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

MSHA PROGRAM POLICY MANUAL VOLUME V
Subpart J Low- and Medium-Voltage Alternating Current Circuits
77.900 Low- and Medium-Voltage Circuits Serving Portable or
Mobile Three-Phase Alternating-Current Equipment;
Circuit Breakers
Low- and medium-voltage circuits supplying power to portable or
mobile three-phase alternating-current equipment 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
stationary and portable or mobile loads, the circuits, extending
to the stationary loads, as well as the circuits extending to the
portable or mobile loads, shall be provided with grounded phase
protection.
Grounded phase protection for resistance-grounded circuits should
be adjusted to operate on as low a value of fault current as
practical; preferably not more than 40 percent of the current
rating of the neutral grounding resistor.
77.902 Low- and Medium-Voltage Ground Check Monitor Circuits
The ground check circuit required by this Section shall cause the
circuit breaker to trip when any of the following occur:
1. The ground check wire is broken. Existing low- and
medium-voltage cable couplers that are not provided with a
terminal contact for the ground check conductor may be
used if suitable means are provided for breaking the
ground check conductor first when the coupler is being
uncoupled.
2. Either the ground wire is broken at any point, or the
impedance of the grounding circuit increases beyond the
amount necessary to cause a 40-volt drop in the grounding
circuit external to the grounding resistor under ground
fault conditions.
77.903 Disconnecting Devices
"Visual evidence," as used in this Section, means that a physical
separation of the current-carrying parts of the disconnecting
device can actually be seen. Consequently, molded case circuit
breakers are not acceptable as disconnecting devices. A
connecting plug on the outby end of a cable with which the cable
is connected to the power source box shall be accepted as a
disconnecting device. Other visible disconnecting means, such as
switches with visibly opened contacts, will also meet the
requirements of this Section.
77.904 Identification of Circuit Breakers
February 2003 (Release V-33) 194