30 CFR § 75.800-2
Approved circuit schemes.
The following circuit schemes will be regarded as providing the necessary
protection to the circuits required by §75.800
(a) Ground check relays may be used for undervoltage protection if
the relay coils are designed to trip the circuit breaker when line voltage
decreases to 40 percent to 60 percent of the nominal line voltage;
(b) Ground trip relays on resistance grounded systems will be
acceptable as grounded phase protection;
(c) One circuit breaker may be used to protect two or more branch
circuits, if the circuit breaker is adjusted to afford overcurrent protection
for the smallest conductor.