GCFI or Ground Fault Circuit Interrupter are also sometimes known as ground fault protected outlets or referred to as GFIs, or Ground Fault Interrupters. A GFCI is a mechanism that offers personal protection against lethal electrical shock or electrocution. It is a special electrical receptacle or outlet that can stop electrical power within milliseconds when an imbalance occurs.
The GCFI outlet is tripped when the current balance between the ungrounded “hot” conductor and the grounded conductor is not the same and that difference exceeds a tolerance which is typically in the range of four to six milliamps. The imbalance indicates that part of the current flowing in the circuit is being diverted such as would be the case if flowing through a human body to ground (the floor or otherwise.)
Class A GFCI outlet mechanisms are used in residential and commercial installations. These devices will trip when current to ground is 6 milliamps or more. They will not trip if the current to ground is less than 4 milliamps if they are functioning to specification. These outlets switch both the phase (hot) and grounded conductors.
These devices do not eliminate all risks of electrical shock. For example if a person touches both circuit conductors at the same time they will still be electrocuted as the imbalance does not exist since the person becomes the balanced load.