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continuous current when used in an unventilated enclosure, and they must also have special 40C
ambient temperature calibration compensation for thermal trip elements. Certain molded-case
circuit breakers, particularly those of, larger frame sizes, are rated for use in enclosures.
(b) Low-voltage power circuit breakers have short-time current ratings. Short-time
current ratings allow for selectivity between circuit breakers in series during short-circuit
conditions, consequently, only the one nearest the fault opens. This short-time rating is based on
a 30-cycle duration test for low-voltage power circuit breakers. Molded-case circuit breakers
sometimes have limited short-time ratings for time durations of approximately 18 cycles. The
solid state trip devices, available to both low voltage power circuit breakers and the larger
three-pole molded case circuit breakers, can include ground fault current sensing and tripping
that is adjusted independently of the phase overcurrent trip devices.
(c) Low-voltage power circuit breakers are designed to permit routine maintenance to
increase long life. In contrast, most molded-case circuit breakers are sealed and maintenance of
the internal mechanism is not possible. Both types allow field adjustment of the tripping
boundaries to some degree, although some varieties of molded-case circuit breakers are
preset and sealed at the factory and are not field adjustable.
4.6.7 Protection. There are two types of overcurrent protection generally emphasized, phase
overcurrent and ground fault. Circuit breakers can be made to provide ground-fault protection
that is generally more sensitive than phase-overcurrent protection. Several low-voltage power
circuit breakers also offer ground fault protection.
(a) Historically, the only means of providing protection against ground-overcurrent
damage has been the use of separately mounted overcurrent relays and current transformers. The
characteristics of the elements, such as high relay burdens and current transformer saturation,
have limited the effectiveness of these schemes. Electronic technology has provided a basis for
design of a ground-fault tripping function that is sensitive enough to detect very low ground
overcurrents, yet is immune to nuisance tripping under numerous system conditions; including
motor starting and phase-to-phase faults. These faults must be detected by the instantaneous or
delay elements to ensure coordination in a selective system. To allow the greatest flexibility of
application in a selective system, there must also be enough adjustment in pickup and time delay
to allow at least two levels of coordination. Two level devices are used extensively to provide
ground-fault protection in air circuit breakers. Three-level coordination is also available, but
careful analysis of system operation is necessary to use more than three levels, as nuisance
tripping must be prevented.
(b) An important aspect of good protection is the simultaneous disconnection of all
phases of a polyphase circuit under short-circuit conditions. This prevents a short circuit from
being backfed from other energized phases. Circuit breakers can provide simultaneous
disconnection of polyphase circuits. The long-time-delay characteristic of low-voltage power


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