Selection of Power Circuit Breaker Rating – Part 1

Note: This is will be part 1 of the selection of power circuit breaker rating series.

The standard power circuit breakers as used in high voltage switchgears will be used here to explain power circuit breaker ratings. The same fundamental principles apply to all other high-voltage power circuit breakers.

The structure of circuit breaker rating is complicated because of the time of operation of the circuit breakers after a short circuit occurs.

The few cycles is required for the power circuit breaker to open the circuit and stop the flow of short-circuit current. This time duration consist of:

1. the protective relays to close their contacts,
2. the circuit-breaker trip coil to move its plunger to release the breaker operating mechanism,
3. the circuit-breaker contacts to part, and
4. the circuit breaker to interrupt the short-circuit current in its arc chamber.

During this period, the short-circuit current produces high mechanical stresses in the circuit breaker and in other parts of the high voltage circuit including the supplied circuit. The stresses are produced almost instantaneously in phase with the current and vary as the square of the current. Therefore, the mechanical stresses are greatest when maximum current is flowing.

The short-circuit current is maximum during the first cycle of the fault because of the presence of the DC component. Motors contribute the most short-circuit current a t that time. Thus, the short-circuit stresses on the circuit breakers and other parts of the circuit are maximum during the first cycle of short-circuit current.

During the time from the inception of the short circuit until the circuit breaker contacts part, the current decreases in magnitude because of the decay of the DC component and the change in motor reactance, as explained previously. Consequently, the current that the circuit breaker must interrupt, four or five cycles after the inception of the short circuit, is generally of less magnitude than the maximum value of the first cycle.

The fact that the current changes in magnitude with time has led to the establishment of two bases for the selection of short-circuit-current ratings on power circuit breakers:

1. the momentary rating or its ability to withstand mechanical stresses due to high short-circuit current and
2. the interrupting rating or its ability to interrupt the flow of short-circuit current within its interrupting element.