TESTS WITH SHORTWALL MINING MACHINES

In making the tests with shortwall mining machines the general procedure was to fasten one end of a flexible test lead under the head of a cap screw on the machine, the other end being free to connect to the instrument. On the mining machine the test lead was taped at intervals to the hand cable of the machine so as to interfere least with handling of the machine by its crew. The two "reference ground" terminals of the instrument were connected to the room track and to a steel rod driven into the mine floor. Figure 20 illustrates the general set-up followed in most of the tests. In reading resistance of room track the machine and track were used as references. No change other than interchange of meter leads was made in the test set-up for these readings.

*

* *

The following conclusions are deduced from the investigation described in this paper:

1. That metal parts of loading and shortwall mining machines merely resting or being maneuvered on the mine floor do not establish a connection with the earth of sufficiently low resistance to make an effective ground.

2. That a single peg or rod driven in the ground or mine floor gives no assurance of making a connection with earth of sufficiently low resistance to be satisfactory as a grounding electrode.

3. That fish-plated, unbonded track in most instances shows a sufficiently low resistance contact with the earth to form a satisfactory grounding electrode. 4. It is further concluded that the only dependable method to be employed in determining the effectiveness of a protective ground, whether for surface or underground equipment, is to measure the resistance of the grounding connection. In other words, although visual inspection or examination helps in determining whether grounding connections have been disturbed or damaged, such inspection of itself does not give any real measure of the effectiveness of protective grounds that are intact. Moreover, to prevent dangerous electric potential differences occurring between earth and frames of machines that are not track-mounted, a separate conductor should be connected between the frame of the machine and a grounding electrode of proved low resistance.

In many instances a conductor joining the frame of a shortwall mining machine to that of its cable-reel truck will prove acceptable for grounding that machine to the track. The presence of coal and dirt under the wheels of the truck could make this method of grounding ineffective. Therefore, a more positive method would be to use a grounding conductor long enough to permit attachment to bonded track, especially if there is any doubt that the room track would make satisfactory contact with other track and with the floor.

ADDITIONAL PROTECTION

When two or more machines are used together in the same working place contact between the frames of the machines, such as a loadingmachine boom touching a shuttle car, may result in electric sparks or flashes due to faulty insulation in one or both machines, unless the frames have been effectively grounded beforehand. With mobile equipment such as a loading machine operating in conjunction with a shuttle car, a detachable bond between the two machines or a detachable bond between each machine and a common grounding conductor would remove the possibility of a difference of potential between the machine frames. Detachable bonds would produce operational difficulties which could be overcome by separate grounding conductors in trailing cables.

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 precautions are shock-preventive measures and are considered necessary in addition to any required frame grounding.

FIRE PROTECTION

Underground electrical stations for permanent installations of pumps, compressors, motor generators or other electrical conversion equipment, and transformers should be placed in well-ventilated, fireproof rooms. Considerable heat is developed by electrical equipment, particularly that operated almost continuously, such as motor-generator sets and rotary converters. In underground substations good ventilation is essential to dissipate the heat and prevent overheating of the machines. Fireproof rooms are also necessary to prevent the spreading of fire if a machine should become overheated and burst into flame.

Fire extinguishers containing chemicals approved for electrical fires should be available at all electric installations. If such extinguishers cannot be obtained, rock dust should be available. Care should be observed in the selection of chemical fire extinguishers for electrical fires. Some extinguishers are filled with liquids that decompose in contact with electric arcs or heated surfaces and give off poisonous fumes. Other extinguishers, notably water or aqueous solutions, are conductors of electricity and present a shock hazard. Before chemical fire extinguishers are purchased the properties of the chemical should be investigated to determine whether it has a solvent or corrosive action on electric machinery. Rock dust is a satisfactory fire extinguisher for electrical fires and has displaced sand to a large extent. Clean, dry sand is satisfactory for extinguishing an electrical fire, but because of its abrasive action, particularly in bearings, it is now considered inferior to rock dust. When possible, suitable chemical fire extinguishers and rock dust should be available for fighting electrical fires.

Clean, dry sand or rock dust and fire extinguishers that do not present either toxic or shock hazards should be placed outside underground electrical stations so as to be out of the smoke in the event of fire in the station.

TRANSFORMERS

Permanent underground transformer stations should be in wellventilated fireproof rooms. Such stations should be provided with doors that close automatically in the event of fire; when practicable, they should be on separate air splits leading to the return. Stations containing transformers filled with flammable oil should be provided with doorsills, or their equivalent, for confining the oil in the event of leakage or explosion. If a fire should occur in such a station, the automatic door should help confine the fire within the station, and the smoke and fumes would go into the return without contaminating the remaining mine atmosphere. Inasmuch as a transformer has no moving parts, it should give little or no trouble if it is large enough for its duty and is installed properly. Suitable warning signs should be posted outside transformer stations to keep unauthorized persons out. The casings of all transformers should be frame-grounded.

Portable transformer stations should be in well-ventilated fireproof housings which are either fully enclosed or fitted with doors that close automatically in the event of fire. Quite often portable trans

743044°-47———7

formers are mounted on mine-car trucks to facilitate movement about the mine; metal boxes with lids that close automatically in case of fire are considered adequate substitutes for fireproof enclosures. The transformer casings should be frame-grounded, and warning signs should be posted. All wiring should be suitably hung and protected, and the disconnecting switches should be properly mounted and guarded.

CONVERSION EQUIPMENT

Conversion equipment consists of motor-generator sets, rotary converters, and mercury-arc rectifiers; it is used to convert alternating current to direct current. Direct current is necessary for supplying power to trolley locomotives and for charging storage batteries. With these exceptions, all other equipment used in coal mines could be operated by alternating current. However, where trolley wires are installed to operate locomotives, other equipment usually is operated by direct current from the trolley circuit rather than to have both alternating- and direct-current circuits in the mine.

Underground stations for motor generators and other conversion equipment should be in well-ventilated fireproof rooms. The equipment should be surrounded by insulating mats or platforms. Meters, rheostats, switches, and circuit breakers should be of safe design and installed on adequately protected switchboards. Motor-generator sets should be frame-grounded. Motor-generator sets or rotary converters may be mounted on trucks so that they are readily portable, and the substation structure may also be portable. Figures 21 and 22 show exterior and interior views of a portable fireproof structure housing a portable motor-generator set.

Manufacturers of rotary converters do not agree that rotary converters should be frame-grounded; the equipment manufacturer should therefore be consulted before a particular rotary converter is framegrounded.

A mercury-arc rectifier converts alternating current to direct current by virtue of the check-valvelike action of mercury-filled vacuum tubes which permit the flow of current in one direction only. Mercuryarc rectifiers are not rotating machines and differ from motor generators and rotary converters, but the same installation and circuit protections apply to them because of possible fire hazards.

STORAGE-BATTERY CHARGING STATIONS

A serious hazard in storage-battery charging stations is the formation of hydrogen from the electrolysis of water. Hydrogen is an explosive gas, easily ignited by spark or flame; therefore, all sources of spark or flame should be eliminated from a battery-charging station. Battery-charging stations should be well-ventilated. If practicable, such stations should be in fireproof rooms, and the air that ventilates them should be conducted directly to return airways. Open flame should not be permitted in a battery-charging room. All electric fixtures, wiring, and charging equipment should be protected to eliminate exposed arcing or sparking.

Storage batteries are used for powering storage-battery locomotives, shuttle cars, and to some extent for power trucks used to operate mining machines and other equipment. An overheated battery cell may start a mine fire if it is covered by a fall of coal.

WIRING AT ELECTRICAL STATIONS

Wiring in underground structures should be insulated and installed properly to minimize the fire hazard. Proper installation includes proper support of wiring on insulators or in conduit and segregation

FIGURE 23.-Master circuit breaker in an underground fireproof enclosure.

of circuits to prevent accidental electrical contact and shock hazards. Lighting wires should be installed as carefully as power wires.

SWITCHES, CIRCUIT BREAKERS, AND SWITCHBOARDS

Underground electrical equipment should be provided with switches of safe design, construction, and installation. Where possible, switches