GFCI breaks on tiny load

[this has been tackled elsewhere, but not answered!]
Plug a drop light into GFCI - the light lights up! Plug drop light into house AC - the light lights up.
Plug an extension cord into GFCI and test with polarity tester - all is well with gnd, hot and neutral.
Plug (same) drop light into above extension cord and the GFCI "blows" (i.e. cuts the circuit breaker).
The resistance of the extension cord only looks like a bigger drop light bulb. So resistance is not the problem.
Plug a "cheater" into extension cord (above) and plug drop light into the cheater (i.e. ground wire is not connected) making sure that there are no possible ground contact points - GFCI "blows" [see above] ! So the grounding is not a problem.
When the GFCI compares the AC current in with the AV current out it is obviously comparing similar phases (a.k.a. cycles). Clearly any phase shift due to capacitance in the extension cord COULD cause what we see.
- Rick
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wrote:

I have seen this happen too. The GFCI circuit only takes about 5ma of ground current to trip it. I think you could get this due to capacitance between conductor in a long extension cord.. I had my trouble tryin to use an electric weed whacker. It doesnt even have a ground connection. I megged the extension cord thinking it may be bad but there was no leakage at 1000 volts. Another extension cord worked fine.
Jimmie
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ground current to trip it. I think you could get this due to capacitance between conductor in a long extension cord.. I had my trouble tryin to use an electric weed whacker. It doesnt even have a ground connection. I megged the extension cord thinking it may be bad but there was no leakage at 1000 volts. Another extension cord worked fine.
I have 2 each 100' (#12 wire) extension cords. I can plug BOTH into an outside GFCI outlet without tripping.
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RickMerrill wrote:

My guess is it is either a crappy extension cord or an flakey extra sensitive GFCI that is the problem.
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On Mon, 01 Jun 2009 11:53:32 -0400, RickMerrill

There is a concern with capacitance in the wiring tripping GFCIs but you need a very long circuit to see it. I suspect you have a lot of leakage in that circuit and the cord pushes it over the edge.
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snipped-for-privacy@aol.com wrote:

There is no leakage in the circuit because a perfectly parallel outlet behaves properly - the problem is entirely due to the cord, but why that is I am not sure. It could be that the cord is an "el cheapo" and has all kinds of leakage problems that don't show up until a real load is put in it. I'm chopping it down from 25' to 6' and see what happens!
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On Mon, 01 Jun 2009 14:36:46 -0400, RickMerrill

I bet the cord has a neutral to ground short in it. or the neutral and ground are swapped Try it with the cheater on the GFCI end but be careful of the ground tab on the cheater. If the light doesn't work there will be 120v on the ground tab..
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snipped-for-privacy@aol.com wrote:

There is no short in the cable. It is fine until a (small)load is added (other than the polarity tester).
Using just 6 feet of the same cable - Blat!
Cheater has NO ground tab ;-)
Polarity tester says the wiring is correct!
Note: the wires in this cord are in parallel; the insulation is yellow. the wires are #16.
???
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On Mon, 01 Jun 2009 17:36:57 -0400, RickMerrill

If the neutral and ground are shorted you will never know until you plug it into a GFCI and put a load on it.
What happened when the 2 pin adapter was plugged into the GFCI and the cord plugged into it?
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On Jun 1, 6:52pm, snipped-for-privacy@aol.com wrote:

That was my first thought too. And it doesn't have to be anywhere near a complete short, just enough to amount to the milliamps needed to trip the GFCI.
On the other hand, I vaguely remember that maybe GFCI's actually try to put a tiny current on the downstream load line all the time to be able to detect this type of fault without any actual load being connected at the far end?

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snipped-for-privacy@optonline.net wrote:

Yes they do. There is info for a GFCI IC at: http://www.national.com/ds/LM/LM1851.pdf To detect a downstream crossed N-G only a second current transformer and a capacitor are added. If the GFCI is wired H-N reversed it will still detect a downstream "N"-G short.
A *really* old GFCI may not have this feature - don't know when it was added to the UL standard.
N-G reverse on the cord as suggested above? (A receptacle tester won't find a N-G reverse.)
If 6 ft of the cord trips the GFCI does the other length of cord trip it?
Do either of the lengths of cord trip other GFCIs?
Sounds like the GFCI is a breaker. That means the cord adds to the capacitance of the rest of the circuit and could push it "over the edge" - also suggested by gfretwell above.

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