30 CFR § 18.50
Protection against external arcs and sparks.
Provision shall be made for maintaining the frames of all off-track machines
and the enclosures of related detached components at safe voltages by using
one or a combination of the following:
(a) A separate conductor(s) in the portable cable in addition to the
power conductors by which the machine frame can be connected to an acceptable
grounding medium, and a separate conductor in all cables connecting related
components not on a common chassis. The cross-sectional area of the
additional conductor(s) shall not be less than 50 percent of that of one
power conductor unless a ground-fault tripping relay is used, in which case
the minimum size may be No. 8 (AWG). Cables smaller than No. 6 (AWG) shall
have an additional conductor(s) of the same size as one power conductor.
(b) A means of actuating a circuit-interrupting device, preferably at
the outby end of the portable cable.
NOTE: The frame to ground potential shall not exceed 40 volts.
(c) A device(s) such as a diode(s) of adequate peak inverse voltage
rating and current-carrying capacity to conduct possible fault current
through the grounded power conductor. Diode installations shall include: (1)
An overcurrent device in series with the diode, the contacts of which are in
the machine's control circuit; and (2) a blocking diode in the control
circuit to prevent operation of the machine with the polarity reversed.
See MSHA's Program Policy Manual