§869 Underground Low- and Medium-voltage Alternating Current Circuits

Summary

Requires low- and medium-voltage three-phase power circuits to have circuit breakers that can safely interrupt faults, and that are tested and kept in good working order as the Secretary requires. Those breakers must protect against low voltage, a grounded phase, short circuits, and too much current. Underground three-phase circuits must have a neutral grounded through a resistor at the power center and a grounding conductor that runs with the power lines to the equipment frames. The resistor must limit ground-fault current to 25 amperes, be rated for the maximum fault current continuously, and be insulated for the system’s phase-to-phase voltage. Ungrounded underground circuits are allowed only if the cable is steel armored or in grounded rigid steel conduit for the whole run. Six months after the law starts, resistance-grounded systems must have a fail-safe ground-check circuit that trips the breaker if the ground or pilot wire is broken; the Secretary may allow up to 12 more months per mine if equipment isn’t available. Cable couplers must break the ground-check wire first and grounds last. Disconnect devices must show visually when power is off. Trailing cables for mobile gear must include ground conductors at least half the size of a power conductor, splices must keep all conductors continuous, single-phase loads must be tied phase-to-phase, circuit breakers must be marked, and medium-voltage trailing cables must have grounds, a ground-check conductor, and metallic shields (or unshielded cables on reels if insulation is rated 2,000 volts or more).