Ignoramus15109 wrote:
Thakns to all. I fixed it. I took the cover off and blew out
accumulated carbon crud. It made a little cloud, I tried not to
breathe and walked out of the garage for a little while to let it
settle.

After that, the drill runs (as it did before), but it no longer trips
the same GFCI breaker. I tried starting and stopping it many times,
shook it while running etc, but the breaker would not break. I made
some pictures, but I doubt anyone would be interested -- just brushes
touching the rotor.

I want to say big thanks to all people who gave me good suggestions!
It was the easiest fix of my life.

I also tested hot to ground with a multimeter, no obvious leaks (I did
not use a megger, but I used the 1M setting of the multimeter).

For the record, I used rubber boots and gloves while I was trying the
drill.

Indeed, there is a big threaded hole on top of the drill, for
inserting a big handle. Maybe it is even meant to be held by 2
people with a big handle.

i

This brings to mind something I encountered a couple years ago. Someone
in a Campground tripped the Camp Group power GFCI when he plugged in his
Travel Trailer. He said it did not happen when he used it at home and
other places (that did not have GFCI).

The problem was that the Ground and Neutral wires were tied together
inside the trailer. Of course the Ground and Neutral were also tied
together in the Campground. Therefore some of his return current was
flowing in the Ground tripping the GFCI.

Same thing was happening in the drill but only a tiny bit was probably
flowing through the carbon dust but it was enough to cause the GFCI to trip.