30 CFR § 77.803
Fail safe ground check circuits on high-voltage resistance grounded systems.
On and after September 30, 1971, all high-voltage, resistance grounded
systems shall include a fail safe ground check circuit or other no less
effective device approved by the Secretary to monitor continuously the
grounding circuit to assure continuity. The fail safe ground check circuit
shall cause the circuit breaker to open when either the ground or ground
check wire is broken.