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

Summary

Require low- and medium-voltage three-phase AC circuits to have circuit breakers that can safely stop fault currents, are tested and kept in good working order, and protect against low voltage, a grounded phase, short circuits, and too much current. Underground three-phase circuits must have a neutral tied to ground through a resistor at the power center, and a grounding wire that runs with the power cables to protect equipment frames. The grounding resistor must limit ground-fault current to 25 amperes, be rated for the largest fault current continuously, and be insulated for the system’s phase-to-phase voltage. The Secretary may allow ungrounded underground circuits only if they are steel armored or run in grounded rigid steel conduit for their full length. Within six months after the rule takes effect, resistance-grounded systems must have a fail-safe ground-check circuit that watches the grounding path and opens the breaker if the ground or pilot check wire breaks; the Secretary may grant a mine-by-mine delay up to 12 months. Cable couplers must break the ground-check conductor first and the ground last. Cables must have visual disconnects with breakers, splices that keep continuity, trailing-cable ground wires at least one-half the size of power conductors, an insulated ground-check conductor after six months (or approved alternative), breakers clearly marked, and medium-voltage trailing cables must include grounding conductors, a ground-check conductor, and metallic shields unless unshielded cable on reels has insulation rated 2,000 volts or more.