RVelectricity – GFCI clarification


By Mike Sokol

(Warning: This may be complicated.)

Dear Readers,
My column last week about how GFCI circuits operate caused some confusion for several readers, so I’m going to clarify it a bit. Reread that column HERE.

Here’s one reader’s comment: 

Dear Mike,
You wrote, “Note that a GFCI breaker doesn’t really need the green/ground wire at all to function. The GFCI detector circuit only cares about what’s going out of the black wire compared to what’s coming back into the white wire.“

I was always told and read that the GFCI breaker was measuring and detecting the difference to ground as it is measuring the “leakage” to ground which is what trips the breaker. The GFCI is detecting the possibility of a person getting shocked from the leakage of current to ground. I do know you’re an expert but I’ll have to run my own test as to whether a GFCI breaker will function properly without a green/ground connected, but I don’t believe it will function, trip. —Karl

And here’s what I responded:

Karl, sorry, but I know for a fact that the green ground wire in the GFCI isn’t part of the sensing circuit. What happens inside of the GFCI is the neutral and hot wires pass through a balanced current transformer which generates a current/voltage whenever there’s a difference between the outgoing current and the incoming current. If the outgoing current and return currents match, then all is well. But if there’s a difference then the circuit knows that the current is going somewhere it shouldn’t. That’s what activates the trip circuity.

Note that if you review NEC 70 (the National Electrical Code) it allows a GFCI to be used to upgrade a non-grounded home branch circuit with a “grounded” outlet, simply because it doesn’t need the ground wire at all to protect you. If your body becomes a fault path of more than 5 mA of current between the hot wire and anything other than the neutral (wet floor, kitchen sink, etc.) that will unbalance the current transformer circuit enough to trip the GFCI. Now it’s true that the self-test may not function without the ground wire connected (at least on some GFCIs), but if you create an external ground leakage of over 5 mA, then it will trip.

Let me know if you want to set up an experiment for your own testing and I’ll help proctor it so you can present your results in a future article here. —Mike

So let’s be clear about this. One of the things that makes me an expert in this field is that I know a lot of other experts, many of whom design and build the products that I write about. When I have any questions at all about how something actually works, I call one of them up for discussion and peer review. This is perhaps the greatest way to learn since peer review is a two-way street that can lead to all sorts of new discoveries and a much deeper level of understanding than is possible from only reading an engineering textbook. These are the engineers and designers who WRITE the textbooks. So rest assured, when I write about something I’m very careful to be accurate due to the dangers of electricity.

And that’s why I know this is how GFCI outlets work. I’ve looked at the schematics and discussed this many times with GFCI designers and read the code until it’s 100% clear to me. A GFCI-protected outlet doesn’t require a ground wire at all to function. That’s because a GFCI has a sensing circuit that compares outgoing current (the hot wire) to incoming current (the neutral wire), and if there’s more than 5 mA (0.005 amperes) of difference, it trips. This sensing circuit is basically a current transformer that has the hot and neutral wires passing through it, and NOT the ground wire.

As you can see from the diagram, the green/ground wire in a GFCI bypasses the sensing circuitry completely and isn’t part of the measurement. That’s because a GFCI is looking for EXTERNAL leakage to ground, which is what occurs if you’re unwisely standing in a puddle of water and your hand touches something electrified. Once the fault current through your body exceeds 5 mA, then the trigger circuit opens up the electrical contacts inside of the GFCI and the flow of current is stopped, saving you from serious shock or worse.

Hope this helps you all understand GFCI operation a little better.

Let’s play safe out there….



Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.

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8 months ago

Thanks Mike. I always wondered how those work. Makes perfect sense now.

Jeff Arthur
8 months ago

Nice diagram, the gfi outlets are a fickle lot . This at least gives us a map .

8 months ago

Very useful, Mike. Thanks for that. I’m sure the diagram isn’t done by you, but I have two “nits” to pick with it: First – it would be useful to label the neutral conductor as that, not “grounded conductor” as in the diagram. It’s not technically wrong, but it might be confusing. Second – (and I may be wrong here), but I suspect the CT circuit only measures currents thru the hot and ground wires, not including the “test” circuit wire as shown on the drawing. Current in the test circuit is what unbalances the hot & neutral and trips the device. Happy to hear if I’m mis-reading this.

8 months ago
Reply to  Don

There are two ways to unbalance the hot and neutral current through the coil. The one shown in this circuit diagram has both legs of the test circuit connected on the source side of the sense coil and runs the test circuit through the coil injecting a small “extra” amount of current in the neutral side of the circuit. The test current travels from the hot side of the source through the coil and back to the neutral side of the source. All three wires have to go through the coil.

It could be wired with one leg of the test circuit (e.g. the hot side) connected on the load side of the sense coil and the other leg of the test circuit connected to the neutral side on the source side of the coil with no test circuit wire through the coil. This would cause a small part of the hot side current through the coil to bypass the coil on its way back to the neutral side of the source causing the neutral current through the coil to be slightly less then the hot side current. The test current travels from hot side of the load (after it has gone through the sense coil) back to the neutral side of the source without going back through the sense coil. Only the two main circuit wires go through the coil. This would more closely simulate a leak to ground on the load side of the coil.

Either way works. It may be that the code specifies which is “official.”