MSHA - Code of Federal Regulations - 30 CFR 75.703-2
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MSHA - Title 30 CFR
30 CFR § 75.703-2
Approved grounding mediums.
For purposes of grounding offtrack direct-current machines, the following grounding mediums are approved:
(a) The grounded polarity of the direct-current power system feeding such machines; or,
(b) The alternating current grounding medium where such machines are fed by an ungrounded direct-current power system originating in a portable rectifier receiving its power from a section power center. However, when such a medium is used, a separate grounding conductor must be employed.