When coordinating circuit breakers, the instantaneous trip setting of the upstream breaker must be considered; specifically, this setting should exceed the maximum prospective short-circuit current at the outgoing terminals of the downstream breaker. If, due to minimal differences in circuit component impedances at the locations of the two breakers, the resulting short-circuit current values are very similar, the upstream breaker may be selected with a short-time delay trip unit.
When the short-circuit current exceeds or equals the instantaneous trip setting of a current-limiting circuit breaker, the breaker will trip within a matter of milliseconds; therefore, downstream protective devices should not rely on such circuit breakers to achieve selective protection requirements.
For circuit breakers equipped with a short-time delay function, setting the time delay to its maximum value results in a reduction of the breaker's breaking capacity. Consequently, in selective protection circuits, it is essential to ensure that the short-time delay breaking capacity of the selected circuit breaker meets the required specifications.
It is also necessary to ensure that the short-circuit time-delay tripping characteristic curve of the upstream circuit breaker does not intersect with the operating time-current characteristic curve of the downstream circuit breaker; furthermore, the short-time delay characteristic curve should not intersect with the instantaneous characteristic curve.
When coordinating circuit breakers with fuses, the coordination between upstream and downstream devices must be carefully considered. The time-current characteristic curve of the circuit breaker should be compared against that of the fuse to ensure that selective protection is maintained in the event of a short-circuit fault.
When circuit breakers are employed for the protection of power distribution lines, it is advisable to select breakers equipped with long-time delay overcurrent trip units. In the event of a single-phase-to-ground fault at the far end of the line, the resulting short-circuit current should be no less than 1.5 times the set current value of the circuit breaker's instantaneous or short-time delay overcurrent trip unit.