diffusion of the gas with the air. Diffusion will take place readily if the apparatus is allowed to remain several hours in a room filled with gas. A machine of the totally enclosed type is much less liable to be filled with gas through diffusion than one having gauze or plate attachments designed to relieve pressure or to permit circulation of air. Diffusion, however, is not the only way by which gas enters the interior of equipment. The metallic parts of electrical apparatus are subject to variations of temperature during periods of rest and operation. Consequently, the atmosphere within expands and contracts, creating an unbalanced pressure with the atmosphere outside. This difference of pressure tends to become equalized by the transfer of gases in and out of the casing. Thus, gas may be drawn into the interior of an electric motor or other electric equipment through very small openings. Finally, if covers or other parts are removed while the apparatus is surrounded with gas the exposed interior soon becomes filled with an explosive mixture.

FACTORS BEARING ON SAFETY

The degree of hazard connected with the introduction of a piece of electrical machinery in a gassy mine and the possible results of an explosion within that machine vary with the following factors:

1. Percentage of gas in the atmosphere inside of and surrounding the machine. 2. Amount, condition, and distribution of coal dust (bituminous) in the mine. 3. Degree of turbulence of the gas within the machine.