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How Do GFCI Outlets Work & What Causes A GFCI to Trip?

4/2/2018

17 Comments

 
Customers often ask our CWC Electric, LLC electricians how GFCI outlets work and what causes them to “trip” or stop working. Troubleshooting non-functioning GFCI outlets are a common electrical service call which can be expensive and frustrating to homeowners.

GFCI stands for Ground Fault Circuit Interrupters and it was designed by Charles Dalziel in 1961. Mr. Dalziel was a professor of electrical engineering at UC Berkeley and wrote The Effects of Electric Shock on Man, a treatise which explained the effects of varying amounts of electricity on humans. Mr. Dalziel apparently knew a thing or two about how much electricity it took to kill a person and developed a device designed to protect human lives. In contrast, circuit breakers were designed to protect equipment and buildings and operate completely differently.

Here’s a mini-lesson on electricity:
​
All electricity attempts to return to its original source. In other words, when an electrician refers to a circuit, he is talking about electricity leaving your panel, flowing through the insulated wires in your house to supply power to any energized items and then following a return path back to the panel. Circuit simply refers to a roughly circular route that starts and finishes at the same place…in this case, your electrical panel.

A GFCI outlet measures the power coming in and then returning to your panel as it completes the circuit. It was designed to detect any difference more than 4 or 5 milliamps and to react quickly (less than one-tenth of a second) to shut down any circuit out of this small range.

For reference, there are 1,000 milliamps in an amp. Any current over 1 milliamp can be felt, 5 milliamps will produce a painful shock, currents at 10-15  milliamps can paralyze or freeze muscles (making it impossible to release an energized object such as a tool, appliance or wire)  and currents as low as 50-100 milliamps can be fatal. If a person is accidentally exposed to electricity, it is very important to stop the flow of electricity into the body as fast as possible. Each year there are an average of 400 home related electrocutions, about 60 which are associated with consumer appliances.

When a GFCI “trips” it is working as designed. If the GFCI’s internal current transformer senses more than a 4-5 milliamp loss, it instantly shuts down the outlet and any outlets it feeds to prevent accidental electrocution. Most often, when a GFCI “trips” it is the result of a faulty appliance plugged into the outlet or an outlet down circuit. Before calling an electrician, you may want to try this simple diagnosis: Unplug all appliances plugged into or down circuit from the GFCI, reset your GFCI by pushing the button in the center of the outlet and then plug the appliances back into the GFCI one at a time and turn them on to see if any of the appliances are causing the appliance to “trip”. If you plug in your coffee maker and the GFCI immediately trips, it is likely you have a faulty appliance and the GFCI is operating as designed.

All man-made objects have a lifespan and eventually wear out. It is recommended that all GFCI’s should be tested monthly to ensure they are operating correctly. Simply push the TEST button to turn power off to the circuit which should cause the RESET button to pop up. You will need to hit RESET to turn the GFCI back on. Do not assume that because an outlet is working that the GFCI protection is in effect. Newer GFCI outlets have an interlock to prevent the outlet from working if the GFCI protection fails however older GFCI outlets do not have this built-in failsafe.

GFCI outlets should be installed in any area where moisture can be potentially present, examples include kitchen countertop outlets, bathroom outlets, all exterior outlets and garages. Your electrician may suggest additional areas where GFCI protection is recommended or required by the National Electrical Code.


​AT CWC Electric, LLC we generally use Leviton, Pass and Seymour or Hubbell brand products. When installing GFCI outlets you should always use a name branded product to ensure safety. 
17 Comments
Sarah Smith link
1/20/2020 03:43:04 pm

Thanks for explaining that a GFCI outlet will measure the power that circulates in your outlet. My husband is trying to figure out why the bathroom outlets aren't working. I will suggest that we find a GFCI or other electrical measuring tool.

Reply
Joe from Alabama
8/4/2020 07:07:11 am

This is an excellent article. Thanks

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Kitty
8/23/2020 05:54:55 am

This is a wonderful article that helped me a lot thank you!

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James Thompson
11/20/2020 08:50:28 pm

Great overview.

Reply
jerry lux
12/17/2020 02:13:40 pm

Since the ground and neutral end at the same place in the panel, is it correct to say that the GFCI.s measure the current flow between the neutral and hot wires, or does the GFCI measure the flow in the ground wire, to detect a problem?

Reply
Thomas
11/5/2021 10:42:38 pm

GFCI measures the difference between the ungrounded conductor (the hot) and the grounded conductor (the neutral). It does not directly measure the grounding conductor, also known as the equipment grounding conductor also known as ground. It basically is looking for current traveling on an unintended path. Electricians/engineers design things so that the current flows along the hot and neutral. This is the intended path. If there is current 'leakage', some or all of the current won't be making it back on the neutral. This will be seen as a drop and cause the GFCI to trip. If some or all the current flows back on the ground, GFCI does not know and does not care. It will trip.

Reply
David
12/30/2020 03:10:52 pm

Very informative and slightly helpful to my situation. I hope you may be able to lend some insight into what recently happened at my house. We had an enormous wind storm in NJ over an evening in December. It was at night. When I woke up the next morning all of my GFCI outlets that had an appliance plugged into them had tripped. They were on different circuits in the house. The ones that didn't trip didn't have anything plugged into them. What are your thoughts on how this could have happened? I reset all of them and everything worked like nothing had happened.

Thanks,
David

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Joe
5/16/2021 08:11:46 pm

When a GFCI trips is there still full current flowing between the ground wire and the hot (Black) wire??

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jl
8/9/2021 09:21:24 pm

The tripping of the GFIC stops all flow of current.

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Thomas
11/5/2021 10:44:58 pm

The intended, or desired, flow of current is on the hot and neutral. If current is flowing on the ground, there is an issue that needs to be resolved.

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Happy Wife!
8/9/2021 07:51:55 pm

Thank you for this article! After a few frustrated minutes of my husband running around the house, trying to figure out why the GFCI kept tripping, I read your article aloud. Two seconds later I hear, "aha!" Thank you, thank you, thank you!

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Sally Billings
10/16/2021 09:10:42 pm

an outside outlet tripped both bathroom outlets on same wall in my 1997 mfg, home. GFI stays on but I hear a brief squeaking noise periodically inside house vicinity of the outlet on outside wall. The outside cord that was wet at end is unplugged. Any danger?

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Eric
11/28/2021 12:02:32 pm

So, because electricity is not my thing. We put up Christmas lights and had them hooked up to outside timers and plugged into our outside outlet by our front door, when they kicked on, the GFCI outlet in our garage tripped. If everything is plugged in and turned on as we go, the lights will stay on. It only trips when all the lights come on at the same time since they’re on a timer. My question is, if I replace the outside outlet would this fix my issue? The cover on the outside outlet isn’t the best and it could have gotten moisture inside it.

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Bill
12/30/2021 06:21:55 pm

I have a sump pump with battery backup that was installed on a gfi. When the power goes out, the battery back up comes on and works fine. However, when the power comes on it trips the gfi, is this normal? Doesn’t make sense to me because if we’re out of town and gfi trips the power won’t be on to run the sump pump or to recharge the battery backup. Please help! Thank you!

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Larry
2/2/2022 07:31:14 pm

I have a pool light that is protected by a GFCI outlet. There is another outlet that is powered by another hot (black) wire. The common (white) wire from the second outlet is attached to the common side of the GFCI outlet. Apparently, someone accidentally "tested" the GFCI outlet.This in turn, caused the second outlet to stop working. Q1 When a GFCI is tripped, does in interrupt both the hot and common circuits? Q2 Does this configuration meet code? Thanks!

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Jessica F
3/15/2022 10:54:52 pm

We have a GFCI outlet/light switch in our bathroom on the north east end of our house that trips when we turn our bedroom (located at the southwest end) ceiling fan off. The ceiling fan is on a dimmer switch that controls the speed. We’ve has multiple electricians come look at it, they replace the switch/outlet in the bathroom & call it good, but can’t explain why this is happening. these are not on the same breaker, and I believe are on opposite sides in the breaker box. Any insight?

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MckimmeCue link
4/14/2022 11:37:27 am

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    Natalie Hogan, Office Manager and Co-Owner, CWC Electric, LLC

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