(a) The following provisions shall apply to alternating current electrical systems serving mobile face equipment in anthracite coal mines. The fundamental components of such a system are (1) the outdoor substation through which power is fed to the mine, (2) high voltage underground transmission system, (3) section transformers or load centers which step the transmission voltage down to machine utilization voltage, and (4) distribution centers used to distribute utilization voltage to mining machinery. The latter may be an integral part of the section transformer or load center (3).

(b) The basic system for both transmission and distribution of alternating current power for mobile face equipment shall be a three-phase four-wire system, with a ground fault current limiting resistor in the neutral circuit and the inby or load end of the neutral resistor solidly grounded. The ground end of the neutral resistor shall be connected to equipment frames through the cable ground conductor to prevent dangerous differences of potential between frame and ground under fault conditions.

(c) The basic system for a completely insulated medium voltage transmission and distribution system furnished from a delta-delta connected bank of transformers serving portable face equipment may be a three-phase four-wire system with ground detectors.