GFI and Shore Power

So...we were visiting a friend in Austin. We were camping in their driveway, like we have done before. I went to plug shore power into an outlet in their garage, like we did before. No matter which plug I tried, I would pop the GFI. This happened even when I shut the 50-amp breakers off in the coach. Since it was a very new house, I couldn't find any outlets without GFI so we wound up boondocking in her driveway.

I am quite familiar with wiring and electrical systems, but I don't know much about GFIs and what causes them to trip. The only difference between the old coach and the new one is that I haven't installed the surge protector in it yet. Any thoughts what is going on here?

GFIs trip when there is a difference between the current going out on the hot conductor and the current returning on the neutral lead.  This means the missing current is returning to ground some other way, possibly through the body of someone who is getting shocked.

Because of this, the amount of leakage current needed to trip a GFI is very small, 30 milliamps, below the threshold of harm to the human body.

It's fairly easy to stay below this threshold if only one or two devices are plugged into a GFI outlet, as it's designed.  But since leakage current is cumulative, you can exceed the threshold by plugging in a whole RV full of electrical stuff, even if everything is working as it should.

But since you said the GFI trips even with the rig's main breakers turned off, that eliminates the majority of the RV from consideration.  You've disconnected everything past the main breaker, so that limits you to the stuff on the shore power side of it.  Your power cord or plug may have a high impedance internal leakage,  or neutral and ground may be tied together at the transfer switch (this sometimes happens because they are supposed to be tied together when the rig is connected to the generator but kept separate when using shore power).  When neutral is tied to ground, part of the return current flows through each wire, upsetting the hot to neutral current balance inside the GFI.

Surge suppressors also can have leakage to ground, which may be significant when added to the residual leakage in the rest of the RV.

If you have an ohmmeter, measure the resistance between the neutral and ground pins on your shore power plug.  On a 50 amp plug, neutral is the center vertical pin, ground is the round one.  You should have infinite resistance between them.  Make sure you aren't touching the probes with your hands since the meter is sensitive enough to measure your body resistance.  If you measure a low resistance, I'd check the transfer switch first, followed by the shore power cord and plug.

I have had the same problem. Often, if I leave the coach plugged in and re-set the gfci it works fine. But sometimes nothing works.

Lou Schneider said:

Because of this, the amount of leakage current needed to trip a GFI is very small, 30 milliamps, below the threshold of harm to the human body.

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Lou,

I thought the GFI leakage current necessary for interrupting the circuit was 5 ma +/- a few tenths of a ma. Has there been a change in this value?

R

I had this same situation happen at a friends new house and never again anywhere else with the same motorhome. Perhaps a problem in the house circuit or that particular GFI?

I tried it on 3 or 4 different circuits with the same result. I'll try it somewhere else and see what happens.

For the short term your only solution is to use a non-GFCI outlet. There likely will be some in the house somewhere, but not outside and probably not in the kitchen or bath areas (the electrical code generally requires GFCI in those areas).

Since the problem occurs with the main 50A breakers off, I would first look for ground leakage in the auto transfer switch. Also check the connections in the power cord itself - it is conceivable that one tiny strand of wire is bridging from neutral to ground pins.

Gary RV Roamer said:

For the short term your only solution is to use a non-GFCI outlet. ..

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Yup. 

There is a state park in the western part of Virginia where we volunteer at every so often and for some bizarre reason all of the power pedestals have 30 amp GFCI outlets.  We will always trip the GFCI and the resolution is to change the breaker in the box  to a non GFCI model - I can get away with this since a) I'm a park volunteer and b) I'm comfortable working around power.

As to why this happens, I think the issue is that the neutral and ground aren't at the same voltage potential (i.e., tied together) in an RV like a house - I could be wrong, that particular memory is a little faded  .

A GFCI is driven strictly by current (amps), so voltage should not affect it. The GFCI logic is simple: if the current returning via the neutral is less than the current feeding in via the hot, it opens the circuit. The missing current has to be going somewhere, and if that somewhere is NOT the neutral wire then a potentially dangerous situation exists.  Typically that other "somewhere" will be the ground wire, cause that's why the ground is there (to catch any stray current).

An RV neutral and ground get tied together when the shore cord is plugged to the power pedestal, i.e. the park power system has the neutral and ground bonded together at its power source and that provides the bonding for the entire system.

So...maybe I should short the ground and neutral in the transfer switch.

Why?  That would simply guarantee you trip any gfci you ever plug to.

There's no good answer other than avoiding GFI outlets whenever possible.

Tying neutral and ground together inside the RV only splits the returning current between the neutral and ground wires.  Since the GFI monitors current on the neutral and hot wires, splitting the returning neutral current between two wires will only guarantee you'll trip any GFI outlet you plug into.

The opposite corollary is lifting the ground at the GFI socket by using a 2 to 3 prong cheater plug.  This sometimes makes a GFI outlet work by eliminating the second return path through the grounding wire.  But, and this is a biggie, it also defeats the grounding protection throughout your RV.

This is a potentially lethal situation, and not something I'd do for the small convenience of having shore power.  Any leakage inside the RV will put voltage on the RV's frame and exposed metal parts.  A properly working ground wire will safely drain this voltage to ground or if the short is large enough, trip the circuit breaker.  Without a working ground wire, the voltage on the RVs frame from even a small amount of leakage is potentially enough to seriously shock or electrocute anyone who gets between the energized RV and ground.

Defeating the ground wire also removes the ground protection from any 3 wire appliance you plug into an RV socket.  If you defeat the safety ground you're totally at the mercy of the GFI operating properly and cutting off the power if someone gets in a hazardous situation.

I suppose I could lift the ground. How many years did we cut the ground off a 3-prong plug to make it fit? Not a comforting thought, though. Next time I'm there, I'll either install a non-gfi breaker for my use or find one. There's got to be one someplace.

Does a washing machine require GFI? Maybe I can use that outlet.

Washing machines do not require a GFI.

mrschwarz said:

I suppose I could lift the ground. How many years did we cut the ground off a 3-prong plug to make it fit? Not a comforting thought, though. Next time I'm there, I'll either install a non-gfi breaker for my use or find one. There's got to be one someplace.

Does a washing machine require GFI? Maybe I can use that outlet.

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Michael:

I think you realize you should locate the source of the leakage rather than just trying to deal with the symptoms.  Sure, a non-GFCI outlet will eliminate the nuisance, but the leakage will remain nonetheless.  IMHO the real risk would occur if you happened to plug into an outlet with a defective (or high resistance) ground connection.  In that situation some parts of your RV would become "hot" relative to ground and you could have a potentially lethal hazard. 

BTW, although washing machines don't have to have GFCI's per NEC, their proximity to sinks often requires it, so that may not be a solution.

Joel

Just to clarify, appliances don't "require GFCI". Branch circuits are required to have GFCI protection if they fall into any of several categories, primarily circuits in wet locations where it is easy for a human to become an alternate ground path.  Common examples are outdoor and bathroom outlets.  You can probably think of others, but this thread isn't intended to explain the details of the National Electrical Code.