Fault clearing time is the
sum of "relay time" and "circuit breaker time". Circuit breaker time is
also called "total break time"
The rapid fault clearing of extra high voltage transmission lines
improves the power system stability. Hence, faster relying and fast
circuit breaker are preferred for extra high voltage transmission lines,
where the circuit breaker time being in order of 2.5 cycles.
For distribution system, such a fast clearing is not necessary.
Discrimination is obtained by "graded time lag:. Hence, slower circuit
breaker, 3 to 5 cycles, are used.
Total breaking time varies between 80-120 ms for circuit breaker up to
12KV and 40-80 ms for circuit breaker above 36KV. It is less than 60 ms
for 145KV, less than 50 ms for 420 KV circuit breaker.
Remember the following time events :
Fault clearing time = relay time + circuit breaker time
Relay time = instant of fault to closure of trip circuit
Circuit breaker time = opening time + arcing time