Looking back at the first post, I've just realized that Bill did specify "a GFCI receptacle" rather than "a receptacle on a GFI-protected circuit. So yes, in that case temporarily bridging the ground screw to the line-side neutral would allow the plug-in tester to check the GFI tripping.
However, as those cheap testers just connect a resistance from hot to ground to check tripping operation, the only thing this would prove over and above using the GFI's own test button is that there is continuity from the ground hole on the recept. to the green screw. (And as the GFI is on a 2-wire circuit, this doesn't matter as the ground screw won't be connected to anything!)
Scott,
I think you'll find the reason that motors etc. sometimes trip a GFI is due to capacitive coupling to the (grounded) frame. I can't see how any combination of reactive components should trip a GFI if just connected in any parallel or series combination between hot and neutral. There needs to be some sort of connection back to ground, be it a direct bond, resistance to ground, capacitive coupling, or whatever.
Roger,
From the practical point of view, I think the GFI's own test button is probably adequate for testing. It's just a simple momentary-contact switch and a resistor in series connected between load-side hot and line-side neutral, and as such is pretty fail-safe.
If any part of that test circuit goes open, the GFI won't trip when the button is pressed and will therefore signal that something is wrong. If the switch shorts out, the breaker will trip and not reset. If the test resistor drifts high in value, then the imbalance will be lowered and again, if it is too low to trip the user will still be alerted that something in the GFI is wrong, even if in this case it is only the test circuit itself.
Should we not be concerned with educating people that these GFI testers will not work properly when used on a GFI recept installed on an old 2-wire circuit? Come to think of it, are there any such warnings on the testers sold in Home Depot and such like?
[This message has been edited by pauluk (edited 08-10-2002).]
