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bearings show signs of rust, disassemble the device
properly adjusted. Make a contact impression and
and carefully clean the rusted surfaces. Light oil can
check the spring pressure following the
be wiped on sparingly to prevent further rusting. Oil
manufacturer's instructions. If information on the
has a tendency to accumulate dust and grit, which may
correct contact pressure is not available, compare the
cause unsatisfactory operation of the device,
contact pressure with that of similar contacts. When
particularly if the device is delicately balanced.
the force is less than the designed value, the contacts
either require replacing because they are worn down,
Arc chutes or boxes should be cleaned by scraping
or the contact springs should be replaced. Always
them with a file if wiping with a cloth is not sufficient.
replace contacts in sets, not singly; and replace contact
Replace or provide new linings when they are broken
screws at the same time. Do not use emery paper or
or burned too deeply. Be certain that arc chutes are
emery cloth to clean contacts, and do not clean
securely fastened and that there is sufficient clearance
contacts when the equipment is energized.
to ensure that no interference occurs when the switch
or contact is opened or closed.
Clean all surfaces of the circuit breaker
mechanism, particularly the insulation surfaces, with
Shunts and flexible connectors, which are flexed
a dry cloth or air hose. Before directing the air on the
by the motion of moving parts, should be replaced
breaker, be certain that the water is blown out of the
when worn, broken, or frayed.
hose, that the air is dry, and that the pressure is not
Operating tests that consist of operating the circuit
over 30 psi. Check the pins, bearings, latches, and all
breakers in the manner in which they are intended to
contact and mechanism springs for excessive wear or
function in service should be conducted regularly. For
corrosion and evidence of overheating. Replace parts
manually operated circuit breakers, simply open and
if necessary.
close the breaker to check the mechanical operation.
Slowly open and close circuit breakers manually
To check both the mechanical operation and the
a few times to be certain that trip shafts, toggle
control wiring, electrically operated circuit breakers
linkages, latches, and all other mechanical parts
should be tested by the operating switch or control.
operate freely and without binding. Be certain that the
Exercise care not to disrupt any electrical power
arcing contacts make-before and break-after the main
supply that is vital to the operation of the ship or to
contacts. If poor alignment, sluggishness, or other
endanger personnel by inadvertently starting motors
abnormal conditions are noted, adjust the contacts
and energizing equipment under repair.
following the manufacturer's instructions.
Metal locking devices are available that can be
Before returning a circuit breaker to service,
attached to the handles of type AQB circuit breakers
inspect all mechanical and electrical connections,
to prevent accidental operation. All breaker handles
including mounting bolts and screws, drawout
are now provided with a 3/32-inch hole permitting
fastening the locking device with a standard cotter pin.
disconnect devices, and control wiring. Tighten where
NSTM, chapter 300, lists the stock numbers for three
necessary. Give the breaker a final cleaning with a
different sizes of breaker handle locking devices.
cloth or compressed air. Operate manually to be
certain that all moving parts function freely. Check the
insulation resistance.
The sealing surfaces of circuit-breaker contactor
Overload relays are provided in motor controllers
and relay magnets should be kept clean and free from
to protect the motor from excessive currents.
rust. Rust on the sealing surface decreases the contact
Excessive motor current causes normally closed
force and may result in overheating of the contact tips.
overload relay contacts to open, which break the
Loud humming or chattering will frequently warn of
circuit to the operating coil of the main contactor, and
this condition. A light machine oil wiped sparingly on
disconnects the motor from the line. Overload relays
the sealing surfaces of the contactor magnet will aid
are of the thermal or magnetic type.
in preventing rust.
Oil should always be used sparingly on circuit
Thermal Overload Relay
breakers, contractors, motor controllers, relays, and
other control equipment. Oil should not be used at all
The thermal type of overload relay is designed to
unless called for in the manufacturer's instructions or
open a circuit when excessive current causes the heater
coils to reach the temperature at which the ratchet
unless oil holes are provided. If working surfaces or

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