GFCI and an Open Ground

BB_TX wrote:

I have looked at a number of web sites and numerous GFCI diagrams and have not seen even one that had any circuit comparing neutral to ground. The only wires showing in those diagrams were hot and neutral. The circuits were comparing current flow difference between hot and neutral.

I'm not surprised. It's close to impossible to find a web site that doesn't totally ignore the downstream ground-neutral fault detection, even though it has been built into GFCIs for well over 10 years. The app note for the GFCI chip describes how an oscillation is induced that indirectly measures the downstream ground-neutral resistance. The grounded neutral detection is resistance based, the trip spec being 2 ohms. Here's one web document that includes the grounded neutral detection: https://www.nema.org/Products/Documents/NEMA-GFCI-2012-Field-Representative-Presentation.pdf

BB_TX wrote:
I have looked at a number of web sites and numerous GFCI diagrams and have not seen even one that had any circuit comparing neutral to ground. The only wires showing in those diagrams were hot and neutral. The circuits were comparing current flow difference between hot and neutral. And I see no way those circuits can detect impedance between neutral wire and ground wire or any additional circuits that could.
Perhaps I have not found the correct diagram to show that. Being an electrical engineer (retired) I really want to understand where that circuit resides and how it works.

I'm an active EE and tried to explain it above. The detection is the same instrument that detects the difference in current between hot and neutral. It's a transformer formed between a sensor coil and the AC wires (inductive coupled). The difference with detecting ground/neutral shorts is that the GFCI needs to introduce a little current into the system.

The diagrams don't show the ground wire in their pictures but they way they work is by inducing current into the ground-neutral loop that is formed between the service panel bonding and the fault. By coupling an AC signal onto that loop via the neutral wire within the GFCI, it will be sensed by the receiving coil. Without the fault, equal currents will be induced in hot/neutral or (none at all with nothing plugged in to complete the circuit).

This ground loop concept is not unlike the 60Hz hum you get with audio equipment. Power supplies couple current into the ground path of the audio cables. The "detector" is the speakers. ;)

DrewE wrote:
.......
It's a question of semantics, really, at this point.

The GFCI will detect a short between the neutral wire and the ground wire downstream from it. Whether or not either one connects to earth is immaterial for that detection. conductor! ........

........ If it's in the RV, then it matters not one whit whether the ground (or neutral, really) is tied to earth or anything else; the GFCI can detect an unacceptably low impedance between the two wires, and trip because of it. In this case, the ground lead is not open ended--it goes back to the GFCI.
.....

I have looked at a number of web sites and numerous GFCI diagrams and have not seen even one that had any circuit comparing neutral to ground. The only wires showing in those diagrams were hot and neutral. The circuits were comparing current flow difference between hot and neutral. And I see no way those circuits can detect impedance between neutral wire and ground wire or any additional circuits that could.
Perhaps I have not found the correct diagram to show that. Being an electrical engineer (retired) I really want to understand where that circuit resides and how it works.

BB_TX wrote:

But the description describes a "grounded neutral" which goes back to the question of how can you have a "grounded neutral" when there is no ground connection at all to the trailer. Current does not flow into an open ended wire. And an ungrounded frame is effectively an open ended conductor.

It's a question of semantics, really, at this point.

The GFCI will detect a short between the neutral wire and the ground wire downstream from it. Whether or not either one connects to earth is immaterial for that detection. It's called a grounded neutral because the neutral wire is connected to the ground wire, using the conventional names for the wires. (Just be glad they didn't go with the terminology of the NEC where it calls them the grounded conductor and the grounding conductor! They're accurate enough descriptions, but it does make understanding what is going on rather tedious at times.)

If the GFCI in question is feeding the RV electrical system as a whole, and the ground lead between it and the RV is broken, then of course it won't trip because of a neutral to chassis ground fault in the RV, as it doesn't see its ground and neutral lines bridged together. If it's in the RV, then it matters not one whit whether the ground (or neutral, really) is tied to earth or anything else; the GFCI can detect an unacceptably low impedance between the two wires, and trip because of it. In this case, the ground lead is not open ended--it goes back to the GFCI.

Nothing in a GFCI's operation has anything to do with stuff being earthed or not.

road-runner wrote:
The downstream neutral-ground fault detection does not use the current imbalance detector, but rather a different mechanism. An example can bee seen here: http://www.onsemi.com/pub/Collateral/FAN4147-D.pdf

Diagram on page 2, description starts near the bottom left of page 5. Older designs did use 120 Hz. signal injection, but still used a separate transformer for detection.

The "different mechanism" described in that document still uses the current imbalance detector; there is only one detector transformer. What I had found was the 120Hz style you mention but it too only had one detector transformer. The one you linked isn't really fundamentally different in that it still relies on inducing current for neutral/ground short detection. Instead of inducing a constant signal, it instead injects an amplified version of what it measures at the imbalance detector. This provides the positive feedback they describe where a small mismatch in current amplifies back and induces a larger one, which is in turn re-detected. The reason for this design is also stated: It's very sensitive to even small (high resistance) connections between neutral and ground because the current mismatch is amplified.

In either design the injection transformer surrounds the neutral and hot wires. With nothing plugged in, no current can be induced in those wires since the circuit is open. However if there is a neutral to ground connection downstream there is now a complete loop formed because the neutral and ground are also connected in the main service panel upstream. The transformer can now cause current to flow around this neutral/ground loop. Since the detection transformer sees this current in the neutral with nothing in the hot to cancel it, it will trip if the current is high enough.

To be honest, I'd never really looked at how GFCI's work until this thread and am thankful for the discussion as it helped me to learn a little in the process.

BB_TX wrote:

But the description describes a "grounded neutral" which goes back to the question of how can you have a "grounded neutral" when there is no ground connection at all to the trailer. Current does not flow into an open ended wire. And an ungrounded frame is effectively an open ended conductor.

For current flow, there can't be a grounded neutral when there's no ground connection. That's what prompted my speculation a bunch of posts ago that if the shore cord's ground pin is not connected, downstream neutral-ground fault detection will be lost.

road-runner wrote:

When I looked at how they do this, I discovered that the GFCI simply induces current into the wires which in turn trips the very same current imbalance detector. So while no external current is flowing, there is some inside the device. Regardless, an RV without a ground path will not trip from current imbalance or shorted neutral & ground.

The downstream neutral-ground fault detection does not use the current imbalance detector, but rather a different mechanism. An example can bee seen here: http://www.onsemi.com/pub/Collateral/FAN4147-D.pdf

Diagram on page 2, description starts near the bottom left of page 5. Older designs did use 120 Hz. signal injection, but still used a separate transformer for detection.

But the description describes a "grounded neutral" which goes back to the question of how can you have a "grounded neutral" when there is no ground connection at all to the trailer. Current does not flow into an open ended wire. And an ungrounded frame is effectively an open ended conductor.

When I looked at how they do this, I discovered that the GFCI simply induces current into the wires which in turn trips the very same current imbalance detector. So while no external current is flowing, there is some inside the device. Regardless, an RV without a ground path will not trip from current imbalance or shorted neutral & ground.

The downstream neutral-ground fault detection does not use the current imbalance detector, but rather a different mechanism. An example can bee seen here: http://www.onsemi.com/pub/Collateral/FAN4147-D.pdf

Diagram on page 2, description starts near the bottom left of page 5. Older designs did use 120 Hz. signal injection, but still used a separate transformer for detection.

Bobbo wrote:

BB_TX wrote:

Dutch_12078 wrote:
As has been stated, a GFCI trips when it detects an imbalance between the hot and neutral. The ground has nothing to do with it. ......

That part I understand and am not questioning the lack of a ground on the GFCI outlet itself. What I don’t understand is how you can have a ground fault in an ungrounded RV that would trip a GFCI outlet. To have a ground fault that would trip a GFCI outlet there must be some path for the errant current to drain to a ground source. Otherwise there would never be an imbalance in the individual currents thru the hot and neutral pins of the outlet.

The GFCI outlet has circuitry to detect a connection downstream from itself of a NEUTRAL/GROUND short even if no current is being used. It will trip. I have a BONDING PLUG for my Honda generator so it will work on my EMS-HW30C equipped trailer. If I plug that into a GFCI outlet, the outlet will trip instantly, even though the BONDING PLUG draws no current whatsoever so there can be no current imbalance.

When I looked at how they do this, I discovered that the GFCI simply induces current into the wires which in turn trips the very same current imbalance detector. So while no external current is flowing, there is some inside the device. Regardless, an RV without a ground path will not trip from current imbalance or shorted neutral & ground.

All theories aside, I just installed a GFI outside yesterday (a recent contractor forgot to replace it). I completely forgot to attach the ground and the GFI worked fine. When I plugged in my tester I saw an open ground and had a :S moment and pulled the GFI out and attached the ground.

Dave