GFCI Question

Little Bill

Little Bill

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Tennessee NEC:2017
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Semi-Retired Electrician
If the grounded conductor is in contact with the EGC downstream of a GFCI receptacle, with no loads on, will the GFCI trip? Downstream is all load side of GFCI.
 
Newer GFCIs have grounded neutral detection, and will trip when the neutral is in contact with the EGC. The current version of UL 943 requires grounded neutral detection and trip.
 
Aa Wayne mentioned, there is additional circuitry inside of GFCIs to detect a neutral to ground fault.
The reason for such detection is that a neutral to EGC connection after the GFCI would allow the
neutral to be a parallel path for ground fault current to flow. With this connection, the fraction of the current in a fault to the EGC that flows back through the neutral in the GFCI will not be sensed by the GFCI, because it is equivalently subtracted from the fault current on the ungrounded (hot) conductor.
 
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Little Bill said:
What if the EGC was not on the GFCI grounding screw?
Click to expand...

The internal GFCI circuitry will have no connection to the grounding screw, and the GFCI function is totally independent of it.
I think the only thing electrically connected to the grounding screw is the yoke of the device that's used for mounting it to a box.
 
synchro said:
The internal GFCI circuitry will have no connection to the grounding screw, and the GFCI function is totally independent of it.
I think the only thing electrically connected to the grounding screw is the yoke of the device that's used for mounting it to a box.
Click to expand...
That is what I thought. Now the question, if the EGC is not connected at the GFCI, how does the GFCI know there is a neutral to ground fault downstream?
 
Little Bill said:
That is what I thought. Now the question, if the EGC is not connected at the GFCI, how does the GFCI know there is a neutral to ground fault downstream?
Click to expand...

I believe that various techniques for neutral to ground fault detection are used. I remember that one method has a separate toroid through which all the circuit conductors pass though, and its windings are driven with a small signal at a higher frequency than 60 Hz. And so it's a little transformer where the branch circuit wires are the secondary of the transformer. If there's a neutral to ground fault downstream, there will be a closed circuit on the secondary of this transformer and so a signal current will flow on the neutral conductor. The other toroid used for detecting ground faults will then sense this current and cause the GFCI to trip.
 
synchro said:
I believe that various techniques for neutral to ground fault detection are used. I remember that one method has a separate toroid through which all the circuit conductors pass though, and its windings are driven with a small signal at a higher frequency than 60 Hz. And so it's a little transformer where the branch circuit wires are the secondary of the transformer. If there's a neutral to ground fault downstream, there will be a closed circuit on the secondary of this transformer and so a signal current will flow on the neutral conductor. The other toroid used for detecting ground faults will then sense this current and cause the GFCI to trip.
Click to expand...
What about with no load?
 
I was working on a panel that has a 30mA GFPE breaker on the feeder main upstream, I had a branch circuit off and cut a cable and to my surprise the GFPE feeder main tripped. Now possibly 30mA flowed in the brief instant the wires were all shorted when i cut the cable but I suspect it had some kind of signal or other way of knowing.
 
Little Bill said:
What about with no load?
Click to expand...

What I was describing did not involve any kind of load. All that was necessary to form a closed circuit around the GFCI is a N to EGC bond at the disconnect and a N to EGC connection downstream of the GFGI. The extra toroid couples a signal current only in the neutral, which then flows back through the EGC in this case. A L-N load is not necessary to detect a L-EGC fault, because the toroid transformer is supplying the signal current that flows through a closed circuit consisting of only the neutral conductor and EGC in this scenario. Of course, the line voltage needs to be present at the GFCI input in order to power its electronics.

Without this extra circuitry dedicated to detecting a N-EGC connection, if such a connection was present a L-G fault would eventually trip the GFCI but at a higher required fault current (depending on the relative resistance of the neutral and EGC, which determines how much fault current gets diverted back through the neutral).
 
Here is what prompted the question.
A fellow has the following set-up.
GFCI receptacle with load side to single pole switch
From switch to a row of duplex receptacles with adapters to plug in a light bulb
He found the GFCI tripped but the switch was off, so no load on the circuit.
GFCI would not reset
Load wires removed & GFCI holds

Due to difficulty getting downstream of the GFCI & switch, he's trying to TS all he can at the GFCI and switch.
He changed the GFCI with a new one, still trips with load wires connected, holds with load wires disconnected.

Obviously there is a ground fault downstream. Question arose as to why it would trip with the switch off.

I gave him several things to check, which would be looking for a fault, both with a meter and visually. Another guy was chiming in with his TS advice. I questioned it as I don't think that is correct. He is saying if you remove the EGC from the GFCI terminal and the GFCI holds, that would tell you you had a fault to the EGC. I don't buy that as the EGC on the device itself isn't monitored through anything inside like the sensor the hot & neutral run through. Am I off base here?
 
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