30 CFR § 77.701-1
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 electric equipment enclosed;
(b) A solid connection to metal waterlines having low resistance to
(c) A solid connection to a grounding conductor extending to a
low-resistance ground field; and,
(d) Any other method of grounding, approved by an authorized
representative of the Secretary, which insures that there is no difference in
potential between such metal enclosures and the earth.