30 CFR § 77.902
Low- and medium-voltage ground check monitor circuits.
On and after September 30, 1971, three-phase low- and medium-voltage
resistance grounded systems to portable and mobile equipment 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 pilot check wire is broken. Cable
couplers shall be constructed to cause the ground check continuity conductor
to break first and the ground conductor last when being uncoupled when pilot
check circuits are used.