On 10/14/2013 6:50 PM, Bill Leonhardt wrote:
In my shop I have two general purpose electrical circuits that I
connect everything to except the big machines. They are 20 amp
(120VAC)circuits with 12 gauge wire. Each circuit starts with a GFCI
outlet and then a chain of normal outlets. All outlets are 20 amp.

....

This evening I was jointing a piece of 1 x 6 maple set to take off
less that 1/64 per pass. After a bit, the GFI would pop. Thinking this
might be the GFCI outlet, I connected the jointer to the second circuit.
Same problem. The only other piece of equipment running was the DC which
is on a separate 240VAC circuit.

The circuit breaker (20 amp)never pops. The GFCI outlet is maybe 6'
from the panel and the second outlet (jointer) is about 8-10' from
that.

The jointer has a 1 hp motor (running at 120VAC)and, when I pulled
the cover off, the motor was barely warm. No dust buildup either
since Ihave this connected to my DC.

....

Any thought why this would cause the GFCI to pop and not the circuit
breaker? Also, what would I look for with respect to the cause. I bought
the jointer new 8 or 9 years ago and it has been lightly used.
....

GFCIs work on the current imbalance between the two legs and trip as low
as 5-6 mA. What this indicates is that the motor now has a leakage path
that ends up being above the GFCI threshold--it probably started off at
just barely under and know with some age there's some insulation
degradation that has let it now be just over instead of just under.

Your choices as I see it are--

a) replace the GFCI w/ a different/newer one and see if goes away for at
least a while,

b) replace the motor w/ one that doesn't have the leakage current. It's
highly unlikely you'll be able to change its characteristics any
although despite no heavy dust buildup you might try blowing it out
thoroughly w/ compressed air to see if internal dust could be
contributing to the problem, or

c) do away w/ the GFCIs

Oh, there's no fluorescent lighting on the circuit is there?

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