30 CFR § 75.803
Fail safe ground check circuits on high-voltage resistance grounded systems.
[STATUTORY PROVISIONS]
On and after September 30, 1970, 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 effective device approved by the Secretary
or his authorized representative to assure such continuity, except that an
extension of time, not in excess of 12 months, may be permitted by the
Secretary on a mine-by-mine basis if he determines that such equipment is not
available.