RVelectricity – Just Ask Mike (J.A.M.): Why surge protectors and GFCI outlets don’t mix

By Mike Sokol

Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM.

Dear Readers,

I keep getting emails like the one below from readers about their RV tripping a GFCI outlet at their home when they plug shore power into it.

Dear Mike,

I just need to charge the batteries and run the refrigerator when my RV is parked next to my house, but when I plug it into the GFCI outlet in the garage it keeps tripping. I have surge protectors in the RV, so that should protect it, right? How can that be happening when I’m using hardly any power at all? —John Q Public

Dear John Q,
Okay, this will be a brief JAM Session on what GFCIs do and how they can interact with surge protectors.

First things first

A GFCI outlet doesn’t protect the wiring from too much current. That’s the job of the circuit breaker in the service panel or pedestal. The GFCI is there to protect a human being from being shocked and killed, nothing else. It does this by measuring any leakage current to ground. When this fault current exceeds 5 mA (5 milliamperes or 0.005 amperes), then it trips.

Second things second

Any surge protector uses something called a MOV (Metal Oxide Varistor), which always has a small amount of leakage from hot to ground. In fact, the Underwriter Laboratories (UL) and the National Electrical Code (NEC) allow surge protector strips to leak up to 3 mA of fault current to ground and still be in compliance. Most appliances are limited to less than 1mA in order to pass UL.

Fault currents are additive

Everything you plug into an electrical outlet leaks a little fault current to the chassis ground and eventually the ground wire in the electrical outlet. Most of the time it’s a lot less than 1mA of current.

But put enough of these ground fault leaks together and once they add up to above 5mA of fault current, any GFCI you plug into is going to eventually trip. So even two surge strips plugged inside of your RV can add up to 6mA of ground fault current, tripping the GFCI in your garage or on the side of the house.

What else?

Old microwave oven transformers can get really leaky (I’ve measured over 10 mA of fault current on one) which can trip a GFCI.

A broken or burned electric water heater element can dump 1 or 2 amperes of current (that’s over 1,000 mA of fault current) into the water and surrounding water tank, which will quickly trip any GFCI you plug your RV into.

And a screw or nail driven through a wire can allow full circuit breaker current (20 amperes or so) to flow into the ground wire. That will instantly trip any GFCI along with the circuit breaker on that branch circuit.

Also, modern RV battery converters, chargers and inverters include line filter capacitors to reduce RFI (Radio Frequency Interference), and have a UL exception allowing them to leak up to 3mA of current to chassis ground. The world is full of electrical noisemakers, and manufacturers are constantly working to keep high frequency interference below levels set by the FCC and others.

What can I do about it?

Troubleshooting random GFCI tripping is beyond the scope of a JAM Session, and I’ve got a few tricks up my sleeve. But NEVER cut off the ground pin of any extension cord trying to stop a GFCI from tripping. And NEVER replace a GFCI outlet with a regular outlet. That’s a huge code violation and potential liability if someone gets shocked.

But if your RV is tripping a GFCI outlet when you plug it into shore power, first make sure you don’t have any surge strips hooked up inside of your RV that could be contributing to these leakage currents. And then turn off all circuit breakers in your RV’s power center and reset the offending GFCI. When you turn the breakers back on one at a time you might get lucky and find the one that’s creating the fault current. That will give you a hint of what’s causing the normal leakage (around 5 mA) or dangerous leakage (over 30 mA) But RVs in general don’t play well with GFCI outlets, so it’s a bit of a conundrum. Much more on this later…

Might I have mo’…

Not in this JAM session. But know that I’ve just found a really interesting line splitter that might be very useful for troubleshooting and cataloging small ground fault leakages. That will be the topic of a full article and video on GFCI troubleshooting later. In the meantime, you can read one of my previous articles on GFCI theory and troubleshooting HERE.

OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.

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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