Question

In my bathroom there is a weird plug that has a "Test" and a "Reset" button. When I push the "Test" button it cuts off the current to the outlet. What is this? How is it different from the fuse in the fuse box?

Answer

That outlet is called a GFCI, or a Ground Fault Circuit Interrupter. It's there to protect people from electrical shock, so it is completely different from a fuse.

Last week one of the questions was, "Why do some appliances have 2 prongs on their outlets and some have 3?", and it talked about fuses. The idea behind a fuse is to protect a house from an electrical fire. If the hot wire were to accidentally touch the neutral wire for some reason (say because a mouse chews through the insulation, or someone drives a nail through the wire while hanging a picture, or because the vacuum cleaner sucks up an outlet cord and cuts it, or because…), an incredible amount of current will flow through the circuit and start heating it up like one of the coils in a toaster. The fuse heats up faster than the wire and burns out before the wire can start a fire.

A GFCI is much more subtle. When you look at a normal 120 volt outlet in the U.S., there are 2 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, and if there is any imbalance then 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 1/30th of a second.

So let's say you are outside with your power drill and it is raining. You are standing on the ground, and since the drill is wet there is a path from the hot wire inside the drill through you to ground (see How the Power System Works for details on grounding). If electricity flows from hot to ground through you, you will potentially die. The GFCI can sense the current flowing through you because not all of the current is flowing from Hot to Neutral as it expects - some of it starts flowing through you to Ground. As soon as the GFCI senses that, it trips the circuit and cuts off the electricity.

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