||In simple terms an RCD looks for a leakage of current from the live wires into the Earth. The earth is attached to all metal work in the property see list below:
Fridge Freezers, Washing Machines, Dishwashers, Hobs, Ovens, Plumbing Pipe work and so on.
RCD's within a domestic environment should be no greater than 30mA (milli Amps). So it looks for a very small amount of Current and if this goes over 30mA it will trip.