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

MSHA PROGRAM POLICY MANUAL VOLUME V
Subpart I Surface High-Voltage Distribution
77.800 High-Voltage Circuits; Circuit Breakers
High-voltage circuits supplying power to portable or mobile
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 circuit(s) extending to the stationary loads,
as well as the circuit(s) 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.
77.800-1 Testing, Examination, and Maintenance of Circuit
Breakers; Procedures
Paragraph (c) does not require the tank surrounding the oil
circuit breakers to be dropped during this examination unless
there is doubt of the qualified person making this examination
as to conditions inside the tank.
77.802 Protection of High-Voltage Circuits; Neutral
Grounding Resistors; Disconnecting Devices
The exception in this Section applies to high-voltage systems
which supply power to stationary surface equipment only.
Consequently, if a high-voltage system supplies power to
equipment located underground in a mine, as well as equipment
located on the surface, the frames of all equipment supplied
power from the system shall be grounded in accordance with
Section 75.802. Likewise, if a high-voltage system supplies
power to portable or mobile equipment located on the surface,
as well as stationary equipment located on the surface, the
frames of all equipment supplied power from the system shall be
grounded in accordance with the resistance grounding
requirements of this Section.
77.803 Fail Safe Ground Check Circuits on High-Voltage
Resistance-Grounded Systems
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; and
February 2003 (Release V-33) 189