cases, dress and clean the contact surface using fine (No.
Circuit breakers have different time delay
00) sandpaper (the use of fine sandpaper prevents
characteristics. Some have a short time, long time, or
scratching the surface of the contact.) Never use emery
instantaneous trip.
cloth or emery paper. Because this copper-oxide film
The adjustments for selective tripping of most
is a partial insulator, follow the sanding procedures by
circuit breakers are made and sealed at the factory.
wiping with a clean cloth moistened with inhibited
Normally, you would not make changes to the circuit
methyl chloroform solvent.
breaker trip settings because changes may completely
NOTE: Ventilate the space when using inhibited
disrupt the circuit breaker protection functions. If there
methyl chloroform to remove all deadly and toxic
is improper tripping action in the compact assemblies,
fumes of the solvent.
you should correct the problem by replacing the entire
breaker.
ARCING CONTACTS.-- The function of arcing
contacts is not necessarily impaired by surface
After circuit breaker covers have been removed,
roughness. You should use a fine file to remove
you should check the interior components, such as
excessively rough spots. Replace arcing contacts when
contacts, overcurrent tripping devices, connections, and
they have been burned severely and cannot be properly
moving mechanical parts.
adjusted. Make a contact impression and check the
Contacts are small metal parts especially selected to
spring pressure following the manufacturer's
resist deterioration and wear from the inherent arcing.
instructions. If information on the correct contact
In a circuit breaker, arcing occurs while its contacts are
pressure is not available, check the contact pressure with
opening and carrying current at the same time. When
that of similar contacts that are functioning properly.
firmly closed, the contacts must not arc.
When the force is less than the designed value, you
should either replace the contacts if they are worn down
The material used to manufacture contacts has been
or replace the contact springs. Remember, always
diligently researched. The result of this research is
replace contacts in sets and replace the contact screws
contacts made from various metals and/or alloys that
at the same time. Do not clean contacts when the
range from pure carbon or copper to pure silver, each
equipment is energized.
used alone and as an alloy with other substances.
CHECKING CIRCUIT BREAKERS.-- Some of
SILVER CONTACT MAINTENANCE.--
the checks you should make on circuit breakers include
Modem circuit breakers have contacts coated with
cleaning the surfaces of the circuit breaker neck,
silver, silver mixed with cadmium oxide, or silver and
checking arcing contacts, oil piston tripping devices,
tungsten. The two silver alloys are extremely hard and
and sealing surfaces of circuit-breaker contactor and
resist being filed. Contacts made of silver or silver
relay magnets.
alloys conduct current when discolored (blackened
during arcing) with silver oxide. Therefore, the
You should clean all surfaces of the circuit breaker
blackened condition doesn't require filing, polishing, or
mechanism with a dry cloth or air hose. When cleaning
removal. However, if the silver contact is severely
the surfaces, pay particular attention to the insulation
pitted or burned, it may require some filing to remove
surfaces. Before directing the air on the breaker, make
raised places on surfaces that prevent intimate and
sure the water is blown out of the hose, the air is dry, and
overall closure of the contact surfaces. In this case, the
the pressure is not over 30 psi. Check the pins, bearings,
contact should be filed by using a fine file or with fine
latching, and all contact and mechanism springs for
sandpaper, No. 00. If necessary, you may use a clean
excessive wear or corrosion and evidence of
cloth moistened with inhibited methyl chloroform.
overheating. Replace parts if necessary.
NOTE: Ventilate the space when using inhibited
Be certain that the arcing contacts make-before and
break-after the main contacts. If poor alignment,
methyl chloroform to remove all deadly and toxic
fumes of the solvent.
sluggishness, or other abnormal conditions are noted,
adjust the contacts following the manufacturer's
COPPER CONTACT MAINTENANCE.--
instructions.
When cleaning and dressing copper contacts, maintain
Oil-piston type of overcurrent tripping devices
the original shape of each contact surface and remove
(grade B timers) are sealed mechanisms and normally
as little copper metal as possible. Inspect the entire
do not require any attention. When oil-film (dashpot)
contact surface and wipe the copper contact surfaces to
remove of the black copper-oxide film. In extreme
overcurrent tripping devices are used, and the dashpot
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