Reply by TopamaxSurvivor

I can usually identify GFCI tripping issues with my ol’ Fluke 27. Motor testing does require a higher voltage meter. I can’t remember if I ever had a motor winding grounding causing the unbalance or not. I have done it too many times to remember them all ;-)) AS I recall, the Fluke usually shows about 3-4 mega ohms or less on a tripping circuit. Usually 6 mega ohms to infinity will be clear. I doubt if this helps much.

If the sander is the only thing tripping the GFCI, it is most likely 99.9% for sure the problem.

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence