a) MCB also has other parameter - short circuit current. If you will pay attention, you will find the numbers on MCB: 6000, 10000, 12000... - this is short circuit current. This current depends on your electricity supply system: transformer, power cables... Basically at living houses MCB with 6000A short circuit current is enough. If you will use 6000 MCB in the system, where short circuit current is ~10000, it is possible the MCB will not trip - the contacts will be welded and something will go on fire. b) You cannot use standard (AC) MCB for DC voltage system: the electric arc is more durable to disruption in direct current because the voltage is constant. So for DC voltage you must use DC MCB. Also you can find universal MCBs: you can use them in AC or in DC system.
