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

MSHA PROGRAM POLICY MANUAL VOLUME V
the phase-to-neutral voltage for potential relaying.
Where an ungrounded high-voltage circuit is accepted for use
underground under the provisions of Section 75.802(b), the circuit
must be provided with grounded-phase protection.
Grounded-phase indicating lights that do not trip the circuit
breaker upon the occurrence of a phase-to-ground fault are not
acceptable as compliance with this Section.
Short-circuit protection can be provided by using the
instantaneous units of overcurrent relays or by using inverse-time
overcurrent relays with minimal time dial settings.
The pickup of the instantaneous unit of an overcurrent relay is
independent of the pickup of the inverse-time unit and is
determined by the position of the top of the screw on the
instantaneous unit.
Overcurrent devices are required in at least two phases of threephase
high-voltage underground distribution circuits.
75.800-3 Testing, Examination, and Maintenance of Circuit
Breakers; Procedures
If the circuit breaker located on the surface provides overload,
short-circuit, undervoltage, and grounded-phase protection for the
entire underground circuit, only that circuit breaker is required
to be tested by this Section. Circuit breakers installed
underground for coordination purposes in such circuits are not
required to be tested; however, each independent ground- check
circuit must be tested in accordance with this Section.
The tests required by paragraph (b)(2) of this Section may be
conducted one of three ways:
1. Primary injection test. This test method involves
sufficient current to cause the circuit breaker to trip
through at least two current transformers associated with
the circuit breaker. Since this method requires that test
connections be made on high-voltage conductors or
terminals, stringent safety procedures must be followed.
This method simultaneously tests the current transformer
ratio, the current transformer secondary wiring, the
operation and calibration of the relays, and the operation
of the circuit breaker tripping circuit.
February 2003 (Release V-33) 75