A ground-fault circuit interrupter (GFCI) is designed to protect you from an electrical shock. These receptacles are used most often outdoors, in kitchens, bathrooms, and any other area where contact with water may be a factor in an electrical shock.
A GFCI constantly monitors electrical current flowing in a circuit to sense any loss of current. If the current flowing through two circuit conductors differs by a very small amount, the GFCI instantly interrupts the current flow to prevent a lethal amount of electricity from reaching someone.
In a normal 120 volt outlet in the United States, there are two vertical slots and then a round hole centered below them. The left slot is slightly larger than the right slot. The left slot is called the “neutral,” the right slot is called the “hot,” and the round hole is the “ground.” If an appliance is working properly, all electricity will flow from the hot to the neutral. A GFCI monitors the amount of current flowing from the hot to the neutral. If there is any imbalance, it trips the circuit. It is able to sense a small difference as 4 or 5 milliamps, and it can react as quickly as one-thirtieth of a second.
As an example, let’s say you are outside with your electric power drill and it is raining. You are standing on the ground, and since the drill is now wet (remember that water is an excellent conductor of electricity) there is a path from the hot wire inside the drill through you to the ground. If the electricity flows from the hot to the ground through you it could be fatal. The GFCI can sense the current flowing through you because not all of the current is flowing from the hot to the neutral as it expects. As soon as the GFCI senses that, it trips the circuit and cuts off the electricity and prevents a serious accident.
