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Contact materials have been subjected to constant
resets the overload unit and the breaker can again be
research, resulting in various products, ranging from
closed.
pure carbon or copper to pure silver, each being used
NLB CIRCUIT BREAKER.-- The NLB circuit
alone and also as alloys with other substances. Modern
breaker is identical to the ALB circuit breaker, except
circuit breakers have contacts coated with silver,
it has no automatic tripping device. It is used only as
silver mixed with cadmium oxide, or silver and
an on-off switch.
tungsten. The two latter silver alloys are extremely
hard and resist being filed. Fortunately, such contacts
Maintenance of Circuit Breakers
made of silver or its alloys conduct current when
discolored (blackened during arcing) with silver
No circuit breaker, regardless of type, should be
oxide. The blackened condition, therefore, requires no
worked on without opening the circuit. Remember that
filing, polishing, or removal. As with a silver contact,
certain terminals may have voltage applied to them
silver oxide is formed during arcing; and it has been
even though the breaker is open. Aboard ship, power
found that the addition of cadmium oxide greatly
may be supplied to either end of the circuit breaker.
improves operation of the contact because it
minimizes the tendency of one contact to weld to
Circuit breakers require careful inspection and
another, retards heavy transfer of one material to
cleaning at least once a year (more frequently if
another, and inhibits erosion.
subjected to unusually severe service conditions).
Usually, a contact containing silver is serviceable
No work should be undertaken on circuit breakers
without first obtaining approval of the electrical or
as long as the total thickness worn away does not
engineer officer.
exceed 0.030 inch.
Before working on a circuit breaker, you should
Severe pitting or burning of a silver contact is
be aware of its time delay characteristics, such as short
another matter. It may require some filing (with a fine
time, long time, or instantaneous trip. The adjustments
file or with fine sandpaper, No. 00) to remove raised
for tripping the circuit breakers are made and sealed
places on surfaces that prevent intimate and overall
at the factory. No unauthorized changes should be
closure of the contact surfaces. If necessary, use a
made to their trip settings because these changes may
CLEAN cloth moistened with INHIBITED methyl
completely disrupt their intended functions of
chloroform. Be very certain to provide ample
protection. If improper tripping action is encountered
ventilation to remove all DEADLY and TOXIC fumes
in the contact assemblies, the fastest and easiest way
of the solvent.
to correct the problem is to replace the entire breaker
When cleaning and dressing copper contacts,
assembly.
maintain the original shape of each contact surface and
Before working on a circuit breaker, de-energize
remove as little copper metal as possible. Inspect and
all control circuits to which it is connected; the
wipe the copper contact surfaces for removal of the
procedure differs somewhat with the type of mounting
black copper-oxide film; in extreme cases, dress and
that is employed. For example, before work is
clean only with fine (No. 00) sandpaper to prevent
performed on drawout circuit breakers, they should be
scratching the surfaces. NEVER use emery cloth or
switched to the open position and removed. Before
emery paper. Because this copper-oxide film is a
working on fixed-mounted circuit breakers, open the
partial insulator, follow the sanding procedure by
disconnecting switches ahead of the breakers. If
wiping with a CLEAN cloth moistened with
disconnecting switches are not provided for isolating
INHIBITED methyl chloroform solvent. Provide
fixed-mounted circuit breakers, de-energize the
VERY LIBERAL ventilation with exhaust fans or
supply bus to the circuit breaker, if practicable, before
with portable blowers to entirely remove all traces of
inspecting, adjusting, replacing parts, or doing any
the deadly fumes of the solvent.
work on the circuit breaker.
Calibration problems on circuit breakers should
Contacts are the small metal parts especially
be handled according to NSTM, chapter 300.
selected to resist deterioration and wear from the
The function of arcing contacts is not necessarily
inherent arcing. The arcing occurs in a circuit breaker
impaired by surface roughness. Remove excessively
while its contacts are opening and carrying current at
rough spots with a fine file. Replace arcing contacts
the same time. When firmly closed, the contacts must
when they have been burned severely and cannot be
not arc.

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