There is never a reason to autoreclose an
electrical circuit breaker following a trip unless there is reason to believe
that the fault is no longer present on the circuit. Historically, when
distribution circuit breakers would trip and result in a circuit outage, the
circuit was patrolled before the circuit breaker was closed.
This practice delayed restoration. Records
were kept of these events. It was discovered that for 85–90% of the
occurrences, no permanent faults were found.
It generally became accepted to autoreclose
these distribution circuit breakers. With the advent of additional protective
devices available to the distribution engineer such as fuses, sectionalizers,
and reclosers with which coordination was necessary, multiple autoreclose
attempts were chosen.
In many areas, three autoreclose attempts
were chosen. This results in four trips to lockout. This practice continued for
several years.
As time went on, load increased and it
became necessary that distribution source transformer size increased as well as
the number of supplied feeders. It is known that when transformers are
subjected to any fault on the secondary that the transformer windings are
stressed.
If the transformer was not designed for the
exposure that is encountered in distribution operation, it is possible that
autoreclosing into a fault that would allow the transformer to contribute its
maximum available short circuit current could result in deformation of the
windings and subsequent arc damage to the transformer core and mounting
structure.
Often, repeated occurrences of these stress
levels resulted in transformer failure. The practice of some utilities is to
block autoreclosing for close-in faults or for faults with a fault current
magnitude in excess of the transformer design capability, in an effort to
mitigate the cumulative effect of these severe faults.
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