What is a Ground Fault Circuit Interrupter (GFCI)?

A ground fault circuit interrupter, also called a GFCI or GFI, is an electrical device designed to monitor current flow and upon detecting a ground fault (“leaking” current), to instantaneously open the circuit and stop the flow of electrical current – thus protecting people from a dangerous electrical shock or electrocution. A GFCI is normally an electrical outlet (also called a receptacle) that can be identified by a test and reset button in the center of it. You have likely seen them in your kitchen or bathroom.

In this post, I will give you a lot of information about GFCI’s. I will explain what a GFCI is, how a GFI works, where GFCI-protected outlets are required, and why GFCI’s sometimes do not work. Before you can understand what a ground fault circuit interrupter is or what it does, you need to know what a ground fault is.                      

What is a Ground Fault?

A ground fault is when electricity takes an unintended path (such as through a person) to ground (a place of lower voltage).

This is similar to what lightning does when it strikes. It is discharging a very high voltage and dissipating it to the ground. Just like heat flows toward colder things, and water flows from a higher location to a lower location, electricity wants to flow from higher voltage to a lower voltage.

Electricity is like water in a lot of ways (See our post about voltage, current and resistance.) Let’s look at an example using water.  Think about a tall water tower that you see in most cities and towns. As long as everything is working correctly, the only way that water can leave the large reservoir at the top of that tower is through the large pipe attached to it.  Let’s suppose that the tower is neglected, and that the reservoir develops cracks and starts to leak. Water will begin to flow through every crack in the tank, as well as through the large pipe. In other words, the water in the top of that tower will take all available paths to get out of the reservoir and to get to a lower elevation.

It’s the same thing when you have a roof leak. Most of the rainwater landing on your roof will flow off of the edge of the roof just like it should. However, if there is a hole of any size in your roof, then some amount of water will find that hole and will leak through it.

Electricity does the same thing – it takes all available paths to get to a place of lower voltage (lower energy). Essentially, it flows downhill just like water does.

When an electrical circuit is working properly, the electrical current is contained within the wiring and cannot escape. It flows from the outlet on the hot wire, to the load (TV, lamp, hair dryer, etc.), and then it flows back to the outlet on the neutral wire. The same amount of current that flows out from the outlet comes back to the outlet.

When a fault develops (such as a damaged wire, an electrical component getting wet, or a person touching a live wire), some of the electricity will then have a new path to take in order to get to a place of lower voltage (normally the ground). This unintentional path is a ground fault. It is similar to that leaking water tower or roof, but instead of water leaking, it is electrical current that is leaking.

How Does a GFCI Work?

A ground fault circuit interrupter has a mechanism inside that compares the amount of electrical current flowing from the outlet to the amount of current flowing back to the outlet. On any properly-operating circuit, the amount of current flowing to the load, a hairdryer for example, will exactly match the amount of current flowing back from the load because the only path for the electricity to flow on are the hot and neutral wires.

Let’s look at an example of a water hose because I think it will help me to explain what I am talking about. If you can measure exactly what is flowing out of a faucet and into a water hose, it should exactly match what is flowing out of the other end of the water hose. In the same way, the exact amount of electrical current that flows into the hairdryer should flow out of the hairdryer because there is no other path for it to take.

Now let’s take that water hose and put a small hole in it somewhere. Now, what flows out of the end of the water hose will be slightly less than what is flowing into the water hose because a little bit of that water will be flowing out of the hole in the hose.

It is this type of situation that a ground fault circuit interrupter is designed to deal with. A GFI outlet is constantly comparing the amount of current flowing out of and back into the outlet, and if there is ever even a very slight difference between those two currents, it will immediately shut off the power at that outlet.

How Does a GFCI Protected Outlet Protect Me?

Let’s look a little more at the hairdryer example. Suppose you pick up that hairdryer, but it had gotten wet inside, or you accidentally drop it in a sink full of water. As soon as the water touches the electrical components inside the hairdryer, the water becomes one more path for the electricity to follow. While most of the electricity will continue to flow through the hairdryer’s cord, some of the electricity will “leak out” of the wires by flowing through the water and to your hand that is holding the hairdryer, and finally through your body. Your body is creating a ground fault, an unintended path for the electricity to flow to ground. It is very possible that the amount of current that will flow through your body would be enough to electrocute you.

This is where the GFCI does its thing. The GFCI will immediately detect the difference in current flow on the hot and neutral wires, and within about 1/10 of a second, it will shut off the power to the outlet. It only takes about a 5 mA (milliamps) difference in the current flow for the GFCI to detect the difference. That is 5/1000ths of an amp. For comparison, the hairdryer will probably have 10 to 15 A (10,000 to 15,000 mA) of current flowing through it, and it only takes a difference of about 5 of those 15,000 milliamps for the GFI to be able to detect it. So, as you can see, GFCI’s are very sensitive and can detect very small differences in current flow. They are truly life-saving devices. According to Bob Villa, since GFCI protection was introduced in the 1970s, electrocutions are down by 83%. (https://www.bobvila.com/articles/gfci-outlets/)

Below are some animated representations of the difference between a GFCI-protected outlet and a regular, not GFCI-protected outlet. 

How Much Current Does It Take to Cause Electrocution?

Studies have shown that electrical current in the range of 100 – 200 milliamps will cause electrocution. Between 10 and 100 milliamps, the victim will receive a very painful shock, but will likely survive. Above 200 milliamps, the heart muscles are essentially “clamped” and will not normally go into ventricular fibrillation, so the victim is likely to survive, but may receive severe burns. You can read more about this at https://www.asc.ohio-state.edu/physics/p616/safety/fatal_current.html.

Where are GFCI-Protected Outlets Required?

GFCI protection is required in the following locations:

1. Kitchens-

• a. All outlets serving kitchen countertops
• b. All outlets within six feet of the kitchen sink. An example of this would be an outlet in the living room below the wall where the sink is (See photo for example.) If an item with a six-foot long cord is plugged in and can reach the kitchen sink (regardless of where it is plugged in), then that outlet must be GFCI protected.
• c. The outlet for the dishwasher

2. Bathrooms – All bathroom outlets must be GFCI protected.
3. Wet bar, laundry room, and utility sinks – All outlets within six feet of a sink in a wet bar, laundry room, or utility sink must GFCI protected.
4. Unfinished basement – All outlets in an unfinished basement must be GFCI protected. Exceptions to this rule are that outlets supplying only a permanently-installed fire or burglary alarm system, and outlets that are not readily accessible that supply a sump pump are not required to be GFCI protected.
5. Outside – All outlets on the exterior of the home (attached to the home, out in the yard, etc.) must be GFCI protected. There is one exception to this rule: an outlet that is not readily accessible and is supplied by a dedicated de-icing or snow-melting equipment is not required to be GFCI protected.
6. Garage and accessory buildings – All outlets in garages and accessory buildings that are located at or below ground level must be GFCI protected.
7. Crawlspaces – All outlets in crawlspaces must be GFCI protected. Basically, crawlspaces have the same GFCI requirements as basements.
8. Pools and Spas – GFCI protection is required for the following:

• a. All pool lights and outlets used for lights
• b. Outlets used for pumps
• c. Any outlet within 20 feet of a pool, spa, or fountain
• d. The power supply for a pool cover

What is the Difference Between a GFCI and a Circuit Breaker?

I’ve heard people ask the question, “Aren’t a circuit breaker and a GFCI the same thing?” The answer is no. It is true that they both shut off electrical power to a circuit or to one or more electrical outlets, so in that respect, they are similar. The difference between a circuit breaker and a GFCI is that they stop the flow of electricity for two totally different reasons. As was explained above, a GFCI shuts off power when there is a ground fault, and it does this to prevent someone from getting electrocuted.

A circuit breaker, on the other hand, shuts off power due to too much current flowing on a circuit. (See Circuit breaker post). If too much current is flowing on a wire, then the wire can overheat and cause a fire, so the main purpose of a circuit breaker is to prevent fires caused by overheating wiring.

GFCIs are designed to prevent electrocution by monitoring for and clearing a ground fault, while circuit breakers are designed to prevent fires due to overheating wires by monitoring for and clearing an over-current event.

NOTE: There are GFCI-protected breakers that serve both as overcurrent protection and as protection against a ground fault. A GFCI breaker will protect every outlet on that circuit.

Do I Have to Install GFCI Outlets in the Home that I am Purchasing or Selling?

If you are planning to purchase a home and your home inspection revealed that the home lacks GFCI protection in some areas or throughout the home, then you may or may not HAVE to install GFCI-protected outlets. If you are planning to sell a home, the same answer applies. To clarify, if it is an older home, then the lack of GFCI outlets is “grandfathered in,” meaning that the home was built before GFCI protection was required or perhaps before GFCI protection was required in certain locations in the home. In this circumstance, your local code enforcement officer or AHJ (Authority Having Jurisdiction) will not require you to install them.

It is possible that your lender will require certain repairs or upgrades to be made to the home before they will loan the purchase money for the home, and these repairs may require the installation of GFCI protection in some areas. If you are selling the home, then you may be required by the buyer’s lender to make certain repairs.

Regardless of if it is required or not, it is definitely a good idea whether you are buying or selling a home to ensure that the home is as safe as possible. GFCI protection will certainly make the home safer.

Can One GFCI Outlet Protect Multiple Outlets?

Yes. It is possible to use one GFCI-protected outlet and to wire several standard outlets to the GFCI. If done properly, this one GFCI-protected receptacle will protect all of the other outlets downstream of it. This is commonly done in homes today. For example, one outlet in the kitchen may be a GFCI outlet, and the other outlets near it may be wired to it. In this way, all of the downstream outlets that are tied to it will also be GFCI protected. I’ve also seen standard outlets in one bathroom tied to a GFI-protected outlet in another bathroom. This is fairly common in many areas.

How Long Do GFCI Outlets Last?

Since GFCI outlets are mechanical devices and not just electrical devices, they tend to eventually fail. They should last at least ten years. Some don’t last ten years, but many last much longer than ten years. It is a good idea to test them regularly, and to replace them if they fail to work.

In the past, when a GFCI outlet failed, it failed in the closed position, meaning that it continued to conduct electricity just like a regular outlet. This was not a good situation because people would think that they were protected, but ended up not being protected because the GFI outlet had failed.

Today, it is required that they fail safe, meaning that they fail in the open position so that they don’t continue to conduct electricity. In this way, the homeowner will know that it has failed and will have to replace it with a new, working GFCI outlet. Please be aware that there are likely many of the older type (fail closed) still installed in many homes across the country, so it would be a very good idea to test all of the GFCI outlets in your home to ensure that they are working properly.

Why Will My GFCI Outlet Not Reset?

There can be a couple of reasons for a GFCI outlet not resetting. The first reason is that it is just worn out, defective, or has failed as described above. If this is the case, the GFCI outlet will just need to be replaced.

The second reason is that the outlet may be installed incorrectly. Since the early 2000’s, it has been required that GFCI’s are designed in such a way that if they are mis-wired, they will not be able to be reset. Due to the two sets of screws on the GFCI outlets, many people, especially handymen and homeowners, who were not familiar with GFCI outlets, were connecting the wires to the wrong screws. When this happens, the GFCI behaves just like a standard outlet, and provides no ground fault protection. This is what led to the rule mentioned above being implemented. Remember, the wires providing the power to the outlet must be connected to the screws labeled “Line.”

If the cause is one of these first two reasons, then you will likely have to call an electrician unless you are familiar enough with basic electrical wiring to change out the outlet yourself.

A third possible reason is that there are several GFCI-protected outlets “chained” together. This means that there are several GFCI’s on the same circuit, and there is a wire running from one to the next one and so forth. When they are wired like this and one trips, it is possible that one or more GFCI outlets upstream also tripped. In this case, to reset the tripped GFCI outlet, you will need to check the other GFCI’s upstream of the tripped one and see if any of those are tripped as well.

A fourth possible reason, though not likely, is that the breaker is also tripped. This is not likely, because if the breaker is tripped there will be multiple outlets and possibly lights not working. It is unlikely that the breaker would only control the one GFCI outlet.

Why Will My GFCI Outlet Not Trip?

The mechanism inside GFCI outlets often wear out. I have seen at least one GFCI outlet that does not trip in about half of the older (20 years of more) homes that I inspect. The majority of the non-tripping GFCI outlets that I have encountered have been on the exterior of home. Dirt and moisture get into these outlets and can eventually cause them to no longer trip. The only fix for this is to replace the outlet.

There are also some older GFCI outlets that will not trip but continue sending out power to whatever is plugged into the outlet. The cause of this is normally that they have been mis-wired by connecting the incoming wires to the “load” screws instead of to the “line” screws. I have also seen some outlets that will not lose power when they are tripped because they have been mis-wired in the same way. Newer outlets are designed to not be able to be reset if they have not been wired properly.

How to Keep My GFCI Outlets Working

Pushing the test button and then the reset button on all the GFCI outlets inside and outside of your home three or four times per year can greatly increase their longevity.

Testing them like this periodically will “exercise” the spring mechanism inside and will help to keep it working much longer than if it is ignored and never tested.

Why Does my GFCI Outlet Keep Tripping?

If you have a GFCI that has tripped multiple times for no apparent reason, then it may be a defective outlet. When an outlet trips for no reason, it is called a nuisance trip, and it normally lives up to its name. If you have such an outlet, the most likely fix is simply to replace it.

How to Install a GFCI Outlet

Installing a GFCI outlet is just about as simple as installing a normal electrical outlet, but there is one big difference. There are two sets of screws on a GFCI-protected outlet, and it is ESSENTIAL that the wires be hooked up to the correct set of screws. I ONLY recommend doing this is you are already familiar with electrical safety and have done some electrical work in the past.

It is possible for one GFCI-protected outlet to protect multiple other regular outlets, and this requires a little more knowledge. If the outlet that you are trying to change out has more than one set of wires (one black, one white, and one bare), then I highly recommend that you get a licensed electrician to do the work for you, because when there are multiple sets of wires, there are just too many variables, and I do not want to try to cover them here. Therefore, the only situation that I will cover will be replacing an outlet with just one set of wires.

1. Turn off the power to the outlet that you are replacing.
2. After making sure that the outlet is dead, disconnect the wires from it.
3. Install the GFCI outlet by doing the following:

• a.     When you remove the GFCI outlet from the box, there may be a piece of yellow tape covering one set of screws. Leave the tape on those screws, and do not use the screws that are covered by the tape. These screws are the “Load” screws and are used when the GFCI outlet is supplying power to and protecting other downstream outlets.
• b.     Note that the set of screws that are not covered should be labeled “Line,” meaning that they are where the wires supplying the power to the outlet should be connected. These are the screws where you will be attaching the wires.

• c.     Connect the black wire to the gold- or brass-colored screw, the white wire to the silver screw, and the bare wire to the green screw.

4. Put the outlet into the box in the wall, screw it into place, and install a cover. Note that it will need a different type of cover than is used for standard outlets.
5. Turn the breaker back on and push the “Reset” button on the outlet.
6. Plug something in and make sure it works.
7. Push the test button to make sure the outlet loses power.
8. Reset the outlet again, and you’re done.

Here is more information on GFCI-protected outlets. 

© 2020 Mike Morgan

This article was written by Mike Morgan, the owner of Morgan Inspection Services. Morgan Inspection Services has been providing home, septic and well inspection services throughout the central Texas area since 2002. He can be reached at 325-998-4663 or at mike@morganinspectionservices.com. No article, or portion thereof, may be reproduced or copied without prior written consent of Mike Morgan.