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

MSHA PROGRAM POLICY MANUAL VOLUME V
setting of d.c. overload relay located in the output leads of
the rectifier bridge. In such instances, the instantaneous
setting of the circuit breaker shall not exceed 82 percent of
the equivalent d.c. overload relay setting.
Reclosing circuit breakers used to protect trolley systems
should be equipped with load-measuring circuits. Reclosing
circuit breakers used to protect multiple-feed systems should
also be equipped with voltage-differential circuits. Loadmeasuring
and voltage-differential circuits are necessary to
prevent the circuit breaker from reclosing on a short circuit or
abnormally high load. Load-measuring circuits should be
adjusted to as low a value of current as practical. In almost
all instances, the system will perform satisfactorily with the
load-measuring circuits adjusted at or below 300 amperes;
however, there may be instances when slightly higher values are
required because of pumps, lights, and other continuous loads on
the system. Voltage-differential circuits should be adjusted to
values equal to or exceeding 85 percent of the system voltage.
The mine operator should establish a program under which all
trolley circuits will be tested and calibrated at least once
every 6 months. The circuit breaker may be tested by loading
the circuit until the current reaches that amount necessary to
cause actuation of the circuit breaker or an external current
source may be used. Circuit breakers equipped with solid state
controls that are actuated by the voltage drop across a shunt
may be tested by disconnecting the control circuit leads from
the shunt and applying the proper amount of voltage to the shunt
leads to cause actuation. Current actuated relays shall be
tested by passing the necessary amount of current through them
to cause actuation. Relays that have a current calibration coil
may be calibrated with an external current calibration source,
provided that: (1) each relay is installed, adjusted,
maintained, tested and calibrated in accordance with the
manufacturer's instructions; and (2) the tolerance of the
calibration current source and the specified relay operating
tolerance shall not exceed 15 percent.
The record required by this Section shall be kept in a book and
should contain the following:
1. Date of test;
2. Name of qualified person making test;
3. Indicated current setting; and
February 2003 (Release V-33) 87