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 earth;
(c) A solid connection to metal waterlines having low resistance to earth;
(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.