Friday, March 25, 2011

Circuit Breaker Time ( total break time)

4. Rated characteristic of circuit breakers 
4.7. Circuit Breaker Time ( total break time)
       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 

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