GFCI Problem with Delta JT360 Jointer

[Kelvin McConnell]

I’m slowing making progress on my shop in the basement. I’m running two 20amp 120v circuits alternating around the shop perimeter (each circuit is a 20amp GFCI w/4 20amp receptacles attached downstream).

That gives you an idea of the setup, so let’s jump to the problem. When I tested the first GFCI circuit last night to make sure it worked, I just plugged in the JT360 jointer since it was the closest tool nearby. It started up fine, but when I turned it off (about a second later) the GFCI flipped. I thought that was odd so I tried the second GFCI circuit and it did the same thing. I reset the circuits and plugged in the dust collector and then an air compressor and they worked ok. I went back to the jointer and plugged it in turned it on and then unplugged it while it was running, everything ok. Something happens in the jointer motor during its shutdown that is flipping the GFCI breaker.

Is this something that I can fix or am I going to have to just plug it into a non-protected outlet? Thks

[Charles Lent]

There may be a problem with the centrifical switch inside the motor that causes a momentary ground connection when it trips back on as the motor slows. You may also have a wiring problem. Check to be sure that the Neutral wire is not being switched instead of the Hot wire. These are just guesses, and dis-assembly and test will be needed to find the real problem.
GFCI circuits are very sensitive and will trip if there is even the slightest leakage current to ground or a difference in current flow between the Hot and Neutral wires. The power switch itself may have a fault in it that would cause this.

Charley

[David G Baker]

Anytime I have had a GFI or GFCI trip I eventually found the problem and each time it was something I had done.
When I did a remodel in my California house I used Home Depot wire clamps that hold the wire as it leaves the box. The bar that screws down to hold the wire was a dimple instead of the standard bar shape. In several areas the dimple had pierced the romex and had made direct contact with the metal of the wires inside. I was very angry because I had installed at least 50 of the fittings. It took quite a while to find the problem and a long time to correct it.
There are quite a few thing that can cause your problem and you should get it taken care of prior to using the equipment.
Another time one tiny bristle of a paint brush touched one of the legs on a receptacle. The tiny dot of paint was very hard to see but I eventually found the dot, reset the GFI and all was well. I suspected the paint because that was the last thing I had done in the area where the circuit ran.

[Rod Sheridan]

Kelvin, at work we have a single phase drill press that does the same thing about 10% of the time.

We measured the insulation resistance of the windings thinking we had a ground fault, however nothing.

It was cured by leaving the table lamp on when you shut it off, which makes me think it's a back EMF that's confusing the GFCI receptacle.

Regards, Rod.