A circuit breaker is required to perform the following three duties: 1. It must be capable of opening the faulty circuit and breaking the fault current. 2. It must be capable of being closed on to a fault 3. Must be capable of carrying fault current for a short time while another breaker is clearing the fault. Depending on the above duties circuit breaker has three ratings braking capacity, making capacity and short time capacity. Breaking capacity It is current that a circuit breaker is capable of breaking at a given recovery voltage under specified conditions. The breaking capacity is always stated at the r.m.s value of fault current at the instant of contact separation. When a fault occurs there is considerable asymmetry in the fault current due to presence of d.c component. The d.c read more

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