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

MSHA PROGRAM POLICY MANUAL VOLUME V
Subpart J Underground Low- and Medium-Voltage Alternating Current
Circuits
75.900 Low- and Medium-Voltage Circuits Serving
Three-Phase Alternating Current Equipment;
Circuit Breakers
Each of the four protective features required by this Section must
be provided for all underground low- and medium-voltage three-phase
a.c. circuits with the exception of trailing cables, which must have
short-circuit, undervoltage, and grounded-phase protection. Circuit
breakers providing short-circuit protection for trailing cables
shall be adjusted so as not to exceed the maximum allowable
instantaneous settings specified under Section 75.601-1. Shortcircuit
and overcurrent protection for circuits shall conform to the
National Electrical Code, 1968. Charts listing sizes and proper
overload and short-circuit protection for motors and motor circuit
conductors can be found in Appendix E of this volume. These tables
shall be used to determine compliance with this Section.
"Adequate interrupting capacity" is the ability of a circuit breaker
to safely interrupt the maximum amount of current that can flow
through its contacts upon occurrence of a short circuit at any point
in the circuit without damage to itself.
Low- and medium-voltage three-phase circuits used underground shall
be protected against the harmful effects of a grounded phase in any
circuit connected to the same transformer secondary. Consequently,
if one bank of transformers supplies power to both underground and
surface loads, both the surface and underground portions of the
circuit(s) shall be provided with grounded-phase protection.
Grounded-phase protective devices for resistance-grounded circuits
should be adjusted to operate on as low a value of fault current as
practical, preferably not more than 50 percent of the current rating
of the neutral grounding resistor.
75.901 Protection of Low- and Medium-Voltage Three-Phase
Circuits Used Underground
When two phase conductors of a three-phase resistance grounded
circuit are used to power single-phase loads, the equipment frames
must be grounded to the grounded side of the grounding resistor.
The grounding circuit shall be provided with a ground check circuit
as required by Section 75.902 and the circuit shall be protected in
accordance with the requirements of Section 75.900.
February 2003 (Release V-33) 81