30 CFR § 75.807
Installation of high-voltage transmission cables.
All underground high-voltage transmission cables shall be installed only in
regularly inspected air courses and haulageways, and shall be covered,
buried, or placed so as to afford protection against damage, guarded where
men regularly work or pass under them unless they are 6 1/2 feet or more
above the floor or rail, securely anchored, properly insulated, and guarded
at ends, and covered, insulated, or placed to prevent contact with trolley
wires and other low-voltage circuits.