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procedures in this manual. In general, certain control
examined to determine that they are in good
circuits, such as those for the starting of motors or
mechanical condition and that there are no loose or
missing parts. The size of overload heaters installed
motor generator sets, or voltmeter switching circuits,
are best tested by using the circuits as they are intended
should be checked to determine that they are of proper
to operate. When testing such circuits, the precautions
size as indicated by the motor nameplate current and
listed in NSTM, chapter 300, should be observed to
heater rating table. Any questionable relays should be
guard against damage to the associated equipment.
checked for proper tripping at the next availability and
replaced. If necessary a description of the various
Protective circuits, such as overcurrent or reverse
types of overload relays can be found in NSTM,
current circuits, usually cannot be tested by actual
chapter 302, "Electric Motors and Controllers."
operation because of the danger to the equipment
involved. These circuits should be visually checked
Circuit Breakers, Contractors, and Relays
and, when possible, relays should be operated
manually to make sure that the rest of the protective
circuit performs its desired functions. Exercise
Circuit breakers should be carefully inspected and
extreme care not to disrupt vital power service or
cleaned at least once a year and more frequently if
damage electrical equipment. Reverse power relays
subjected to unusually severe service conditions; that
should be checked under actual operating conditions.
is, during ships overhaul period. A special inspection
With two generators operating in parallel, the
should be made after a circuit breaker has opened a
generator whose reverse power relay is to be checked
heavy short circuit.
should be made to take power from the other
To test normally operated circuit breakers, simply
generator. The reverse power relay should trip the
open and close the breaker to check mechanical
generator circuit breaker in 10 seconds or less after the
operation. For electrically operated circuit breakers,
reverse power relay starts to operate. If the relay fails
the test should be made with the operating switch or
to function, the generator circuit breaker should be
control to check both mechanical operation and
tripped manually to prevent damage to the prime
control wiring. Care must be exercised during these
mover. To make a generator act as a load, it is
operating tests not to disrupt any electric power supply
necessary to restrict the flow of steam or fuel. This can
vital to the operation of the ship, nor to endanger ship's
be accomplished by reducing the speed control setting
personnel by inadvertently starting or energizing
slowly until the generator begins to absorb power and
equipment being repaired.
act as a motor.
POWER REMOVAL.-- Before working on a
circuit breaker, control circuits to which it is
Bus Transfer Equipment
connected should be de-energized. Drawout circuit
breakers should be switched to the open position and
Bus transfer equipment should be tested weekly.
removed before any work is done on them.
For manual bus transfer equipment, manually transfer
Disconnecting switches ahead of fixed-mounted
a load from one power source to another, and check
circuit breakers should be opened before any work is
the mechanical operation and mechanical interlocks.
done on the circuit breaker. Where disconnecting
For semiautomatic equipment, the test should also
switches are not provided to isolate fixed-mounted
include operation by the control push buttons. For
circuit breakers, the supply bus to the circuit breaker
automatic equipment, check the operation with the
should be de-energized, if practical, before inspecting,
control switches. The test should include operation
adjusting, or replacing parts or doing any work on the
initiated by cutting off power (opening a feeder circuit
circuit breaker. If the bus cannot be de-energized,
breaker) to see if an automatic transfer takes place.
observe the safety precautions of working on an
When testing bus transfer equipment, you should
energized circuit, These precautions can be found in
follow the same precautions given for testing circuit
NSTM, chapter 300.
breakers. The tests and inspections are normally
outlined on MRCs.
CONTACT CLEANING.-- Contacts in circuit
breakers, contractors relays, and other switching
Overload Relays
equipment should be clean, free from severe pitting or
burning, and properly aligned, Occasional opening
During periodic inspections of motor controllers,
and closing of contacts will aid cleaning and sealing.
Remove surface dirt, dust, or grease with a clean cloth
or at least once a year, overload relays should be

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