We’ll explain it, but first, we need to explain the role of the fuse or circuit breaker. Fuses and circuit breakers are designed to protect a house against an electrical fire. Without a fuse, a fire could start if the neutral wire of an electrical system came into contact with the hot wire. This can happen when a mouse chews through the insulation when someone drives a nail through the wire while hanging a picture, or a vacuum cleaner sucks up an outlet cord and cuts it. When the neutral and hot wire touch, it heats up like the coils in a toaster. The fuse or circuit breaker is designed to heat up faster than the wire and will interrupt the electrical flow through the wire to prevent the fire.
A GFCI outlet is much more subtle. When you look at 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. The left slot is called “neutral,” the right slot is called “hot” and the hole below them is called “ground.” If an appliance is working properly, all electricity that the appliance uses will flow from hot to neutral. A GFCI monitors the amount of current flowing from hot to neutral. If there is any imbalance, it trips the circuit. It is able to sense a mismatch as small as 4 or 5 milliamps, and it can react as quickly as one-thirtieth of a second.