30 CFR § 77.802
Protection of high-voltage circuits; neutral grounding resistors; disconnecting devices.
High-voltage circuits supplying portable or mobile equipment shall contain
either a direct or derived neutral which shall be grounded through a suitable
resistor at the source transformers, and a grounding circuit, originating at
the grounded side of the grounding resistor, shall extend along with the
power conductors and serve as a grounding conductor for the frames of all
high-voltage equipment supplied power from that circuit, except that the
Secretary or his authorized representative may permit other high-voltage
circuits to feed stationary electrical equipment, if he finds that such
exception will not pose a hazard to the miners. Disconnecting devices shall
be installed and so equipped or designed in such a manner that it can be
determined by visual observation that the power is disconnected.