Maintenance of Circuit Breakers

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.

2-28