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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