Equipment Leakage, commonly referred to as earth leakage or ground fault, is an unintentional electrical connection between an energized conductor and "ground". Ground is an arbitrarily decided point whose voltage is taken to zero, in many situations, this point is the actual ground or earth.
A Ground Fault Circuit Interrupter, or GFCI, is a device whose function is to interrupt the electric circuit to the load when a fault current to ground exceeds a predetermined value, which Carling refers to as the Equipment Leakage Trip Current. This value is less than that required to operate the overcurrent protective device of the supply circuit.
There are two basic types of ground fault interrupters. The first type is designed for people protection, which means that it must cause the circuit to open at very low levels of fault current (about 6 milliamps). The interrupter senses any unbalance in the circuit which would be a leakage to ground on one of the circuit conductors, but not on the other. After the leakage is detected, an interconnected overcurrent device trips open the faulted circuit.
The second type of ground fault interrupter is designed for equipment protection. The equipment leakage circuit interrupter is similar to the first type except that the minimum trip level is set above the 6 milliamps value.