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How well will a GFCI circuit be able to detect current flows, if both the "hot" input and the (supposedly) "neutral" input to a GFCI outlet are energized?

The reason I am interested in this is that I am renting an old (1915) townhome and have discovered that an (ungrounded) wall outlet and a ceiling fixture are both controlled by a wall switch that opens the netural wire instead of the hot wire. The result is that when switch is off, the direct neutral line is open, but the netrual input of the GFCI is connected to hot. When I reported this to the landlord, their electrical contractor put in a GFCI, but they do not want to correct the open neutral. (I think it may be a knob and tube circuit with only the neutral line going through the wall switch, so fixing it may require pulling apart the wall and/or rerunning the entire circuit.)

For what it is worth, when the switch is off (and both inputs are energized), the GFCI test button will trigger a disconnect, but the reset button is not functional. But I'm not sure if the test button guarantees that the GFCI is providing effective protection in this scenario, hence my question: will a GFCI circuit work when both inputs are energized and it has no real neutral and no ground?

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It isn't real clear from your question: Is the outlet (which is where the GFCI is presumably installed) switched or unswitched? –  Dave Tweed Aug 16 '13 at 20:42
    
Both the outlet and the light fixture are downstream from and get turned "off" by the switch opening neutral. –  Jeff Aug 16 '13 at 21:30
    
And yes, the GFCI is integrated with the outlet receptacle. –  Jeff Aug 16 '13 at 21:53
    
If the junction box housing the switch receives both the hot and neutral, from upstream, it should be easy to fix. It should; I think it is against code (USA, Canada) for the two conductors of the same circuit to go through different routes in the walls. E.g. a just a neutral going to a switch and then toward the appliances, with the hot taking some other path that is nowhere near the switch. –  Kaz Aug 16 '13 at 22:10
    
@Kaz: I'm pretty certain the US codes explicitly allow for a hot/neutral pair to go to a light fixture, and for unswitched and switched hot to proceed from there to a junction box without a neutral tagging along. It's interesting to note that in that case, even though both wires going to the switch are on the hot side, the current through them should be equal and opposite. –  supercat Aug 16 '13 at 22:17
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2 Answers

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A GFCI is only measuring currents and does not care about the voltages on either of the two wires going through it.

As long as the net current is always zero, it really doesn't matter how it is switched on and off.

The real problem with a "switched neutral" is that the contacts inside the fixture are live with respect to ground even when the switch is off. If you should accidentally touch the contacts while grounded, the GFCI will protect you by tripping. But if the fixture were wired correctly, that particular risk wouldn't exist at all.

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Thanks for the warning. I actually don't think the GFCI was wired to put the light fixture downstream, so I think it is still unprotected. –  Jeff Aug 16 '13 at 21:51
    
For a GFCI to provide protection, the line neutral wire must be connected any time the line hot side is connected. Disconnecting just the neutral side is very dangerous, since even in the presence of lethal fault currents the GFCI would have insufficient power to operate the solenoid to disconnect the mains. –  supercat Aug 16 '13 at 22:55
    
@supercat: Yes, I was concerned about that too, but the OP states that the "test" button does trip the device even when the switch is off. I'm not really sure how that's working... –  Dave Tweed Aug 17 '13 at 1:07
    
I just tried the test button again with the switch off (i.e., neutral open) and the test button does indeed trip the GFCI. –  Jeff Aug 17 '13 at 2:15
    
@Jeff: If the light isn't downstream and there's no load plugged into the GFCI, then the test resistor might pass enough current to trip the solenoid. What happens if you plug in a low-resistance load like a 100W light fixture and have the fixture's switch turned on but the wall switch off? –  supercat Aug 19 '13 at 16:47
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The situation you describe is dangerous and should be totally unacceptable.

A GFCI whose hot can be connected to mains hot without the neutral connection being connected to mains neutral cannot be relied upon to provide any protection. Suppose that a switched-off light bulb represents a 10-ohm resistance between the GFCI's load hot and load neutral terminals. Suppose further that a 450-ohm path develops between the base of that bulb and ground while the GFCI neutral is switched off. What's going to happen?

The GFCI hot terminals are going to be at mains potential, and the neutral terminals are going to be at about 98% of mains potential. Even with 250mA flowing through the fault path, the potential difference between the GFCI hot and neutral terminals will only be about 2.5 volts. Consequently, it's entirely possible that the GFCI won't trip even with a fault current that's well above a level generally regarded as lethal.

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In the scenario you are describing, you say that the GFCI would not detect the current flowing out, because the currents on the load hot and load neutral terminals are similar, but Dave Tweed suggests that a GFCI detects "net current" flows. In your scenario, both flows go out from the GFCI through the bulb to ground. Can a GFCI not detect this? –  Jeff Aug 17 '13 at 2:39
    
GFIs trigger on any current imbalance over 6 mA, often 4 to 5. (0.004 Amps) See 1 and 2 –  wallyk Aug 19 '13 at 14:52
    
@jeff: A typical GFCI includes a relatively sensitive sensing circuit which, when a current imbalances is detected, feeds mains power into a solenoid which then opens the contact. In the scenario I described, the sensing circuit would have no trouble "detecting" 250mA, but would be unable to open the contact in response. –  supercat Aug 19 '13 at 15:11
    
@supercat I see what you are saying. I had thought you were saying that the 2.5V difference wouldn't be enough for the detector circuit, but you are saying the 2.5V difference wouldn't allow the GFCI to draw enough power from the mains to physically open the contact. I share that concern, but I'm having trouble imaging how the test button is working unless the GFCI device is designed to store sufficient potential energy operate the disconnect. –  Jeff Aug 23 '13 at 19:48
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