30 CFR § 75.701-1
Approved methods of grounding of equipment receiving power from ungrounded alternating current power systems.
For purposes of grounding metallic frames, casings and other enclosures of
equipment receiving power from ungrounded alternating current power systems,
the following methods of grounding will be approved:
(a) A solid connection between the metallic frame, casing, or other
metal enclosure and the grounded metallic sheath, armor, or conduit enclosing
the power conductor feeding the electrical equipment enclosed;
(b) A solid connection to a borehole casing having low resistance to
(c) A solid connection to metal waterlines having low resistance to
(d) A solid connection to a grounding conductor extending to a low
resistance ground field located on the surface;
(e) Any other method of grounding, approved by an authorized
representative of the Secretary, which ensures that there is no difference in
potential between such metal enclosures and the earth.