How Ground Fault Interrupter (GFI) is used as a Safety Device

The function of a ground fault interrupter (GFI) is to disconnect the power if a current leakage to ground is detected. GFI is also termed to as Ground fault circuit interrupter (GFCI).

We have two types of ground fault interrupters: those designed for life protection and those designed for equipment protection.

GFI used for life protection is employed to prevent shock hazard, specifically in bathrooms and around water. Without a GFI, a person who touches a hot wire while standing in the water or on wet floor will get shocked because his/her body provides an electrical path to the ground. The GFI (GFCI) circuit opens a circuit breaker quickly if it senses that more current is flowing in the hot wire than in the neutral wire, indicating that the current has found an alternate path to the ground i.e. through a person’s body. The figure 1(a) below illustrates how a GFI is used for life protection.

You can notice in the figure above, the power line conductors pass through toroidal coil. If this coil senses any (current) unbalance, a solid-state circuit directs the circuit breaker to open.

Related: Single-phase and Three-phase Power Systems

GFI used for equipment protection is employed to prevent equipment damage and the possibility of fire due to the internal arcing. Under normal circumstances, a current should not flow in the safety ground wire. For instance, in a single-phase motor, the motor current should flow only through the hot and neutral wires. Only if there is a short circuit or fault between the hot wire and the motor chassis, will there be a current in the ground wire (note, ground wire is usually connected to the chassis or metal frame). The fault might not draw enough current to trigger the overload device but it could be a fire hazard. The GFI monitors the current in the ground wire and opens a circuit breaker if the current exceeds the preset level.