2-26
TIME DELAY RATINGS
Circuit breakers, like fuses, are rated by the amount of time delay. In circuit breakers the ratings are
instantaneous, short time delay, and longtime delay. The delay times of circuit breakers can be used to
provide for SELECTIVE TRIPPING.
Selective tripping is used to cause the circuit breaker closest to the faulty circuit to trip. This will
remove power from the faulty circuit without affecting other, nonfaulty circuits. Figure 2-22 should help
you understand selective tripping.
Figure 2-22.—Use of circuit breakers in a power distribution system.
Figure 2-22 shows a power distribution system using circuit breakers for protection. Circuit breaker
1 (CB1) has the entire current for all seven loads through it. CB2 feeds loads 1, 2, 3, and 4 (through CB4,
CB5, CB6, and CB7), and CB3 feeds loads 5, 6, and 7 (through CB8, CB9, and CB10). If all the circuit
breakers were rated with the same time delay, an overload on load 5 could cause CB1, CB3, and CB8 to
trip. This would remove power from all seven loads, even though load 5 was the only circuit with an
overload.
Selective tripping would have CB1 rated as long time delay, CB2 and CB3 rated as short time delay,
and CB4 through CB10 rated as instantaneous. With this arrangement, if load 5 had an overload, only
CB8 would trip. CB8 would remove the power from load 5 before CB1 or CB3 could react to the
overload. In this way, only load 5 would be affected and the other circuits would continue to operate.
PHYSICAL TYPES OF CIRCUIT BREAKERS
All the circuit breakers presented so far in this chapter have been physically large, designed to
control large amounts of power, and used a type of toggle operating mechanism. Not all circuit breakers
are of this type. The circuit breaker in figure 2-23 is physically large and controls large amounts of power;
but the operating mechanism is not a toggle. Except for the difference in the operating mechanism, this
circuit breaker is identical to the circuit breakers already presented.
