How Do Ground Fault Interrupters Work?

Electricity is a powerful and potentially dangerous force if mishandled. While we have utilized electricity in our homes for years, many of the advancements in electrical technology have focused on making electrical energy safer and easier to use with confidence. One such safety advancement is one that you probably have in your home today but may not realize or think about: a ground fault interrupter (or GFI, for short). These safety devices are critical in keeping you and your home safe when using electricity in certain areas of your home, so it is important to understand what these devices do, how they work, and why you need to have them.

What Is a GFI?

The plain and simple answer is that a GFI is a device that provides protection against shock injuries or electrocution. Electricity is constantly looking for the shortest and easiest path to reach the ground, and will flow quickly through any path it can find, even a small one. In the case of a typical home situation, that can be something as simple as someone with wet hands grabbing a live, wet plug—the water on their hands forms a connection for electricity to travel along, creating a fast and low-resistance path to the ground. Unfortunately, this path means traveling through the person, and that can cause serious injury.

Ground faults can be caused by a number of different problems, but the most common ones are either unexpected wiring faults or accidental connections through a conductive material. For example, a ground wire coming loose in your electrical system and contacting a grounded piece of metal will cause a ground fault, and could cause a power surge along the impacted circuit. A GFI is a specialized piece of equipment that is designed to detect these faults and shut off power to protect you and your home.

How Do GFIs Work?

GFIs work on a pretty simple principle, but the way they do it is rather complicated. To put it simply, a GFI is constantly “watching” the amount of current that flows through it. When it sees an unexpected increase in current, it quickly reacts and cuts off the electrical current flowing through that circuit, sort of like what a circuit breaker does. However, a GFI is not like a circuit breaker—circuit breakers will only shut off when the amount of current flowing through it exceeds the amount the breaker is rated for. A GFI will shut off with as little as five to ten milliamps of current difference.

What does that mean in practical terms? It means a properly-functioning GFI can shut off in less than one-tenth of a second, and that can make the difference between an unpleasant but otherwise harmless shock and a potentially fatal electrical accident.

“But wait,” you might ask. “Wouldn’t plugging something into a GFI increase current and cause it to shut off?” Believe it or not, no. The process we described above is incredibly simplified, and the truth is that GFIs are actually much smarter than that. A GFI’s circuitry is only looking out for unexpected blips or aberrations in current flow, such as sudden, large increases caused by a ground fault. Simply plugging a device in and turning it on, even if it is a high-power device, generally creates a much more gradual increase in current levels, and thus the GFI doesn’t shut off. However, as GFIs age, they become less perfect with this detection. You might find that a GFI starts shutting off when you plug something that consumes a lot of energy in, such as a vacuum cleaner or hair dryer, even when nothing is wrong otherwise. You might also find that your GFI doesn’t shut off when you test it anymore. In either case, you should replace it right away.

Types of GFIs.

There are three primary types of GFIs available, two of which are commonly found in homes. The first is the one you are probably most familiar with seeing: an outlet GFI. Outlet GFIs are installed directly in receptacles themselves, and this makes them by far the easiest and most convenient method of protection. They are pretty easy to identify as well: if you have an outlet with two buttons on it, one labeled “reset” and the other labeled “test”, then that outlet is GFI protected. Should your GFI trip, simply unplug everything from that outlet and press the reset button. You should also test your GFI every month to make sure it is still functioning normally.

The second most common type of GFI is a GFI-equipped circuit breaker. This is a specialized type of circuit breaker installed in your electrical panel that also contains a GFI for added safety and protection against ground faults. They are useful for situations where a receptacle-installed GFI might not be possible.