Tolerances.
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19 years ago
Odd GFCI behavior - explanation?
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- posted
19 years ago
Installed two Leviton GFCIs yesterday, each one for that outlet only, not protecting anything else downstream. One works just fine. The other one doesn't: it resets OK and powers up, plug-in circuit tester shows it's wired correctly, test button _on_the_plug-in_tester_ trips the GFCI, it resets OK, but the test button on the GFCI itself does *not* trip it. Cycled through this repeatedly, with no change in behavior.
Took the misbehaving one back to the retailer and exchanged it. Installed the replacement: identical behavior.
Swapped it with the other GFCI to see if the problem would follow the unit, or stay with the box. *Both* units work properly.
Speculations as to what's going on? I'm at a complete loss to explain this.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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19 years ago
Intermittent ground in the flaky outlet box? Offhand, you'd think that since a ground isn't necessary for GFCI operation, you wouldn't need one for testing, but I'm at a loss (at least until the caffeine kicks in) how one could test GFCI operation without a ground to fault to...
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19 years ago
While that might explain the test button on the GFCI not working, how about the test button on my plug-in tester? That tripped the GFCI _every_time_.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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19 years ago
Gah. OK, how 'bout there's already some ground leakage from the neutral to ground (capacitive coupling or something), the "test" button on the GFCI is set to fault the hot to ground, and the "test" load is close enough to the hairy edge that it can't quite swing the differential current back over the "trip" level. The tester, OTOH, uses a larger load, so it swings it all the way over to the trip level. Now, how the heck a neutral (even a floating one) could leak enough current to ground for that to happen is left as an exercise for the student (meaning, I can't really think of a way).
OK, so I'm blowing a smoke cloud that can be seen for miles. Damned if I can come up with a reasonable explanation, either...
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19 years ago
testing,
A GFCI's test button is designed to trip the GFCI even without a ground. It does this by shorting one of the output leads through a resistor to an input lead, or something like that (so the output leads are imbalanced).
A GFCI tester cannot do that (because it doesn't have access to the "input leads"), so it simply runs some current from the hot wire to the ground (and hence it needs a ground to operate properly).
It seems to me that if the circuit has an open neutral, the GFCI test button wouldn't trip it, but the standalone tester _would_ trip it.
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19 years ago
only, not
Installed the
Doug,
I've found that some of these require a lot of pressure with the thumbnail to trip.
Ed
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19 years ago
Doug To undestand why the two test produced different results you need to understand how each test load is applied. The plug in tester shunts the test current to the equipment grounding conductor of the circuit. The built in test button shunts the test current from the ungrounded current carrying conductor (the Hot) on load side of the monitoring coil to the grounded current carrying conductor (the neutral) on the line side of the monitoring coil. What you had was a high resistance neutral connection on the line side of the GFCI that would not carry enough of the test current to trip the GFCI. Since the plug in tester was shunting the test current to the EGC it had a solid path for the test current. The internal tester was trying to use the neutral and was not getting a low impedance path back to the power source. Something you did in changing out the units has restored the neutral path and now it is of low enough impedance to trip the GFCI. The bad news is that may well be temporary. You may have an intermittent bad termination or splice in the neutral. An Ideal Suretest or a manually applied dummy load with a good meter can test my theory.
-- Tom Horne
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19 years ago
installation manual on one I just got says to plug in a lamp when testing. It wasn't clear if this was just so you know it worked or because it needs a load to function correctly. Does it work any differently when you have something plugged in and turned on.
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19 years ago
But the plug-in tester showed everything correct, and 120V appliances plugged into the GFCI recep operate normally. This would eliminate an open neutral.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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19 years ago
Yep, I know. I set the end of a screwdriver on it, and leaned into it pretty heavy, after three tries with the thumbnail failed. Still no trip.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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19 years ago
Made no difference.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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19 years ago
According to Doug Miller :
If the plug-in tester showed that the hot, neutral and ground were on the right prongs, it can trip the GFCI, and two prong devices act normally, then I'm totally stumped.
Oh, waitasec, are you sure you got the load and line wires the right way around? I think you said it was the dead-end of the circuit - no feedthru, you're _sure_ the power feed is on the "line" side?
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19 years ago
Yes, yes, and yes.
Me too.
Absolutely certain. The unit ships from the factory with yellow tape over the load terminals, and I did not remove the tape. I also verified that the terminals that I connected the wires to were labelled "LINE".
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
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19 years ago
According to Doug Miller :
Assuming other stuff from this thread (you've swapped GFCI's, and the problem "stays" with _this_ receptacle box, so the GFCI is known good), clutching at straws time:
- as per someone else's posting, an intermittent neutral
- something to do with physical stress on the box (and clamp) - are there any other wires/wirenuts in the box? Are all the connections recently done/solid? Try moving the connections around. Try pushing the outlet test button with/without the tester plugged in. "Lean on" the outlet around while pushing the test button. etc.
[You're checking for loose connections that make/break depending on being pushed a bit.]- evil gremlins violating the laws of physics
- Double check that the neutral and hot are the right way around
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19 years ago
Not quite... I returned the (apparently defective) GFCI to the retailer and exchanged it for a new unit - same problem. Swapped it with another, identical, GFCI already installed and working elsewhere in the house, and both of them now work normally.
If that were the case, though, I would have expected to see the lights on the tester at least flicker. Nada. On steady the whole time.
No other wires in the box, just the one cable. No wirenuts at all. No connections to move around. Pressed the outlet test button with, and without, the tester plugged in - no change. Leaned on it, too - no change.
Yep. Tried that. No effect.
Don't know how to test for that. :-)
Tester shows the receptacle is wired correctly - and the tester *does* light up in the pattern indicating hot-neutral reverse on outlets that were known to be reversed. (A previous homeowner wasn't quite as careful as he should've been with some of his projects.)
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?