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How a GFCI operates and why it requires no ground.

Magnetic field of wire with current flowing into page.

Magnetic field around a wire with the conventional current flowing into the page.

Way back, a long time ago, I started introducing the idea of inductance in a post called “Through the Looking Glass – Duality – The Inductor“. To start talking about inductance it is necessary to talk about electromagnetism and that is where I used the picture to the left. As current flows through a wire it creates a magnetic field around the wire. The strength of the field depends upon the amount of current and the direction of the field, clockwise or anti-clockwise, depends upon the direction of the current flow.

Now imagine a toroid, an iron based metallic doughnut, with the wire running through the center of the toroid.   The iron would concentrate the magnetic field within it, because iron conducts magnetism better than free air conducts magnetic fields.  Now imagine that we have several turns of wire wrapped around the toroid.  If the current in the wire is alternating current, we have formed a transformer with a one turn primary, the main conductor, and several turns on the secondary.  That is the basis for a device called a Current Transformer, (CT).   CT’s are commonly used in the power industry and industry in general to measure the amount of current when large currents are involved.  It is much easier to handle instrument wiring in the single digit Ampere range (usually 5 A for CT’s than it is to use wiring designed for 100’s or even 1000’s of Amperes.

Now, to get back on subject, 20A 120V house wiring,  Imagine that both the outgoing (hot) and incoming (neutral) wire were routed through the center of the toroid.  If both have exactly the same current flowing but in opposite directions, the two magnetic fields would cancel and there would be no magnetic field flowing through the toroid.  That is exactly how a GFCI operates.  It measures the difference between the current in the hot wire and the neutral wire.   If more current is going out the hot wire than is coming back through the neutral wire then the current must be going to ground through some other path.  The secondary windings on the toroid are connected to an electromagnet that develops enough pull to open a switch and interrupts the circuit.  GFCI is simply an acronym for Ground Fault Circuit Interrupter.   A GFCI will trip if the current difference is greater than 4 to 6 mA.  A GFCI will protect a person from a fatal shock if current is flowing through the person to ground.  IT WILL NOT PROTECT if the person contacts both the hot and neutral wires because they are simply part of the circuit in that case.   Luckily, most shocks occur from the hot wire to a person grounded so the GFCI provides protection.

GFCI’s come in two types.  the one most people are familiar with is a GFCI receptacle, but it is possible to buy a GFCI circuit breaker.   The receptacle type is used most often because it is less expensive, it can be reset without going to the main circuit breaker panel, and it can protect just part of the circuit.   The GFCI receptacle has two sets of terminals.   One set is marked “Line” and the other set is marked “Load”.   The receptacle can not only provide protection to things plugged into it but it can also provide protection to other devices on the circuit by extending the circuit through the “Load” terminals.  This can be a good and a bad thing.   It saves money by using standard receptacles instead of the more expensive GFCI receptacles.  The problem is if the GFCI trips and now you have dead devices, it may take some time to find the tripped GFCI.   The second problem is then sorting out what is actually causing the GFCI to trip.   For that reason when I wired my son’s house, I installed several GFCI’s on his back porch all on one common circuit, but fed each one off the “Line” terminals of the previous receptacle.   At that point the addional cost was nothing to the total cost of the house.

You will notice in the discussion of the how the GFCI operates no mention was made of the ground circuit.  That is because it is not necessary for the operation of a GFCI.   As a matter of fact.  current code allows replacement of the old 2 wire receptacles with 3 wire receptacles and without a ground if the following two requirements are met.  First, the receptacles have to be marked  “No Equipment Ground” and be GFCI Protected.
In general GFCI’ receptacles must be installed near sinks and other sources of water,  for outdoor outlets, and for garages and other places with bare concrete.   Please Note:  I did say “in general”… refer to the actual NEC for the specifics.

My personal war story about GFCI’s is my garage did not have one in it.  It was a very hot July or August day and I was working underneath a car.  I had short legged pants on and was sweating profusely.  My wet hand was on the concrete floor of the garage as I was crawling on the floor and my knee came in contact with a bare wire on a fan.   I did not think I was going to get off of that.   Needless to say, my garage now has a GFCI in it, and the fan wire was quickly replaced.  While roaming the net, I found this excellent document by NEMA about a GFCI.

A three wire plug.

A three wire plug.

One last thing I want to point out about the plugs used to plug into the receptacles.   First you will notice the ground pin on the three wire plug is longer than the other two connectors.   This is because the ground is connected before the other two wires when plugging in the plug.   On the two wire polarized plug the neutral wire isconnected to the wide connector.

A Polarized 2 wire plug.

A Polarized 2 wire plug.






Now a war story to kind of summarize everything about grounding.  Recently I built a hydroponic system to grow some vegetables   Part of that system includes a pump to pump nutrients to the plants and I needed a timer to control when the pump would operate.   All of the timers I could find were two wire timers, but the pump is a 3 wire pump and the manufacture had explicit instructions that the pump must be grounded.   I had a 3 wire extension cord to connect from the outdoor GFCI to the location of the pump, but somehow I needed to get from 3 wire to 2 wire and back to 3 wire while maintaining a ground.   The way I did it was to use a standard receptacle and mount it in an outlet box.  I used a short 3 wire plug and used it to feed the receptacle.  I then cut the jumper between the two sets of terminals on the receptacle.  This made the top half of the receptacle powered, but the bottom half was not. but since the ground connection feeds both halves both were adequately grounded.   Next I wired a 2 wire polarized plug to the bottom half of the receptacle making sure to connect the correct wire, the one connected to the wide prong, to the neutral terminal.  I plugged in the timer to the top receptacle and the 2 wire plug was plugged into the timer.   The very small pump was connected to the bottom receptacle.

The Work-Around I forgot to talk about.

I promised to talk about a common problem and a work around I do to avoid it.   In industrial systems, everything is grounded together using the very large copper wire so lightening strikes are normally not a problem.   But on residential and commercial buildings things are often not so lucky or clean.   The worst case I can think of is the case of a commercial installation with two separate building and each one had a separate power service.   However they had two fire alarm systems with a common communications between the two buildings.   Shortly after installation, lightening hit close to one building causing a momentary rise in the ground potential at that building and wiped out the the communications circuits and the wiring between the buildings because this was very small diameter wiring.   The work-around?   This was digital communications between the two alarm panels so copper communications wiring was replaced with fiber optics cable so there was no actual circuit between the two buildings.   I think probably very soon I will have a situation where I will have a similar problem to deal with on a home project.

There is a whole lot more that could be talked about in grounding and some people make it their whole careers.   However, I think we have dealt with it enough for just about anything we will do.


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2 comments to How a GFCI operates and why it requires no ground.

  • Hugh

    Why I would NOT use GCFI/RCD in a storage unit that holds a chest/deep freezer: After 2 weeks of not fishing I decided to go out. I went to the chest freezer in the garage and noticed a conspicuous “lack of noise” coming from the freezer. This chest freezer stored all my bait and other odds’n’sods related to fishing (rigged baits and ice buckets etc). Needless to say the GCFI had tripped. I knew what had happened immediately. I knew it was wrong, but I did open the lid – for less than 2 secs. The air was still and warm. It’s amazing how many flies are awoken because upon opening the second time there was a swarm. So, having refrozen the freezer and contents I was left with a most awful smell. Clorox works – to a certain extent. So I changed the GCFI socket to a regular socket (no protection) and the freezer has been on for over 3 years. Moral of the story – never use a GCFI socket and leave it unattended.

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