(1) Protective fence or enclosure. Unless surface transformers are isolated by an elevation of at least eight feet above the ground, they shall be enclosed in a transformer house or surrounded by a fence at least six feet high and a horizontal distance of at least eight feet from any live part. All metal enclosures shall be effectively grounded. The entrance to enclosure shall be kept locked at all times, unless authorized persons are present.

(2) All transformers containing flammable oil and installed where they present a fire hazard, shall be provided with means to confine the oil in the event of a leak or rupture of the transformer casing.

(3) Primary or incoming line lightning arrestors.

(4) Positive disconnecting means on the incoming or primary line with circuit breaker or fuses to interrupt safely any current, normal or abnormal, which may be encountered.

(5) Transformer bank to convert the incoming or primary voltage to the transmission voltage. The use of auto-transformers for this purpose is prohibited. Secondary or underground transmission voltage shall not exceed seven thousand two hundred volts, nominal, phase to phase. The transformer may be connected¹ delta-wye, wye-delta or delta-delta. Wye-wye connections shall not be used because of voltage instability under some conditions of load. In the event that the secondary winding is delta-connected, the neutral necessary for the four wire transmission circuit shall be derived by use of three-phase “zig-zag” or grounding transformer except as provided in section 1131(c).² Where such grounding transformers are used, they shall be capable of carrying full ground fault current with same time rating as the ground resistor to which it is connected. Should the substation primary or supply voltage equal the mine transmission voltage, the main transformer bank may be omitted and the “zig-zag” transformer used to derive a system neutral if one is not otherwise available.

(6) Secondary lightning arrestors.

(7) Ground fault-current limiting resistor capable of continuously limiting ground fault current to fifty amperes or less. The resistor shall be adequately insulated and shall be protected by a grounded fence or screen unless mounted eight feet or more above ground.

(8) A secondary or mine feeder circuit breaker with interrupting capacity adequate for any possible condition of fault and no less than the short circuit capacity of the system supplying power to the breaker. Positive disconnect means shall be provided on the input side of the breaker. Use of automatic reclosing circuit breakers is prohibited. Breaker automatic tripping shall be through protective relays.

(9) Surge protection or station ground bed to which shall be connected all lightning³ arrestor grounds, substation equipment frame grounds, fence (if metallic) and substation structure (if metallic). There shall be no direct connection between this ground bed and either the grounded side of the mine direct current system or the neutral ground bed described below.

(10) Neutral or primary ground bed located at least fifty feet away from the station ground at its closest point and to which shall be connected only the inby or load end of the neutral current limiting resistor. To prevent current transformer core saturation by stray direct current return currents, or neutral conductor damage, there shall be no direct or metallic connection between any point of the alternating current neutral circuit and the mine direct current ground.

(11) All surface and underground substations, battery charging stations, pump stations and⁴ compressor stations shall be kept free of nonessential combustible materials and refuse.

(12) Suitable danger signs shall be posted conspicuously at all electrically operated stations.

(13) All housings enclosing oil switch gear shall be effectively ventilated.

(14) Reverse-current protection shall be provided at all synchronous motor-direct current generator stations where two or more generators are connected in parallel, and battery charging stations, to prevent a feedback in the case of power failure.

(15) Insulating mats or other non-conducting material shall be kept in place at each switchboard power control switch, and at stationary machinery where shock hazards exist.

(16) Resistors or rheostats shall be installed in such a manner as not to create a fire hazard and shall be guarded adequately against personal contact.

(17) Switchboards shall be well lighted, front and rear.

(18) Switch rooms and substations shall be kept free of all refuse, debris and combustible material not in service.