(1) Protective fence or enclosure.

(2) Primary or incoming line lightning arrestors.

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

(4) 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 15,000 volts, nominal, phase to phase. The transformer may be connected 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 the use of a three-phase zig-zag or grounding transformer. Where grounding transformers are used, they shall be of sufficient capacity to carry maximum ground fault current continuously. 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.

(5) Secondary lightning arrestors.

(6) Ground fault-current limiting resistor capable of continuously limiting ground fault current to 25 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.

(7) 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 and output side of the breaker. Use of automatic reclosing circuit breakers is prohibited. Breaker automatic tripping shall be through protective relays and shall provide, as a minimum, tripping by undervoltage, instantaneous and inverse time limit phase overcurrent, ground fault current not exceeding 15 amperes and ground-continuity check not exceeding seven amperes. The ground-continuity check circuit shall continuously monitor the integrity of the neutral circuit leading underground and shall cause the breaker to open when either the ground or pilot check wire is broken. An ammeter capable of reading current in each phase and a voltmeter capable of reading phase-to-phase voltage shall be provided at the circuit breaker.

(8) Surge protection or station ground bed to which shall be connected all lightning 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.

(9) Neutral or primary ground bed located at least 25 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 high-voltage alternating current neutral circuit and the mine direct-current ground.

(10) Ground bed resistance shall be measured at least every six months and appropriate action taken to assure the maintenance of four ohms or less of ground bed resistance. A record of these resistance measurements shall be kept in a book provided for that purpose.