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

MSHA PROGRAM POLICY MANUAL VOLUME V
Ungrounded three-phase low- and medium-voltage circuits may be
permitted in an underground coal mine to feed stationary equipment
only after an investigation has been made by electrical personnel
and after that investigation finds that the use of such circuits in
a particular mine does not pose a hazard to the miners.
When two phase conductors of an ungrounded surface three-phase
circuit are taken underground for use as a control circuit, this
circuit exhibits all the hazards inherent in an ungrounded threephase
circuit and shall be judged accordingly. If an operator
wishes to install an ungrounded circuit underground, including
the control circuit described above, application should be made
to the District Manager. The District Manager shall assign
electrical inspectors or electrical engineers to an investigation
of the application, and, if it is determined that no hazard is
created by the installation of the ungrounded circuit, the
District Manager shall notify the operator in writing of the
acceptability of the circuit and of any restrictions imposed on
the operation of the circuit.
Ungrounded circuits feeding portable distribution boxes or power
centers that supply power to mobile equipment are not to be
accepted.
In all cases where ungrounded circuits are accepted for use
underground, the circuit breaker protecting the circuit must be
equipped with grounded-phase protection. Ground-indicating
lights that do not trip the circuit breaker when a phase-toground
fault occurs are not acceptable.
75.902 Low- and Medium-Voltage Ground Check Monitor Circuits
The following criteria shall be used for determining compliance
of ground check circuits in low- and medium-voltage systems
supplying power only to stationary equipment.
1. If a ground check conductor is used, the ground check
circuit will trip the circuit breaker when the ground
check conductor is broken.
2. The ground check circuit will trip the circuit breaker if
the ground wire is broken at any point in the grounding
circuit. If low resistance parallel paths for fault
current and monitoring current are present, the ground
check circuit will be acceptable if it is designed to
trip the circuit breaker when 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 fault conditions.
February 2003 (Release V-33) 82