Gfci tripping with no ground lead, how is that possible

[Bill D]

I am testing a small single phase motor. Right now I have hot and neutral connected but the ground wire is just floating. Not touching anything. there is no wiring box or terminals so easier to leave ground alone until it is running.
it is sitting on dry concrete, outdoors. When i plug it in it jumps then trips the GFCi off. I do not understand why the gfci trips. It should be equal current in and out of the motor with no imbalance going anywhere. Even if the metal outer case was electrically hot I would think the concrete would be a good enough insulator since it has not rained for several months. But the humidity is high today 39%.
I thought for a GFCI to trip current in had to be unbalanced to current out. Normally that imbalance would be flowing to ground. AFAIK a direct short from hot to neutral will not rip a GCFI off since current in and out is equal.
Bill D

 

[Bill D]

I will test tomorrow on a pice of dry wood with some plastic under that. If not then I will hook up the ground lead temporarily.
Bill D

 

[4GSR]

Check the resistance between the motor leads and the motor housing. If it registers anything, it is shorted. Place the motor on a non-gfic circuit and turn on. If you get fireworks or a electrifying shock from the motor, it has a ground fault. The motor needs to be checked with a megger, that would be the safe way to determine if it is safe or not or if it will run.

 

[memphisjed]

Not a safe bypass:
Bridge ground wire and neutral somewhere between motor and gf outlet.
If everything in motor is good your fine, if not bad things can happen: The gf will not trip.

Not a safe bypass! Works. Again, not a safe bypass!

 

[boslab]

Neutral earth short?a few milliamperes will trip in a LE short over here.
Mark

 

[Bill D]

Tried the motor again this AM. Sitting up on a five gallon plastic bucket so no electrical connection to ground. Same deal a few degrees of twist then the gfci tripped out. I connected the ground wire and no change.
Shifted to a non gfci outlet and it ran fine for a minute or two.
Old motor with rubber and fabric covered leads. I do not doubt the winding insulation is not good. But I do not understand any imbalance between current flow in and out of the motor. With no connection to ground where are the extra electrons going?
Bill D

 

[sfriedberg]

With no connection to ground where are the extra electrons going?

If you are really confident there can't possibly be any ground leakage of more than 1 or 2 milliamps (don't forget to consider the power cord itself, including possible internal faults and crud and dirt on its exterior jacket!), there are still some possibilities. Here's a page about it.

 

[Joe Gwinn]

I am testing a small single phase motor. Right now I have hot and neutral connected but the ground wire is just floating. Not touching anything. there is no wiring box or terminals so easier to leave ground alone until it is running.
it is sitting on dry concrete, outdoors. When i plug it in it jumps then trips the GFCi off. I do not understand why the gfci trips. It should be equal current in and out of the motor with no imbalance going anywhere. Even if the metal outer case was electrically hot I would think the concrete would be a good enough insulator since it has not rained for several months. But the humidity is high today 39%.
I thought for a GFCI to trip current in had to be unbalanced to current out. Normally that imbalance would be flowing to ground. AFAIK a direct short from hot to neutral will not trip a GCFI off since current in and out is equal.
Bill D

As you say above, GFCIs measure the difference in current going into the motor and coming back out. Any current that takes a side path, will upset this balance.

As others have said, it doesn't take much current unbalance (between hot and neutral) to trip a GFCI.

I would guess there is current leakage from winding to motor frame, and thence from frame to ground via the "dry" concrete. Outdoor concrete is never really dry.

Test this by putting the motor up on a bit of reasonably thick rigid plastic sheet that rests on the concrete, versus motor directly on the concrete. If the trips cease, the motor frame will be hot, perhaps at full voltage. Check with voltmeter, not with hand. Or better, an ordinary 100 watt incandescent lamp. If the lamp does not light, it's a leakage, versus a hard short. If the voltmeter has a suitable current range, measure the leakage current to ground. (Don't try this without first doing the lamp test, to protect the voltmeter.)

 

[Gordon Heaton]

Have you tried a different GFCI? I've seen several go bad...

 

[Rickyb]

As 4gsr said, get a megger.

 

[candeservices]

Finally all those days of waiting will finally pay off. GFCI's do not require a ground to work infact a gfi is the only replacement for an outlet that does not have a ground barb(think older built home). Second part is GFI's on motor loads are very sensitive and nuisance trip due to the large inrush that is applied to them, or there is a very small voltage leak across the coils and that is exactly what the gfi's were designed to look for. The leak does not have to go to ground the gfi is looking for power coming in on the hot and the exact same power coming back on the neutral, if these two readings do not match the gfi trips. I would think that motor insulations are going bad the extra electrons are charging the motor ceasing and when the gfi trips are then draining back on the neutral.

 

[Bill D]

Problem solved. Stupid computer power cord had a bad color code which I read wrong. It had brown, blue and blue with a green stripe. Green was not ground
So I had it wired from hot to ground with neutral not connected to anything. Full power went from hot to ground
. At least I know the GFCI works.
I would have sworn I checked the cord with a meter to make sure which was ground. Taking short cuts in the heat when it was 105 yesterday. Hotter in the sun where the motor was.
Bill D

 

[Gordon Heaton]

Thanks for the follow-up post.

 

[jim rozen]

Brown = black = hot
Blue = white = neutral return = ground*ed*
BL/GN = green = ground*ing* wire

Or did the manufacturer goof up?

 

[candeservices]

Those are over sea's color code or European.