MSHA - Code of Federal Regulations - 30 CFR 75.701-3
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MSHA - Title 30 CFR
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 approved:
(a) A solid connection to the mine track;
(b) A solid connection to the grounded power conductor of the system;
(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.