Final answer:
A GFCI works based on electromagnetic induction to compare the currents in the live/hot and neutral wires. If the currents are not equal, it means that there is a leakage current, which can present a hazard. A GFCI is set to interrupt the circuit if the leakage current exceeds 5 mA.
Step-by-step explanation:
A ground fault circuit interrupter (GFCI) is a safety device found in updated kitchen and bathroom wiring that works based on electromagnetic induction. GFCIs compare the currents in the live/hot and neutral wires. When live/hot and neutral currents are not equal, it is almost always because current in the neutral is less than in the live/hot wire.
Then some of the current, called a leakage current, is returning to the voltage source by a path other than through the neutral wire. It is assumed that this path presents a hazard. GFCIs are usually set to interrupt the circuit if the leakage current is greater than 5 mA, the accepted maximum harmless shock. Even if the leakage current goes safely to ground through an intact ground wire, the GFCI will trip, forcing repair of the leakage.