30 CFR § 77.906
Trailing cables supplying power to low-voltage mobile equipment; ground wires and ground check wires.
On and after September 30, 1971, all trailing cables supplying power to
portable or mobile equipment from low-voltage three-phase resistance grounded
power systems shall contain one or more ground conductors having a
cross-sectional area of not less than one-half the power conductor. Such
trailing cables shall include an insulated conductor for the ground
continuity check circuit except where a no less effective device has been
approved by the Secretary to assure continuity. Splices made in low-voltage
trailing cables shall provide continuity of all components.
Subpart K--Ground Control