Code of Federal Regulations Title 30. Mineral Resources § 30.75.701–3 Approved methods of grounding metallic frames, casings and other enclosures of electric equipment receiving power from direct current power systems with one polarity grounded

(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.