Introduction to Circuit and Motor Protection – CIRCUIT PROTECTION (7)

Circuit Breakers
As defined by NEMA, a circuit breaker is a device that opens and closes a circuit by nonautomatic means if used as a disconnect.

In other words, a circuit breaker opens the circuit automatically because of predetermined overcurrents created by an overload or a short circuit. A circuit breaker trips a mechanism inside which disconnects the circuit from the overcurrent condition (see Figure 19).

The basic part of a circuit breaker, a circuit interrupter, is shown in Figure 20 for a three-pole circuit. This section of the breaker, which is nonautomatic, is followed by the representation of the automatic breaker section.
Circuit breakers use two types of tripping elements: a bimetal, or thermal tripping element, or a
magnetic tripping element. Circuit breakers occasionally use both types of tripping elements. These devices can trip instantly in what is called inverse time (see Figure 21).

Inverse time means that the time it takes the breaker to trip is inversely proportional to the amount of current. The higher the amount of current, the less time needed for the circuit breaker to trip.
For instance, a 20-amp breaker may take several minutes to trip a current of 25 amps (see Figure 22). However, if the same 20-amp breaker is subjected to a 50-amp current, it will trip in a fraction of a second.

to be continued…………


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