What is the operation of earth leakage relay?

An earth leakage relay is an electronic or electromechanical device that detects the current flowing to earth (Americans call this ground). If this earth leakage current is above a preset level (and carries on for a preset time) the relay is "tripped" - this may switch on an alarm signal - or more usually it simply disconnects the supply. Currents may flow to earth due to a fault condition or because of lots of individual items connected to the supply have a small "leakage" to earth.