30 CFR § 75.701-3
Approved methods of grounding metallic frames, casings and other enclosures of electric equipment receiving power from direct current power systems with one polarity grounded.
For the purpose of grounding metallic frames, casings and enclosures of any
electric equipment or device-receiving power from a direct-current power
system with one polarity grounded, the following methods of grounding will be
(a) A solid connection to the mine track;
(b) A solid connection to the grounded power conductor of the
(c) Silicon diode grounding; however, this method shall be employed
only when such devices are installed in accordance with the requirements set
forth in paragraph (d) of §75.703-3; and
(d) 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.