Jadnashua
Retired Defense Industry Engineer xxx
They're designed to trip when they see about a 5ma (0.005A) or more delta between the hot and neutral...enough to potentially feel, but not necessarily hurt you under the right circumstances. In a properly working circuit, all the current that goes out on the hot should return on the neutral, thus, the loads are even. AN old motor which has a lot of dust from the brushes, or some florescent lights, and other things can leak but not trip a breaker. They can trip a GFCI. That's why some people say they'd never use a GFCI protected circuit on a refrigerator or freezer...the GFCI could trip. But, if they're working properly, it never should. If they've worn enough where they do trip one, they're a safety hazard that should either be repaired or replaced. But, a lot of old ones get relegated to the basement or garage, and eventually will have issues. Things don't last forever, and the spoiled food is cheaper than a funeral.