30 CFR § 77.901 Protection of low- and medium-voltage three-phase circuits.
(a) Low- and medium-voltage circuits supplying power to portable or
mobile three-phase alternating equipment shall contain:
(1) Either a direct or derived neutral grounded through a
suitable resistor at the power source;
(2) A grounding circuit originating at the grounded side
of the grounding resistor which extends along with the power conductors and
serves as a grounding conductor for the frames of all the electric equipment
supplied power from the circuit.
(b) Grounding resistors, where required, shall be of an ohmic value
which limits the ground fault current to no more than 25 amperes. Such
grounding resistors shall be rated for maximum fault current continuously and
provide insulation from ground for a voltage equal to the phase-to-phase
voltage of the system.
(c) Low- and medium-voltage circuits supplying power to three-phase
alternating current stationary electric equipment shall comply with the
National Electric Code.