GFCI and an Open Ground

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.

.

BFL13 wrote:
...

Once I found that cut and got it all proper, no more GFCI popping from that cause. The receptacle was ok, it was that white wire touching a grounded metal part behind it.

If that metal box were not "grounded", would the GFCI still pop?

OR --if there is no ground, how can you have a "ground fault"?

Most likely it would trip, but not due to a current imbalance between the hot and neutral lines. Modern GFCIs have special circuitry to detect faults between the ground and neutral lines downstream even absent any applied load (and thus no current imbalance). If the box is connected to the GFCIs ground, which it presumably is by virtue of the GFCI's mounting screws, then it would trip for that reason.

Likewise, if the ground lead is intact back to the service entrance where it is bonded to neutral, a fault path for the current exists whether or not it is properly earthed.

In the case of an RV with a missing ground connection in the shore power cord, a hot to ground fault would in theory not trip the GFCI as there is no fault current, at least until someone walks up and mounts the entry steps with bare feet. At that point, a GFCI would trip and stop a nasty shock or worse.

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.

I agree. You cannot have a ground fault without a 3rd conductor back to the source. The intent of a GFCI is to prevent the ground wire or a human to earth ground from becoming that conductor. If the only electrical connection between an insulated RV and the power source are the hot and neutral, a GFCI cannot trip. Current will either flow equally in the 2 conductors or not at all.

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.

I had a GFCI episode inside the RV that turned out to be the white wire going into the back of the GFCI receptacle having a cut in its covering, farther back before getting to the receptacle, so some copper was showing. This copper was touching the metal box the receptacle was in, and that metal box was of course "grounded" with the usual bare copper wire for that.

Once I found that cut and got it all proper, no more GFCI popping from that cause. The receptacle was ok, it was that white wire touching a grounded metal part behind it.

If that metal box were not "grounded", would the GFCI still pop?

OR --if there is no ground, how can you have a "ground fault"?

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. The only reason there's a ground connection at all on a GFCI outlet is for the third pin safety ground for the three wire devices plugged into it. If you look at a GFCI circuit breaker for example, it only has an internal hot connection, and an external neutral connection. If you look at the eight schematics included with this 1998 GFCI patent for instance, notice that in every case, the ground is simply a pass through with no connection to the detection circuitry at any point.

BFL13 wrote:
The GCFI "does not need a ground to function", but if you have a GFCI popping, ISTR you can stop that by snipping your shore cord's ground prong. (Not a good idea!)

How can both of those things be true?

Confusing to me also. I can see a non grounded GFCI functioning in a sticks and bricks built structure where there can be alternate paths to ground. Metal plumbing would obviously provide a path. But if an RV has an ungrounded frame sitting on rubber tires and wood/plastic blocks, how can you have a ground fault? For a ground fault to occur, there must be some path to ground for the stray current to drain off causing the current differential between hot and neutral.

Educate me.

The GCFI "does not need a ground to function", but if you have a GFCI popping, ISTR you can stop that by snipping your shore cord's ground prong. (Not a good idea!)

How can both of those things be true?

(Where the popping GFCI is back at the pedestal, not a GFCI inside the RV)

LittleBill wrote:
well considering neutral and ground are tied together at the panel, it would be considered faulted 100% of the time

What if one of them shorts out? Had an issue with a receptacle a while back. Found that the neutral arced out at the buss bar. I am not giving electrical advise, just stating an issue I had and have no idea why it arced. Loose maybe?