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 breaking 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 component dies away
rapidly.
Breaking
capacity is expressed in MVA by taking into account the rated breaking
current and rated service voltage. Thus if I is the rated breaking
current in amperes and V is rated service line voltage in volts, then
for a three phase circuit breaking capacity = √ 3 * V * I * 10 –6 MVA.
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