Miniature Circuit Breaker (MCB 3) :
These are miniature, moulded case ac air break circuit breakers intended for protecting industrial commercial and domestic installations as per IS 8828-1996
Such circuit breakers are available having voltage rating up to 240/415 V rms and current ratings up to 300 A associated with breaking capacities up to 2400 A at 035 power factor
At first sight these breaking capacities might appear small, but as in the case of moulded case circuit breakers, the possible short circuit current is limited by the impedances on the source side of the breaker.
If the source consists of direct connection to the low voltage side of a small transformer, the short circuit current is likely to be in this range, though the power factor might too severe
In addition, the faults in MCB circuits are to a large extent limited by cable resistance. On a single-phase-to-earth fault, 5 meter of 10 mm aluminium cable will limit the short circuit current generally to below 5000 A.
Current limiting technology has resulted in the development of MCB with fault current handling capacity up to 10 kA, high enough for most of the applications.
The fault energy in these MCBs is substantially reduced. Under fault conditions, the contacts are physically pulled apart and electro dynamic forces set up by the fault current help to extinguish arc in less than half a cycle (generally 3-5 ms).
