Program Policy Manual - VOLUME V - Mine Safety and Health ...

MSHA PROGRAM POLICY MANUAL VOLUME V
Subpart I Underground High-Voltage Distribution
75.800 High-Voltage Circuits; Circuit Breakers
A suitable circuit breaker is one that is:
1. Capable of interrupting the maximum fault current to
which it may be subjected without damage to itself;
2. Capable of carrying the continuous current imposed upon
it without damage to itself; and
3. Rated for not less than the voltage of the circuit.
"Undervoltage protection" is defined in the IEEE Standard
Dictionary of Electrical and Electronic Terms (IEEE Standards 100-
1972) as "the effect of a device, operative on the reduction or
failure of voltage, to cause and maintain the interruption of
power to the main circuit." The principal purpose for
undervoltage protection is to prevent automatic restarting of
equipment when power is restored after an outage. Undervoltage
protection can be provided by an undervoltage trip coil on the
circuit breaker or by an undervoltage relay or a ground-check
relay connected into the circuit breaker trip circuit. When an
undervoltage relay or ground-check relay is used to operate the
shunt trip coil on a circuit breaker, a stored-energy tripping
source must be provided (i.e., capacitor trip or battery trip) to
insure that the circuit breaker will trip during a power outage.
The undervoltage relay may either be an induction or attraction
type and must trip the circuit breaker when the line voltage
decreases to 40 percent of nominal or less when power is lost.
A high-voltage circuit extending underground shall be protected
against the harmful effects of a grounded phase in the underground
circuit and in any surface circuit supplied from the same set of
transformer windings. Consequently, if one set of transformer
windings supplies resistance-grounded power to both underground
and surface loads, the circuit(s) extending to the surface loads
must also be provided with grounded-phase protection. Fuses may
be used to provide grounded-phase protection only for small
control transformers installed in the same substation as the
transformers that supply the resistance- grounded circuit. In all
other cases, circuit breakers equipped with grounded-phase
protective devices must be used to provide the required groundedphase
protection.
Grounded-phase relays should be adjusted to operate on as low a
value of current or voltage as practical. In order to provide
safe, reliable relaying, settings should not exceed 50 percent of
the maximum fault current for current relaying or 50 percent of
February 2003 (Release V-33) 74