Saturday, December 3, 2022


RVelectricity – No~Shock~Zone Part 10: GFCI testing


By Mike Sokol

For information on how to support my RVelectricity and No~Shock~Zone articles, seminars and videos, please click visit the I Like Mike Campaign.

Dear Readers,
As promised, here’s Part 10 of my original No~Shock~Zone series on RV Electricity. Astute readers will recognize that Part 9 is missing, but that was only a recap of previous articles, so I’ll save it to the end of this redux.

Understanding and Preventing RV Electrical Damage

Copyright Mike Sokol 2010/2019 – All Rights Reserved

If you’ve read the survey we did in July 2010 at, you may remember that 21% of RV owners who responded have been shocked by their vehicle. What follows is #10 in a 12-part series about basic electricity for RV users and how to protect yourself and your family from shocks and possible electrocution. In addition, this series could protect your RV’s appliances, entertainment systems and computers from going up in smoke.

This series of articles is provided as a helpful educational assist in your RV travels, and is not intended to have you circumvent an electrician. The author, and will not be held liable or responsible for any injury resulting from reader error or misuse of the information contained in these articles. If you feel you have a dangerous electrical condition in your RV or at a campground, make sure to contact a qualified, licensed electrician.

GFCI review

If you don’t already know what a GFCI (Ground Fault Circuit Interrupter) circuit breaker is, please read Part 8 on basic theory and operation of this lifesaving device. GFCI devices are among the least understood of all electrical safety circuits, but their function is really pretty simple once you understand their basic operating principles.

It’s all about balance

Here’s the illustration we used in Part 8 to demonstrate what a GFCI is looking for electrically. Note that a perfectly isolated electrical appliance should have exactly the same amount of electrical current going out and coming back in. For example, if an appliance draws 7.000 amperes of current from the black/hot wire, then exactly 7.000 amperes of return current should be coming back in the white wire. However, if there’s any secondary connection to the earth/ground from something like our happy camper poking a piece of metal in a socket while standing on the ground, there will now be more current going out the black wire than is returning from the white wire.

GFCI breakers in America are designed to trip when there’s any more than 5 milliamperes (5/1000 of an amp) of difference between the black and white wires. 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.

Nuisance tripping

What bothers many campers and homeowners about GFCI breakers is that they’ll occasionally trip for no apparent reason. So if you plug your shore power to the GFCI outlet in your garage to run your RV refrigerator while you’re stocking for an extended trip next week, you may come back the following  day to find the power out and your food spoiled. Nobody was in the RV and nothing looks out of place. Or you plug in a power drill to your exterior RV outlet and BAM! – it trips before you can pull the trigger on the drill. Why would that happen when the drill runs just fine in your basement workshop? Those sorts of situations are what make home and RV owners suspicious of GFCIs and want to replace them with a non-protected outlet – which, I might add, is illegal to do.

Less than perfect?

What could cause an appliance or electrical circuit to behave badly and fool a GFCI into tripping? Glad you asked. Every appliance has at least two separate wires connecting it to the power outlet, and many will have a third “green” wire known as the safety ground. The purpose of this ground wire is to drain off any electrical leakage within the appliance itself that might occur from deteriorated insulation, a pinched wire or perhaps a failed component such as a power transformer or light bulb socket with water inside.

This deterioration or component failure often occurs in old electrical appliances. So if you’re plugging in a Fender guitar amp from your teenage years, the amplifier probably has a lot of heat damage to the power transformer from those extended bar jams of “Smoke on the Water.” That overheating is what causes that peculiar “burnt transformer” smell that we also associate with a bad fluorescent light ballast.

An appliance’s or amplifier’s insulation breakdown doesn’t always result in a complete short circuit that would trip a regular 20-amp circuit breaker. It can be like a small leak in a pipe that’s dripping water just a bit. So let’s assume that there’s 10 milliamperes of electrical leakage from the hot/black wire of the power cord to the chassis of your amplifier. That’s way less than the 3 or 4 amperes of current your amp is drawing from the circuit to run the tubes and power the speakers. Therefore, a 20-amp circuit breaker thinks that all is well.

However, plug that same guitar amplifier into a GFCI breaker and it sees there are 4.010 amperes of current going to the black wire and only 4.000 amperes of current coming back from the white wire. Where did those extra 10 milliamperes (0.010 amps) of current go? Well, back through the green wire that ties to the white wire way back at the electrical panel. But since the GFCI doesn’t know or care if that extra 10 mA of current was properly disposed of via the green ground wire or your hand touching the electrified chassis of the amplifier, it trips the breaker in an attempt to save your life. Let’s not call this nuisance tripping, but rather life-saving tripping.

Charting fault combinations

So here are two ways to determine which of your appliances is tripping the GFCI. The first is pretty simple. Just unplug every appliance from its own electrical outlet and begin plugging them back in one at a time and turning them on. You’ll want to cycle the appliance on and off a few times since there can be ground fault current “spikes” when you turn on a microwave or light switch.

But here’s where it gets tricky: ground fault currents are additive. So if you have two appliances that are each leaking 4 mA (milliamperes) of current to ground, turning on either one of them won’t trip your GFCI, but turning on both appliances at the same time will allow their fault currents of 4 mA and 4 mA to add up to 8 mA. And, of course, 8 mA is greater than the 5 mA limit of the GFCI breaker so it trips.

This might require a little detective work, but I usually make a simple chart or spreadsheet of all my appliances and turn them on singly and in combination with every other possible appliance. Charting seems like an unnecessary step, but that’s when you’ll see obvious combinations that cause a problem, like in my chart above.  It shows that when I turned on both the porch light and the microwave, the GFCI would trip. Now I know there’s something electrically leaking to ground in both the porch light and microwave. The porch light might have a water leak in the wall, which is letting moisture into an unsealed electrical box, while the microwave could have a chafed wire from bouncing down the highway for the last few years. Both problems should be corrected since they’ll only get worse, not better, with age.

Measure it

There is, however, an even better method if you’re an RV tech or can borrow or buy a clamp-on ammeter such as the Fluke shown in the picture. To do this properly you’ll need a meter that can display down to 0.001 amperes, which is 1 mA resolution. Clamp-on ammeters have a current transformer that looks for current flowing through the wires placed inside their jaws. However, if you simply clamp them around the entire power cord of an appliance, you’ll be summing the current going out from the black wire with the currents returning from the green and white wires and you won’t know the actual ground leakage current. Because the GFCI ignores the green wire current in its own leakage calculations, we need to do the same thing with our clamp-on ammeter to get the real current levels involved.

You can do this ground fault leakage test by sacrificing a short extension cord to make a test cable. (Don’t you feel like a scientist, now?)

With the extension cord unplugged from everything, just slit off the outer covering, being careful not to nick the insulation of the black, white or green wires. Get rid of any nylon filler and untwist the group of wires until you get something that looks like the picture to the right.

This is perfectly safe to use for testing, but because you’ve removed the outer protective layer of insulation, you’ll need to retire this particular extension cord from your regular hookup inventory. That why I typically do this modification to a short 6-ft. extension cord which I then keep on my test bench.

This modification allows you to plug your appliances one at a time into a non-GFCI outlet using your test cable to see how much current is leaking back to ground.

Clamp the ammeter around the black (hot) and white (neutral) wires as shown in the picture, keeping the green (ground) wire out of the jaws. or alternately, you can clamp the meter around the green ground wire. Either way will yield the same results.

Your ammeter will now be registering how much current is going out the black wire minus how much is coming back in the white wire. So any currents you read on the meter will be the ground leakage that can cause the GFCI to trip from that appliance. Or if you’ve clamped around the green ground wire you’ll be reading the amount of current leaking to the EGC ground wire, which will be the outgoing current minus the incoming current. Pretty neat!

Note that there’s going to be a certain amount of leakage to ground from anything plugged into a wall outlet. So less than 1 mA is not a problem. If you have a high resolution clamp meter like this one ($1,000+) you can read down to 0.1 mA of resolution, showing 0.0008 amps which is 0.8 mA of current flow, just less than 1 mA. That by itself shouldn’t cause a GFCI to trip. But you can see that if you have five appliances plugged into a single GFCI (like a campsite 20-amp receptacle) and each appliance is leaking around 1 mA of current to ground, and since 5 times 1 mA = 5 mA total leakage current, that GFCI breaker is going to trip whenever it feels like doing so.

Now, if you don’t have an expensive clamp ammeter like mine that registers down to 0.1 mA of current, all is not lost. Most clamp meters under $100 will only register current down to 0.01 mA which is 10 milli-amps. To increase its resolution all you have to do is wrap the green wire in your test cord around the jaw of any clamp meter 10 times, and that will amplify the sensitivity of the meter by a factor of 10. So with a 10X wrap when your meter is reading 10 mA of current (0.01 amp) there’s actually only 1 mA (0.001 amp) of leakage current, and a reading of 30 mA (0.03 amp) on your clamp meter is actually only 3 mA (0.003 amp) of leakage current from your appliance, etc. Neat trick, eh?

One way to check your entire RV for GFCI leakage current is to use a 15-amp dogbone adapter to plug it into this short test cord with all your RV circuit breakers and appliances off. Then turn on one circuit breaker and appliance at a time and monitor the amount of leakage current you’re seeing on the clamp meter. With the exception of the converter/inverter (which by code can leak up to 3 mA to ground and still be within limits), any other appliance in your RV which leaks over 1 mA of current to ground (reading 10 mA or 0.01 on your meter with the 10X modification) is suspect and should be evaluated for internal power supply ground–leakage problems and probably replaced.

Sorry, but troubleshooting each appliance for what’s causing its own ground fault leakage is beyond the scope of this article, but once you’ve identified the problem, you can either replace or repair each item, checking again with your clamp-on ammeter to confirm you’ve fixed the current leak to ground. Once your RV’s total ground leakage current is below 5 mA, then your random shore power GFCI tripping should become a thing of the past.

Wrap up

GFCI breakers nearly always trip for a reason, and that reason is that they see an imbalance between how much current is going out to an appliance from the black wire compared to how much is coming back in from the white wire. If you clamp an ammeter around the black and white wires at the same time, any current flow detected will be ground leakage within the appliance itself. Over 5 mA of leakage to ground is supposed to trip a residential GFCI, so it’s only doing its job.

Future shock

Part 11 of this series will cover extension cord wiring and testing, so we’ll see you then.

Let’s play safe out there….


RV Electricity – No~Shock~Zone by Mike Sokol – Issue 24The best book on RV electricity, hands down!
Mike Sokol is America’s leading expert on RV electricity. Mike has taken his 50+ years of experience to write this book about RV electricity that nearly anyone can understand. Covers the basics of Voltage, Amperage, Wattage and Grounding, with additional chapters on RV Hot-Skin testing, GFCI operation, portable generator hookups and troubleshooting RV electrical systems. This should be essential reading for all RVers. Learn more or order.

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

For information on how to support my RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign.


Did you enjoy this article?

Click on a star to rate it!

Average rating 0 / 5. Vote count: 0

No votes so far! Be the first to rate this post.


Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Inline Feedbacks
View all comments
Robert E Staples
3 years ago

This man is a genius!

Tommy Molnar
3 years ago

Yeah, and I’m NOT. Some of this stuff is SO over my pay grade, I just scratch my head and revel in how my trailer so far, has had none of these types of trouble.

RV Staff(@rvstaff)
3 years ago

I’ll second that! Thanks, Robert. 😀 —Diane at

Karl Eby
3 years ago

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 to be 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 weather a GFCI breaker will function properly without a green/ground connected, but I don’t believe it will function, trip.

Stephen Kight
3 years ago
Reply to  Karl Eby

Save yourself some time. Mike is right. What goes out on the black has to come back on the white, otherwise there is leakage to ground. The water pipe system in a house or a barefoot person standing in wet grass can be the ground that the current leaks to . The green wire in the electrical system ultimately connects to these things. It doesn’t have to leak directly to the green wire and most of the time it doesn’t. Often GFCI receptacles are used to replace the old 2 wire, ungrounded receptacles because they don’t need a ground hooked directly to them but they will protect against leakage to other grounds that exist. Rarely does someone touch the green wire.

Subscribe to our newsletter

Every Saturday and Sunday morning. Serving RVers for more than 20 years.