An MCB (Miniature Circuit Breaker) is the switch in your distribution board that automatically cuts power to a circuit when it detects an overload or short circuit. It protects your wiring from overheating and prevents many electrical fires — and unlike an old fuse, you simply switch it back on after fixing the fault.
Each circuit (lights, sockets, AC, geyser) usually has its own MCB rated to match the wire size and current it protects. That pairing matters: an MCB that is too large for the wire won't trip in time. This is one reason cable sizing should be done properly — see how to choose the right cable.
MCBs are protective devices, not a substitute for good wiring. Pair correctly-rated MCBs with quality house wires, and have the board set up by a licensed electrician.