How Ground Fault Protection Works

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What You'll Need
GFCI Receptacle Tester
Night light or lamp
What You'll Need
GFCI Receptacle Tester
Night light or lamp

A GFCI is a ground fault circuit interrupter, an inexpensive electrical device that, if installed in household branch circuits, could prevent more than two-thirds of the approximately 300 electrocutions still occurring each year in and around the home. Installation of the device could also prevent thousands of burn and electric shock injuries each year.

The GFCI is designed to protect people from severe or fatal electric shocks. Because a GFCI detects ground faults, it can also prevent some electrical fires and reduce the severity of others by interrupting the flow of electric current. Read on to learn more about this device and how it functions.

The Problem

Have you ever experienced an electric shock? If you have, the shock probably happened because your hand or some other part of your body contacted a source of electrical current and your body provided a path for the electrical current to go to the ground.

An unintentional electric path between a source of current and a grounded surface is referred to as a "ground fault." Ground faults occur when current is leaking somewhere; in effect, electricity is escaping to the ground. How it leaks is very important. If your body provides a path to the ground for this leakage, you could be electrocuted.

There are a number of examples of accidents that underscore this hazard. For example, two children, ages five and six, were electrocuted in Texas when a plugged-in hair dryer fell into the tub in which they were bathing. Also, a three-year-old Kansas girl was electrocuted when she touched a faulty countertop. These two electrocutions occurred because the electrical current escaping from the appliance traveled through the victim to ground (in these cases, the grounded plumbing fixtures). Had a GFCI been installed, these deaths would probably have been prevented because a GFCI would have sensed the current flowing to ground and would have switched off the power before the electrocution occurred.

How the GFCI Works

In the home's wiring system, the GFCI constantly monitors electricity flowing in a circuit, to sense any loss of current. If the current flowing through the circuit differs by a small amount from that returning, the GFCI quickly switches off power to that circuit. The GFCI interrupts power faster than a blink of an eye to prevent any lethal dose of electricity. You may receive a painful shock, but you should not receive serious injury.

Here's how it may work in your house. Suppose a bare wire inside an appliance touches the metal case. The case is then charged with electricity. If you touch the appliance with one hand while the other hand is touching a grounded metal object, like a water faucet, you will receive a shock. If the appliance is plugged into an outlet protected by a GFCI, the power will be shut off before a fatal shock would occur.

Availability of GFCIs

Three common types of ground fault circuit interrupters are available for home use.

Receptacle Type

This type of GFCI is used in place of the standard duplex receptacle found throughout the house. It fits into the standard outlet box and protects you against ground faults whenever an electrical product is plugged into the outlet. Most receptacle-type GFCls can be installed so that they also protect other electrical outlets further "downstream" in the branch circuit.

Circuit Breaker Type

In homes equipped with circuit breakers rather than fuses, a circuit breaker GFCI may be installed in a panel box to give protection to selected circuits. The circuit breaker GFCI serves a dual purpose. Not only will it shut off electricity in the event of a ground fault, but it will also trip when a short circuit or an overload occurs. Protection covers the wiring and each outlet, lighting fixture, heater, etc. served by the branch circuit protected by the GFCI in the panel box. The downside is that they're over double the price of a regular breaker of the same capacity.

Portable Type

Where permanent GFCls are not practical, portable GFCls may be used. One type contains the GFCI circuitry in a plastic enclosure with plug blades in the back and receptacle slots in the front. It can be plugged into a receptacle; then, the electrical product is plugged into the GFCI. Another portable type is an extension cord combined with a GFCI. It adds flexibility in using receptacles that are not protected.

Where GFCIs Should Be Considered

In homes built to comply with the National Electrical Code (the Code), GFCI protection is required for most outdoor receptacles (since 1973), bathroom receptacle circuits (since 1975), garage wall outlets (since 1978), kitchen receptacles (since 1987), and all receptacles in crawl spaces and unfinished basements (since 1990). Owners of homes that do not have GFCls installed in areas specified in the latest version of the Code should consider having them installed.

For broad protection, GFCI circuit breakers may be added in many panels of older homes to replace ordinary circuit breakers. For homes protected by fuses, you are limited to receptacle or portable-type GFCIs, and these may be installed in areas of greatest exposure such as the bathroom, kitchen, basement, garage, and outdoor circuits.

A GFCI should be used whenever operating electrically powered garden equipment (mower, hedge trimmer, edger, etc.). Consumers can obtain similar protection by using GFCIs with electric tools (drills, saws, sanders, etc.) for DIY work in and around the house.

Installing GFCIs

Circuit breaker and receptacle-type GFCIs may be installed in your home by a qualified electrician. Receptacle-type GFCIs may also be installed by knowledgeable consumers familiar with electrical wiring practices who follow the instructions accompanying the device. When in doubt about the proper procedure, contact a qualified electrician. Do not attempt to install it yourself.

The portable GFCI requires no special knowledge or equipment to install.

Testing the GFCIs

All GFCIs should be tested once a month to make sure they are working properly and are protecting you from fatal shock. They should also be tested right after installation to make sure they are working properly and protecting the circuit. Professional electricians will have a special device called GFCI Receptacle Tester for this initial test. They can be purchased at any hardware store for under $20.

Another method to test the receptacle GFCI, first, plug a night light or lamp into the outlet. The light should be on. Then, press the "TEST" button on the GFCI. The GFCI's "RESET" button should pop out, and the light should go out.

If the reset button pops out but the light does not go out, the GFCI has been improperly wired. Contact an electrician to correct the wiring errors. If the reset button does not pop out, the GFCI is defective and should be replaced, also by a professional so you're certain to have satisfactory results.

If the GFCI is functioning properly and the lamp goes out, press the reset button to restore power to the outlet.

In the event you have installed your own GFCI, this test method will not always be adequate for testing whether it has been installed correctly. You should always have a professional conduct the initial test to keep yourself safe and to make sure it's working.