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.