83 STAT. ]
PUBLIC LAW 91-173-DEC. 30, 1969
determines, based on existing physical conditions, that such instalhition will be more accessible at a greater distance and Avill not pose any hazard to the miners. (c) The grounding resistor, where required, shall be of the proper ohmic value to limit the voltage drop in the grounding circuit external to the resistor to not more than 100 volts under fault conditions. The grounding resistor shall be rated for maximum fault current continuously and insulated from ground for a voltage equal to the phase-tophase voltage of the system. (d) Six months after the operative date of this title, high-voltage, resistance grounded systems shall include a fail safe ground check circuit to monitor continuously the grounding circuit to assure continuity and the fail safe ground check circuit shall cause the circuit breaker to open when either the ground or pilot check wire is broken, or other no less effecti\'e device approved by the Secretary or his authorized representative to assure such continuity, except that an extension of time, not in excess of twelve months, may be permitted by the Secretary on a mine-by-mine basis if he determines that such equipment is not available. (e)(1) Underground high-voltage cables used in resistance grounded systems shall be equipped with metallic shields around each power conductor, with one or more ground conductors having a total crosssectional area of not less than one-half the power conductor, and w4th an insulated internal or external conductor not smaller than No. 8 (AWG) for the ground continuity check circuit. (2) All such cables shall be adequate for the intended current and voltage. Splices made in such cables shall provide continuity of all components. (f) Couplers that are used with medium-voltage or high-voltage power circuits shall be of the three-phase type with a full metallic shell, except that the Secretary may permit, under such guidelines as he may prescribe, no less effective couplers constructed of materials other than metal. Couplers shall be adequate for the voltage and current expected. All exposed metal on the metallic couplers shall be grounded to the ground conductor in the cable. The coupler shall be constructed so that the ground check continuity conductor shall be broken first and the ground conductors shall be broken last when the coupler is being uncoupled. (g) Single-phase loads, such as transformer primaries, shall be connected phase to phase. (h) All underground high-voltage transmission cables shall be installed only in regularly inspected aircourses and haulageways, and shall be covered, buried, or placed so as to afford protection against damage, guarded where men regularly work or pass under them unless they are six and one-half feet or more above the floor or rail, securely anchored, properly insulated, and guarded at ends, and covered, insulated, or placed to prevent contact with trolley wires and other low-voltage circuits. (i) Disconnecting devices shall be installed at the beginning of branch lines in high-voltage circuits and equipped or designed in such a manner that it can be determined by visual observation that the circuit is deenergized when the switches are open. (j) Circuit breakers and disconnecting switches underground shall be marked for identification. (k) I n the case of high-volt age cables used as trailing cables, temporary splices shall not be used and all permanent splices shall be
781
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