Code of Federal Regulations Title 30. Mineral Resources § 30.77.701–1 Approved methods of grounding of equipment receiving power from ungrounded alternating current power systems

(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 earth;

(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.