Circuit Breaker breaking capacity expresses the highest rms value of the short circuit current that the circuit breaker is capable of breaking under specified conditions of transient recovery voltage and power frequency voltage.
Transient recovery voltage: The power frequency recovery voltage which appears across the breaker contacts after the transient oscillations die out and final extinction of the arc has resulted in all the poles.
When short circuit occurs it can be seen from the short circuit wave that the rms value of the current varies with time on account of presence of dc component of the current which decays with time.
The moment when fault occurs maximum fault current is observed and decays with respect to time. When fault occurs relay senses the fault and gives trip command to circuit breaker, this process will take few cycles of operation. Hence the actual fault current interrupted by the circuit breaker is less than the initial value of the short circuit current.
Let at the instant of separation of contacts.
AC component of short circuit current, Iac = x
DC component of short circuit current, Idc = y
Symmetrical breaking current = RMS value of ac component of short circuit current at the instant of separation of the contacts.
Symmetrical Breaking current = x/(2)1/2
Assymetrical breaking current = RMS value of the combined sums of ac and dc components
Generally Breaking Capcity in MVA is given as (3)1/2 x rated voltage in kV x rated breaking current in kA.
The breaking current can be also defined in terms of kA for specified conditions of TRV:
Symmetrical breaking current capacity:
It is the value of symmetrical breaking current of the circuit breaker which the circuit breaker is capable of breaking at a stated recovery voltage and a stated reference restriking voltage under prescribed condition
Asymmetrical breaking capacity:
The asymmetrical breaking capacity of the circuit breaker is the value of asymmetrical breaking current which the circuit breaker is capable of breaking at a stated recovery voltage and a stated reference restriking voltage under prescribed condition