You’ve seen them around your house, usually in your bathroom and kitchen or outside.

They’re electrical outlets with a “test” and “reset” button on them. Maybe you’ve had to press the “reset” button every now and then on one when it seemed to randomly stop working.

Those outlets with the buttons are called GFCI outlets. GFCI stands for “ground fault circuit interrupter.”

GFCI outlets serve a vital function: protecting you from dying from an electrical shock.

If you’ve ever wondered how those “outlets with the buttons on them” work, read on. Below you’ll find the primer on the GFCI outlet you didn’t know you needed.

Understanding Ground Faults

To understand how a GFCI outlet works, it helps to have a rudimentary understanding of how an outlet works in general. So let’s start off with a very elementary outline of how electrical outlets function. (Note: the outlets we’ll be focusing on here are the outlets you’ll find in North America. Other countries have other setups).

Take a look at an outlet. You’re probably very familiar with its configuration of two vertical slots and a hole. But do you know anything about these openings? Let’s zoom in a bit more:

Hot wire slot. The slot on the right is shorter than the slot on the left. The right slot is connected to the “hot wire.” This is where your electricity leaves the outlet to power your devices.

Neutral wire slot. The long slot on the left is connected to the neutral wire. Electricity leaves the hot wire slot, travels to power your device, and then travels back to the neutral wire slot, thus making an electrical circuit.

Ground wire hole. The hole on the bottom goes to your ground wire. Usually, there’s no electricity flowing through the ground wire. Electricity will only flow through the ground wire if there’s a breakdown in how the hot and neutral wires typically carry the electrical current.

In a normal situation, when you plug an electric razor into your outlet near the sink, 120 volts of electricity leave the hot wire to power your device. A neutral wire from your razor takes 120 volts of electric current back to the neutral wire slot on your outlet.

A nice electrical circuit is formed, powering your razor and keeping you safe from electric shock.

But what happens if you drop your electric razor into a sink full of water?

Well, water is a fantastic conductor of electricity thanks to the ions in it from dissolved minerals and salts.

Instead of the electricity flowing back through the neutral wire to the neutral slot on your outlet, the electricity will flow through the water until it gets to the ground.

When the current of electricity strays from a planned path to the ground, it’s called a ground fault.

You can probably see why ground faults could be potentially dangerous. If you were to pick up that electric razor from the sink full of water, you’re picking up a device that no longer has a safe, complete circuit between hot and neutral. The water is now carrying an electrical current. You grab that thing out of the sink, and — POW! — you’re going to get a shock that will send you to kingdom come.

How GFCIs Work

GFCI outlets can detect ground faults and quickly shut off the power from the outlet, potentially saving you from getting electrocuted.

GFCI outlets constantly monitor the electricity leaving the hot wire slot and returning back through the neutral wire slot. In a properly functioning outlet, 120 volts should leave the hot wire, and 120 volts should return to the neutral wire.

If the GFCI outlet senses a mismatch as small as 4 to 5 milliamps between the hot and neutral wire, it interprets this as a potential ground fault, and the GFCI outlet will shut the power off from the outlet immediately (we’re talking in as little as 1/30th of a second).

By shutting off the power