MSHA - Code of Federal Regulations - 30 CFR 77.701-2
30 CFR § 77.701-2
Approved methods of grounding metallic frames, casings, and other enclosures of electric equipment receiving power from a direct-current power system.
(a) The following methods of grounding metallic frames, casings, and
other enclosures of electric equipment receiving power from a direct-current
power system with one polarity grounded will be approved:
(1) A solid connection to the grounded power conductor of
the system; and,
(2) Any other method, approved by an authorized
representative of the Secretary, which insures that there is no difference
in potential between such metal enclosures and the earth.
(b) A method of grounding of metallic frames, casings, and other
enclosures of electric equipment receiving power from a direct-current power
system other than a system with one polarity grounded, will be approved by an
authorized representative of the Secretary if the method insures that there
is no difference in potential between such frames, casings, and other
enclosures, and the earth.