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"Circuit
Breaker Servicing and Repair" With
trilingual summary
Circuit breakers are essential components of all industrial and
commercial electric power systems. Of the air-magnetic, vacuum,
oil, or SF6 types, at voltages from 480 through 34,500, they
protect against damaging short-circuit or ground faults, and in some
applications (particularly in power generating stations) also serve
as motor starting contactors.
Transformers and most rotating machines spend most of their
lifetimes in actual operation. A circuit breaker, in contrast, may
be inactive for months or years. Its often complex operating
mechanism is therefore subject to malfunction caused by deterioration
of lubricant and by corrosion. Though often neglected, an important
maintenance activity is periodic manual operation to make sure all
parts move freely.
However, that does not test the electrical devices that initiate
breaker opening and closing. The breaker may be incapable of
tripping open during a short-circuit. Industrial plant surveys have
repeatedly found many circuit breakers unable to provide fault
protection because of neglect. Even when a breaker does interrupt
fault current, its operation may be slowed so as to greatly increase
the arc energy supplied to the downstream fault.
Many electrical service centers have therefore made circuit breaker
servicing and repair an important part of their business, Such work
usually includes making sure parts are properly lubricated and any
defective hardware is replaced; refinishing or replacing pitted or
worn contacts; cleaning or replacing damaged acr chutes; and
replacing old electromechanical trip units with microprocessor-based
electronic auxiliaries. Some shops also modernize air circuit
breakers by replacing the contact assemblies with vacuum
interrupters.
Repaired or rebuilt breakers are tested in several ways, including
primary current injection (in which a fault is simulated by passing
several values of a-c current through each breaker pole at six or
more times rated breaker amperes, and comparing the observed trip
times with the breaker's design characteristic). Secondary injection
is also used -- a test in which current flows only in the breaker
trip circuit.
Circuit breaker maintenance guidelines are available in standards of
the National Fire Protection Association (No. 70B); the IEEE (Nos.
493, 902, 1015, C37.10); and the International Electrical Testing
Association (Maintenance Testing Specifications).
From “Circuit
Breaker Servicing and Repair"
to be published in the
Electrical Apparatus June 2007 issue
. Visit our online webstore to
order
copies. © 2007
Barks Publications, Inc. All Rights Reserved.
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