We just had electrical work done in our house, and now my fiance is having problems using his circular saw. The new GFCI receptacle in the garage trips as soon as he starts it. Same thing happens when he plugs it into the new GFCI receptacle in the bathroom.
Well, you say, there’s probably a ground fault in the saw. Get a new one.
But here’s the weird thing. If he uses my (fairly new, hardly used) grounded extension cord, the GFCIs trip as described above. But if he uses an old 3-prong extension cord of his in series with mine, the GFCI doesn’t trip. The saw works fine.
Why would this be?
Here’s my theory. The GFCI detects a current differential between the hot and neutral wires, right? If his saw has a ground fault and a functioning ground wire, then some current is flowing through the ground wire, enough to trip the GFCI. But when he connects the old, beat up extension cord, it probably doesn’t have a functioning ground wire — maybe it’s broken — and so, no current returns through the ground wire. If my fiance is not grounded, either, then no current is exiting through a ground path, it all returns through the neutral, and the GFCI doesn’t trip.
Not a good situation. I’ve already told him he needs a new saw, but he thinks that because it works fine, he doesn’t need to replace it.
Any electricians here know if my analysis of the situation is correct? Any other plausible explanations for why the GFCI doesn’t trip with the old extension cord?
Thanks,
Rebeccah
Replies
Your analysis sounds basically correct. He could verify by getting a "suicide plug" (3-prong to 2-prong adapter) and verifying that the saw runs with that in place, but doesn't run if the wire on the adapter is grounded to a water pipe or some such.
Likely there's a high-resistance path to ground inside the saw -- not enough to (quite) cause a shock, but enough to trip the GFCI. Since this high-resistance path could suddenly become a low-resistance path (and since the resistance could be changed if your BF has sweaty hands), it's a pretty unsafe to run the saw without it being *properly* grounded, *unless* it's plugged into a GFCI. But who wants to DEPEND on a GFCI (or a ground, for that matter) to save your life?
The other possibility (not real likely) is that the saw produces a starting surge that trips the GFCI, and having an extra-long extension cord "suppresses" the surge.
Probably there's a tool repair place somewhere nearby that has an "megger" meter to check for current leakage in a power tool. He should get his saw checked there.
Buy your BF an "octopus" multi-outlet adapter with built-in GFCI for his birthday or Christmas.
Thanks, Dan.
I'm going to print this out and show it to him.
Rebeccah
I could be wrong, but I think the GFCI measures the current between the hot and the neutral, not the hot and the ground. There may be a fault in the new cord.
That's what she said, hot and neutral. If the unit trips when the saw's plugged directly into the outlet, and also trips plugged into the new cord, there's nothing pointing at the new cord.
And if the new cord were defective, one would expect a trip when the new cord was plugged in, without the saw.
It could also be miss wired. Crossed neutral with another circuit or neutral connected to ground down stream.
Will trip on any load. Like wise it might be a bad GFCI.
But if I rememver the symptoms that it does not trip if an ungrounded extension cord was used. That would rule out the bad GFCI/wiring.
And it could be a bad-right-from-the-box GFCI fixture, too.
New GFCI cheaper than a new saw (if not as fun to "break in" . . . )Occupational hazard of my occupation not being around (sorry Bubba)
That would have to be two bad GFCI receptacles... he tried plugging the saw into the bathroom, too, with the same result.
And other stuff (lights, boom box, sump pump) plugged into the GFCI work fine.
I think it's the saw.