§869 Underground low- and medium-voltage alternating current circuits

Summary

Require circuit breakers on low- and medium-voltage three-phase AC circuits that can interrupt fault currents. The breakers must be tested and kept in working order as the Secretary requires, and must protect against low voltage, a grounded phase, short circuits, and too much current. Underground three-phase circuits must have a neutral grounded at the power center through a resistor, and a grounding conductor must run with the power wires to bond 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. Six months after this law takes effect, resistance-grounded systems must have a fail-safe ground-check circuit that trips the breaker if the ground or pilot check wire is broken (the Secretary may allow up to a 12-month extension per mine). Disconnects must show visually when power is off. Trailing cables for mobile equipment must include ground conductors at least half the size of the power conductors, and six months after the law takes effect must also include an insulated ground-check conductor or an approved equivalent (12-month mine-by-mine extension allowed). Splices must keep all conductors continuous. Single-phase loads must be tied phase-to-phase. Circuit breakers must be clearly identified. Medium-voltage trailing cables must have grounding conductors, a ground-check conductor, and metal shields around power conductors or a grounded outer shield, except cables on reels may omit shields if insulation is rated 2,000 volts or more.