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How does a miniature circuit breaker actually work?
Miniature circuit breakers (MCBs) ensure electrical safety in homes, offices, and other buildings as well as for industrial applications by protecting electrical installations against overloads and short circuits. Once a fault is detected, the miniature circuit breaker automatically switches off the electrical circuit to prevent damage to wires and to avoid the risk of fire. Warranting reliability and safety for people and assets, MCBs are equipped with two tripping mechanisms: the delayed thermal tripping mechanism for overload protection and the magnetic tripping mechanism for short circuit protection.

Just imagine you live in an apartment built maybe sometime in the 60s. You’ve just decided to get your annual spring clean over and done within record time tonight. In preparation, you’ve compiled your favorite MP3 playlist on your PC and it’s being streamed in all rooms. You’ve filled the dishwasher and turned it on, the washing machine is full of clothes and you’ve flicked the switch, you’ve pulled the vacuum cleaner out of its corner and plugged it in.

Not long after, you hear a quiet ‘CLICK’ from the corner (or, in the case of older apartments, from out in the hallway). The lights are out, the music is only playing in a couple of rooms or not at all, the washing machine and the dishwasher are off and the vacuum cleaner has fallen silent.

You’re annoyed, and you try to figure out what’s going on. Usually, ‘the cause’ can be located quite quickly: that ‘thing’ in the box on the wall is off. So, you switch it back on, and off you go. Moments later, the circuit breaker once again clicks to cut off the overload. In a pinch, you might even get the bright idea that you might be able to jam the switch in the ON position so that you can just finish vacuuming your apartment.

What happened?
Annoyingly, doing that won’t let you effectively clean your apartment nowadays. Our ABB miniature circuit breaker will, thankfully, protect your wires from both overloads and short-circuits even if the switch has been forced into the ON position, due to what we call a ‘trip-free’ MCB. Even if your finger keeps the switch in the on position, the internal mechanism will trip and ensure safe disconnection of the overloaded circuit.

What exactly does an MCB do?
The short answer: it protects wires from overloads and short-circuits. When we look on the inside of an MCB we can see how that actually works.

http://www.shcetelec.com/circuit-breaker/residual-current-circuit-breaker/