30 CFR § 75.814
Electrical protection.
(a) High-voltage circuits must be protected against short circuits,
overloads, ground faults, and undervoltages by circuit-interrupting
devices of adequate interrupting capacity as follows:
(1) Current settings of short-circuit protective devices must not
exceed the setting specified in approval documentation, or seventy-five
percent of the minimum available phase-to-phase short-circuit current,
whichever is less.
2) Time-delay settings of short-circuit protective devices used to
protect any cable extending from the section power center to a motor-
starter enclosure must not exceed the settings specified in approval
documentation, or 0.25-second, whichever is less. Time delay settings
of short-circuit protective devices used to protect motor and shearer
circuits must not exceed the settings specified in approval
documentation, or 3 cycles, whichever is less.
3) Ground-fault currents must be limited by a neutral grounding
resistor to not more than--
i) 6.5 amperes when the nominal voltage of the power circuit is
2,400 volts or less; or
ii) 3.75 amperes when the nominal voltage of the power circuit
exceeds 2,400 volts.
4) High-voltage circuits extending from the section power center
must be provided with--
i) Ground-fault protection set to cause deenergization at not more
than 40 percent of the current rating of the neutral grounding
resistor;
ii) A backup ground-fault detection device to cause deenergization
when a ground fault occurs with the neutral grounding resistor open;
and
iii) Thermal protection for the grounding resistor that will
deenergize the longwall power center if the resistor is subjected to a
sustained ground fault. The thermal protection must operate at either
50 percent of the maximum temperature rise of the grounding resistor,
or 150 deg. C (302 deg. F), whichever is less, and must open the
ground-wire monitor circuit for the high-voltage circuit supplying the
section power center. The thermal protection must not be dependent upon
control power and may consist of a current transformer and overcurrent
relay.
5) High-voltage motor and shearer circuits must be provided with
instantaneous ground-fault protection set at not more than 0.125-
ampere.
6) Time-delay settings of ground-fault protective devices used to
provide coordination with the instantaneous ground-fault protection of
motor and shearer circuits must not exceed 0.25-second.
7) Undervoltage protection must be provided by a device which
operates on loss of voltage to cause and maintain the interruption of
power to a circuit to prevent automatic restarting of the equipment.
b) Current transformers used for the ground-fault protection
specified in paragraphs (a)(4)(i) and (5) of this section must be
single window-type and must be installed to encircle all three phase
conductors. Equipment safety grounding conductors must not pass through
or be connected in series with ground-fault current transformers.
c) Each ground-fault current device specified in paragraphs
(a)(4)(i) and (5) of this section must be provided with a test circuit
that will inject a primary current of 50 percent or less of the current
rating of the grounding resistor through the current transformer and
cause each corresponding circuit-interrupting device to open.
d) Circuit-interrupting devices must not reclose automatically.
e) Where two or more high-voltage cables are used to supply power
to a common bus in a high-voltage enclosure, each cable must be
provided with ground-wire monitoring. The ground-wire monitoring
circuits must cause deenergization of each cable when either the
ground-monitor or grounding conductor(s) of any cable become severed or
open. On or after May 10, 2002, parallel connected cables on newly
installed longwalls must be protected as follows:
1) When one circuit-interrupting device is used to protect
parallel connected cables, the circuit-interrupting device must be
electrically interlocked with the cables so that the device will open
when any cable is disconnected; or
2) When two or more parallel circuit-interrupting devices are used
to protect parallel connected cables, the circuit-interrupting devices
must be mechanically and electrically interlocked. Mechanical
interlocking must cause all devices to open simultaneously and
electrical interlocking must cause all devices to open when any cable
is disconnected.
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