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Old 06-19-2012, 08:14 PM   #16
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GFI problem


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Originally Posted by curiousB

Not true. GFCIs are differential transformers that sense imbalance in current across hot and neutral conductors. If there is no load there is no current. Therefore they won't trip.
GFCIs have a second toroidal coil that injects a small 120Hz signal which induces equal voltages on the H and N wires. If the N and G are touching on the load side of the GFCI current will flow through the N to the G via the short and back to the N via the connection at the service panel. Since there is no current flow on the H wire, there is a current imbalance and the GFCI will trip with NO LOAD.

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Old 06-20-2012, 05:42 AM   #17
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GFI problem


Does the GFCI trip regardless of which receptacle (on the GFCI or in one of the downstream boxex) the load is plugged into?

Does the GFCI trip when you plug something in but before you turn it on?

Does the GFCI trip when you press your thumb on the receptacle without plugging anything in?

Do all brands of GFCI units have a second toroidal coil that maintains a small current flow that would trip if neutral had a fault to ground downstream with no other load drawn?

OT: How much power is consumed by the dozen or so GFCI units in a home wired to the 2011 National Electric Code, when, say, you are on vacation and unplugged everything?
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Last edited by AllanJ; 06-20-2012 at 05:48 AM.
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Old 06-20-2012, 07:30 AM   #18
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GFI problem


Just tried a test on 1 of my bathrooms GFCI:

This is a dedicated 20 amp receptacle with no connections to the load side, the lights and fan are connected to the line terminals.

Tested with the TEST push button and my GFCI receptacle tester, GFCI tripped as expected.

Installed a temporary jumper from the neutral to the ground, GFCI did not trip.

Plugged night light in, GFCI immediately tripped and would not reset.

Removed night light and receptacle would then reset.

This receptacle was installed 7 years ago when I bought this house but it might be older than that as it is one I had in stock.

Did not have a new GFCI available to test. I don't know if this second toroidal coil feature is something that has been added to newer GFCIs.
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Old 06-20-2012, 12:44 PM   #19
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GFI problem


Quote:
Originally Posted by HouseHelper View Post
GFCIs have a second toroidal coil that injects a small 120Hz signal which induces equal voltages on the H and N wires. If the N and G are touching on the load side of the GFCI current will flow through the N to the G via the short and back to the N via the connection at the service panel. Since there is no current flow on the H wire, there is a current imbalance and the GFCI will trip with NO LOAD.

I wasn't aware of this. I don't believe it is all GFCIs but it does exist. Do you know if it is a mandatory UL requirement or just a competitive differentiator? I learned something new today.

In any event it isn't related to OP as his situation is related to plugging in a load and that event tripping the GFCI. The crossed neutral to ground fault would be detected with just power in the circuit and no load if the GFCI he has supports that feature.
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Old 06-22-2012, 03:08 PM   #20
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GFI problem


Since my last post on this thread (#18) I have been bothered by what others have said regarding a ground neutral short causing a GFCI to trip with no load. As I said in post 18, my test did not cause a trip. These GFCI's were installed in 2005 and may have been a few years older than that. Today I purchased a brand new Leviton Renu 20-Amp White on White Tamper Resistant GFCI and installed in my bathroom. I tested by plugging in a 3 conductor plug with the ground neutral shorted. The GFCI tripped immediately without a load and would not reset until I removed the plug.

Apparently newer GFCIs have circuitry to detect a ground neutral short without a load on the circuit. I apologize if my previous post has mislead anyone.

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