30 CFR § 75.902
Low- and medium-voltage ground check monitor circuits.
On or before September 30, 1970, low- and medium-voltage resistance grounded
systems shall include a fail-safe ground check circuit to monitor
continuously the grounding circuit to assure continuity which 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 extention 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. Cable couplers shall be constructed so that the ground check
continuity conductor shall be broken first and the ground conductors shall
be broken last when the coupler is being uncoupled.