should be in one piece so that the annular space will be filled uniformly; the piece should be long enough that when compressed it will occupy a space of not less than one-half inch along the cable. Figure 6 shows a section of a stuffing box with a cable packed in it.

Care should be taken that the packing nut does not bind before compressing the asbestos packing, which should grip the cable tightly. After the packing nut has been adjusted it should be secured against unscrewing.

PROPERLY SEALED COMPARTMENT

A properly sealed compartment is one that has seals in places where they should be. Figure 6 shows the stuffing box with cable gland tightly in place and sealed. The sealing wire is threaded through holes in the bolts as shown; the ends are brought together and held by a lead seal. The seal must be mutilated or the sealing wire cut before the gland can be removed; thus, the opening of sealed compartments by unauthorized persons can be detected. Locks may be used on some compartments instead of seal wires, but the latter have the advantage of having no moving parts to become clogged with rust and dirt; moreover, locks occupy more space than seals.

PROPERLY GUARDED WIRING

The wiring between motors, controllers, rheostats, and other parts of a permissible machine must be protected against abrasion, cuts, damage by moving parts, or damage by objects that may fall or be thrown upon a machine. Such damage may result in short circuits and arcs that could ignite flammable gas; therefore, adequate mechanical safeguards are necessary to protect all wires and cables between motors and other electrical accessories.

A common practice is to enclose the wires and cables in conduit; compressed-air-hose conduit has been used extensively for this purpose. Figure 7 shows the use of hose conduit as applied to the wiring of a permissible storage-battery-locomotive chassis. Two main conduits running from the controller are shown in the figure; the smaller conduit contains a cable connected to a motor, and the larger contains a cable connected to a rheostat. A third conduit, shown in the photograph, contains a cable that connects the controller with the storage batteries by means of a plug. These conduits keep the wiring away from the drive chain and brake mechanism and also keep oil and moisture from the wiring.

Hose conduit used to protect the wiring of a permissible machine should be held securely not only at the ends but should be supported by suitable clamps at such other points as necessary to keep the unsupported length of conduit to a minimum and to prevent disconnection of the conduit from its fastenings due to motion or vibration of the machine. These points are determined by such factors as the length and position of the conduit and its proximity to moving parts. Figure 7 illustrates some special conduit clamps. One of these clamps keeps the conduit out of contact with the sprocket chain connecting two axles. Another clamp is shaped like an inverted channel and prevents the cross bar of a brake rigging from rubbing on the conduit.

743044°-47-5

FIGURE 7.-Permissible storage-battery-locomotive chassis showing hose conduit.

PROTECTION FOR WIRES AND TRAILING CABLES

Wiring should be protected adequately against short circuits and overloads, as well as against mechanical damage. Such protection is necessary to minimize the possibility of arcing or sparking external to explosion-proof compartments.

Various methods of protection have been devised by the several manufacturers of permissible equipment. A fuse is the simplest device for protecting against overloads and short circuits, but it must have the proper rating to serve the intended purpose. It is not unusual coal-mine practice for an electrician or mechanic to replace a blown fuse with one that is too large for the needed protection. The blowing of the original fuse may have been caused by overloading the machine or, because of low voltage, application of excessive current to obtain the power required. Another common fault is neglecting to keep the contacts clean and tight between a fuse and its clips, causing high resistance that may result in heating and failure. The correct procedure is to remove the cause of blowing the fuse rather than to apply a larger fuse; this might allow overheating of the wires and damage to the insulation or blown-out or burned-out armatures, fields, or rheostats, all of which are gas-ignition hazards. Nails and copper wires as large as 4-0 trolley wire have been used in place of fuses. The practice cannot be too strongly condemned not only because it is unsafe but also because it is uneconomical. It has not been unusual to find that fuse-protected trolley taps for trailing cables have been discarded and the cables, which would have been protected by a fuse, have burned.

Various types of circuit breakers and overload relays to protect wiring are used in conjunction with permissible machines. If such devices are set too high, the protection against overload will be lost. Therefore, it is important that circuit breakers and relays be set correctly at all times or, if fuse protection is used, that the fuses should be of the proper size.

UNDERGROUND ELECTRICAL EQUIPMENT

Electric appliances, machines, and conductors should be large enough for the required work so that they will function efficiently without overheating. Overheating of electrical parts has been an important source of fires and explosions.

TRAILING CABLES

The possibility of igniting flammable gas must be guarded against when trailing cables are used; fuse protection is therefore important: 27 Fuses in trailing-cable circuits give threefold protection: (1) Protection of the motors from overload; (2) protection of the supplying system from shortcircuit overload when the cable or motor circuit is damaged or fails; (3) limitation of the current and its duration of flow, thereby reducing the arcing at the point of failure and the probability of igniting the cable insulation and sheath, which in turn might cause an explosion or a mine fire. * *

*

The cable is fused at the power end and usually is connected to the power circuit in a fresh-air entry. Its length varies from 200 to 500 feet. Almost the whole length of the cable is exposed to damage by being run over, by roof falls, and by being hit with tools, timbers, etc. Thus, a cable may be severed, short-circuited, or sufficiently exposed to cause grounding of a conductor.

27 Hooker, A. B., and Coggeshall, E. J., Do Fuses Protect Against Methane Ignitions? Bureau of Mines Rept. of Investigations 3179, 1932, 13 pp.

Short circuits may be due to faulty cable insulation or a poorly made splice that lets the conductors come together or in contact with the ground; they may also be due to excessive damage to the cable in spite of its original insulation and sheath. The short-circuit current is usually limited only by fuses or a circuit breaker at the power end.

Short circuit by severing or grounding of a cable in the presence of methane is always a hazard. If no gas is present, the hazard is relative, depending upon the amount of current flow and conditions at the point of failure. Thus, if the cable is crushed on the rail or severed by a conducting material, there will be an

FIGURE 8.--Mounted samples of specially recommended trailing cables.

electric arc that may ignite the cable rubber, which in turn may ignite coal dust or other combustible and cause a mine fire. If the cable is severed by nonconducting material, such as slate from a roof fall, a direct short circuit may not occur, but the normal current flow will be interrupted and the resulting arc may bridge the severed ends of the conductors, causing flame that may ignite surrounding combustible material.

Explosive mixtures of methane and air may be ignited by the smallest spark which might result from breaking any lighting or power circuit.

HAZARDS

similarly operated is common practice in bituminous-coal mines. Trailing cables are necessities for portable electric equipment other than those machines operated with storage batteries. Such cables, because of their liability to failure from being run over by mobile equipment, present a hazard from arcs when used to operate permissible equipment. Trailing cable may produce arcs from short circuits caused by falls of rock that crush or break the cable. They also present shock hazards if handled during normal operation of machines because of the possibility of faulty insulation.

The following conditions in coal mines render the upkeep of trailing cables difficult: 28

(1) Mechanical injury to the insulation by blows from tools, falling coal, or rock; (2) crushing by being run over by coal cars or the wheels of other trackmounted machines; (3) abrasion due to reeling and unreeling and wear over

sharp edges of coal and rock; (4) occasional strains on insulation and conductors due to excessive tension by reeling devices; (5) damage of insulation by action of mine water through which cables are often dragged; and (6) abuse from excessive current overloads due to the use of undersized conductors or to low-voltage conditions.

The ideal trailing cable should have an outer covering that insulates against electric shock, excludes moisture, withstands abrasion, and protects the conductors against mechanical blows and an inner conductor covering to give adequate insulation between conductors. Depending upon service conditions, each cable may have one, two, three, and (in special instances) four or more conductors.

REQUIREMENTS

Trailing cables for portable machines should be flexible in addition to being adequately insulated. For this reason all-rubber insulated cable is recommended for use with permissible machines. At the time

28 Ilsley, L. C., Hooker, A. B., and Coggeshall, E. J., Rubber-Sheathed Trailing Cables: Bureau of Mines Bull. 358, 1932, p .2.