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. 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. ;)