A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by excess current from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. MCBs are designed to safely isolate at full load current. Breaking a DC current is harder than breaking an AC current. When contacts open, an arc is drawn. In AC this is usually extinguished by the zero crossing, but DC arcs are not self-extinguishing. This means they can only be extinguished by cooling or mechanical interruption. DC switchgear – MCBs, switches, starters etc. – has to open faster and further than AC, and may incorporate Air blasts or moving mechanical shields to interrupt the arc.
