30 CFR § 75.802 Protection of high-voltage circuits extending underground.
(a) Except as provided in paragraph (b) of this section, high-voltage
circuits extending underground and supplying portable, mobile, or, stationary
high-voltage 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.
(b) Notwithstanding the requirements of paragraph (a) of this
section, the Secretary or his authorized representative may permit ungrounded
high-voltage circuits to be extended underground to feed stationary electric
(1) Such circuits are either steel armored or installed in
grounded, rigid steel conduit throughout their entire length; or,
(2) The voltage of such circuits is nominally 2,400 volts
or less phase-to-phase and the cables used in such circuits are equipped with
metallic shields around each power conductor, and contain one or more ground
conductors having a total cross sectional area of not less than one-half the
power conductor; and,
(3) Upon a finding by the Secretary or his authorized
representative that the use of the circuits described in paragraphs (b)(1)
and (2) of this section does not pose a hazard to the miners.
(c) Within 100 feet of the point on the surface where high-voltage
circuits enter the underground portion of the mine, 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, except
that the Secretary or his authorized representative may permit such devices
to be installed at a greater distance from such area of the mine if he
determines, based on existing physical conditions, that such installation
will be more accessible at a greater distance and will not pose any hazard
to the miners.