With a long run of extension cords and adaptors this issue comes up often. Grab a voltmeter, put it on the AC measurment mode and place one probe in the ground and the other on the TT. Now turn your microwave on and measure while going. The voltage will probably go up a bit.
Long runs of extension cords, (and everyone follows those rules right?), cautioned on drills and other items is often ignored and your TT can draw a lot of current if the batteries are low or you are running an AC for example.
The issue is the resistance of the wires causing a voltage drop and the TT is a series resistor.This resistance can be from poor house wiring, worn outlet contacts, long runs of wires, and loose connections. First step is to get away from light weight cords etc.
Not making light of a possible dangerous situation. Just don't chase ghosts.
