gfci and n+g short

You are correct. It should have said that a *downstream* N-G short would trip it, not an upstream short. This is a standard function in GFCIs made today. By design, a GFCI will trip with an imbalance of current L-N, or a downstream N-G fault. They now also must sense reversed line polarity. The L-N imbalance can be caused by a leakage current path, or improper wiring such as a multi-wire branch circuit.

There are situations where other circuit wires installed in close proximity to the GFCI can occasionally cause a false trip due to their effect on the sensing transformer. There are also occasional reports of tripping caused by voltage transients or a nearby source of RF energy. I can regularly trip two GFCI breakers randomly at my facility by intermittently connecting a solenoid voltage tester (Wiggy) to a third non-GFCI circuit fed from the same sub-panel!

From the information I have seen over the years, I believe that failure to trip is a more widespread problem, primarily due to MOV failures causing destruction of the adjacent circuitry. The new UL standard requires much greater tolerance to transients, and the MOVs are being isolated from the rest of the circuitry to reduce the likelihood of this problem occuring.

Ben Miller

A downstream N-G short will trip one but the upstream side is not monitored.

| I was doing some reading on gfci's and they referred to a neutral to ground | short upstream of the gfci would trip it. | | aren't the neutral and ground buss connected and wouldn't this be seen as a | short ? | | also are there any known devices or configurations that trip gfci's even | when they are functioning properly ?

There can be confusion as to which is "up" and which is "down". To avoid that risk, I'll use different terms.

Ground an neutral are bonded together at the service entrance. From there power goes through a branch breaker and eventually to a GFCI receptacle and is connected to the "line" side. The receptacle outlets, as well as the screws labeled "load" on the other side, are thus protected.

The GFCI device looks for a current leakage by some means, usually with a small current transformer with both the hot and neutral wires running in parallel through the single CT. As long as all the current going out one wire comes back through the other, they cancel and the CT sense zero net current and all is well. As soon as the neutral is connected to the ground wire at the load, then the current that would normally just flow on the neutral alone can now also flow on the ground wire. And some of it actually will flow on the ground wire. However much does flow on the ground wire, it is that much which does not flow on the neutral, and does not cancel out the hot wire current, which leaves the CT with a net current. When that current is equal to or higher than some value between 2 milliamps and 6 milliamps, the GFCI is supposed to open the circuit and prevent the current flowing.

If neutral and ground are connected together on the load side but there is no load, the GFCI may, or may not, trip open as a result. If other circuits are sufficiently out of balance between the two opposite hot wires in the split single phase system, then that voltage on the neutral can go out to the load and back over the ground (parallaling it's already low resistance path back to the transformer neutral connection). It can be enough to trip the GFCI. But if things are in balance, there may be zero voltage on the neutral and it won't see anything to trip on.

Connections between neutral and ground that are on the line side of the GFCI won't be detected since they won't result in any current through the CT of the GFCI.

GFCI devices in portable cords and extension devices also include things to detect damage that such cords could have happen such as an open ground. I've not seen how this is done, but I would suspect that a high resistance is connected to the hot wire and to the ground wire through a current transformer or other sensor. The a circuit detects if the current is flowing and if there is also voltage present. If voltage is present and that tiny current is NOT flowing, then it opens the circuit. This is a bit more tricky because you have to make sure that plugging or unplugging the cord does not false trip the circuit by having voltage detected before the test current is detected. So there might be a time delay in there. This kind of test for ground wire continuity is needed because otherwise damage to the ground wire would not prevent load operation, though without the protection the ground wire provides. Cascading two such cords could be tricky if the test current on one can trip the GFCI of the other. So I would think they would have to have test currents well below the 2 ma level. But that could also make it more susceptable to problems like RF energy.

Battery operated power tools are obviously safer, but not all tools can be practically run on batteries.

No load is actually required. They do it "proactively" by injecting a current onto the neutral wire by means of a second toroidal transformer. If there is an N-G fault, the injected current will flow through the large resulting loop between the fault and the service panel, with only the N current going through the sensing transformer. This causes a net sensing current, and trips the circuit.

Ben

No load is actually required. They do it "proactively" by injecting a current onto the neutral wire by means of a second toroidal transformer. If there is an N-G fault, the injected current will flow through the large resulting loop between the fault and the service panel, with only the N current going through the sensing transformer. This causes a net sensing current, and trips the circuit.

My apologies if this shows up twice. My news server seems to be doing funny things to my messages, so I am not always sure if they are being posted or not.

Ben

|> Connections between neutral and ground that are on the line side of the |> GFCI won't be detected since they won't result in any current through the |> CT of the GFCI. | | No load is actually required. They do it "proactively" by injecting a | current onto the neutral wire by means of a second toroidal transformer. If | there is an N-G fault, the injected current will flow through the large | resulting loop between the fault and the service panel, with only the N | current going through the sensing transformer. This causes a net sensing | current, and trips the circuit.

Assuming the required N-G bond at the service entrance has not failed and opened up, etc.

How would you wire this on a 3-wire dual voltage circuit? I would seem to me that if two such devices wired in cascade did this, and chose different phases to power the current injection, it could cancel.

| My apologies if this shows up twice. My news server seems to be doing funny | things to my messages, so I am not always sure if they are being posted or | not.

N/P

The circuits I have seen use a half-wave signal for the injection (one diode in series with the coil). If the polarity & phasing of the coils is standardized (I don't know offhand if it is), then it would seem like another device on the opposite phase of a multi-wire circuit would either add to the signal or fill in the missing half phase, either of which would produce the required current.

Ben Miller

|> How would you wire this on a 3-wire dual voltage circuit? I would seem to |> me that if two such devices wired in cascade did this, and chose different |> phases to power the current injection, it could cancel. | | The circuits I have seen use a half-wave signal for the injection (one diode | in series with the coil). If the polarity & phasing of the coils is | standardized (I don't know offhand if it is), then it would seem like | another device on the opposite phase of a multi-wire circuit would either | add to the signal or fill in the missing half phase, either of which would | produce the required current.

Ah, RFI!