Saturday, June 10, 2023


Why does the GFCI hum when the inverter is on?

gary-736Dear Gary,
I replaced the GFCI in my Class A motorhome and the new one hums when the inverter is on. Any idea why? —Denise D.

Dear Denise,
Maybe because the new one doesn’t know the words? Or, more likely, it might have something to do with an internal line balance-detecting transformer. This, of course, is assuming no modifications were made to the wiring at the inverter itself. None would have been required if the inverter was installed correctly to begin with. The humming could also be related to the type of inverter in the system. Is it a true sine wave inverter? Or perhaps it has a modified square wave output form. 

All GFCIs have a set of contact terminals labeled “Line” and another set labeled “Load.” The black and the white wire (hot and neutral) from both the line and the load must be connected to the correct terminals on the GFCI. If, for instance, the hot wire from the line is wired to the “Line” terminal, but the neutral wire from the line is wired to the “Load” terminal, it will confuse the GFCI. The main purpose of the GFCI is to monitor the balance of that circuit between the black and white conductors. Miswiring the hot or the neutral at the GFCI may render it inoperable, but not to the point of actually causing it to trip, nor will it likely trip the circuit breaker for that circuit. 

Here’s something you can check. Be sure the coach is not plugged in and the inverter is off before proceeding. With the GFCI removed from the receptacle box, there should be two sets of Romex conductors located in the make-up box. The black and white wires from each must be kept relevant to each other. In other words, the same pair of black and white conductors must attach to the correct (line or load) terminals on the GFCI. With the motorhome plugged in, the “line” set will be energized. The “load” set is everything else downstream of the GFCI and will not be hot when the GFCI is tripped.

There is a remote chance the new GFCI is faulty, but chances are it is simply a case of incorrect wiring. I’ve actually seen all the white wires bundled into one wire nut behind the GFCI – a no-no for GFCI wiring. It is best to have a pro RV service tech take a look if you are unfamiliar with working on a live circuit. The circuit must be energized at a certain point in order to differentiate between the line – hot – and the load – not hot – sets of wires at the GFCI.

I know this may sound a bit confusing, but a pro tech with an accurate VOM will be able to quickly discern the problem with a few simple measurements. 

Read more from Gary Bunzer at the See Gary’s videos about RV repair and maintenance.




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4 years ago

I would first try using a GFIC receptacle tester, just seems less complicated!

Tommy B
4 years ago

Or it could be that the hum happens because a modified sine wave chops off some on the wave thus creating a momentary loss of power to the coil that holds the relay on/off. Happens 60 times a second.

Mike Sokol
4 years ago
Reply to  Tommy B

You’re right except for a few details. (I do and teach this for a living). The modified sine wave does chop off the top (and bottom) of the sine wave which creates all kinds of harmonics above the 60 Hz sine wave frequency. There’s a lot of 3rd order harmonics generated, so you’re hearing 180 Hz, 300 Hz, 420 Hz, etc…. And this happens 120 times a second (not just 60 times) since the 60 Hz waveform has both a top and bottom peak during each “wave”. Once I get the RVelectricity forum and educational site up and running I’ll have room to post details of how this works. I’ll even be able to show you “triplen” harmonic currents caused by harmonics related to clipped sine waves. That’s some really fun stuff….

Geek Alert – Read this if you dare:

Rob Cooley
4 years ago
Reply to  Mike Sokol

Definitely sounds true since a square wave is made made up of even harmonics.

Mike Sokol
4 years ago
Reply to  Rob Cooley

Very close, but not 100% exactly right. Sorry….

Perfect square waves are made up of “odd-order” harmonics, while triangle waves are primarily “even-order harmonics”. And of course sawtooth waves are a combination of even and odd harmonics. Yes, I teach this in my electronic music synthesis class. See below:

Using Fourier expansion with cycle frequency f over time t, an ideal square wave with an amplitude of 1 can be represented as an infinite sum of sinusoidal waves: The ideal square wave contains only components of odd-integer harmonic frequencies such as 3rd, 5th, 7th (of the form 2π(2k − 1)f). Sawtooth waves and real-world signals contain all integer harmonics such as 2nd, 3rd, 4th, 5th, 6th, 7th, etc…

Rob Cooley
4 years ago
Reply to  Mike Sokol

I had s 50/50 chance on that one. I should of looked it up.

Mike Sokol
4 years ago
Reply to  Rob Cooley

Actually there’s a few music synth textbooks that mistakenly say that square waves contain even order harmonics, when I had been experimenting with Moog oscillators of all types for the past 40 years or so. So I THOUGHT I knew what I was talking about in my electronics music class, but there it was in black and white in a textbook. So rather than assume I’d been wrong for the past 4 decades, I looked up the Fourier transform, and there was the math to prove I was right. Whew…

Actually I was quite impressed that you understood the concept of even-order harmonics as it’s applied to switched inverteres and power supplies. Someday I’ll go into the math of Triplen Harmonics as it relates to 3-phase neutrals overheating. It’s really cool stuff, but way over the heads of even most electricians I know. But you would probably have a blast with the concept.

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