RCBOs (residual current circuit breakers, in the UK), also called GFCIs (ground fault circuit interupters, in the USA) monitor the difference in current flowing into and returning from an electrical load. If any difference is detected (because a fault condition has caused some current to flow to earth) the device will trip within 20 - 40ms.

A conventional mains circuit breaker operates like a fuse, and will trip when the current exceeds the rated value, and like a fuse the breaker will trip when the excess current is many, many times the rated current. Thus it is considerably slower to trip than an RCBO. However unlike fuses, circuit breakers can be reset.