I think you have it, that is how a GFCI works. In the case of an inverter that is floating use of a ground is a bit of an oxymoron.
The principle is that every electron that comes out of one connection has to go back to the other connection. If the GFCI is directly connected to the two connections on the inverter, then two connections to the load the current should always be the same. There is no other path for the electrons to take. As Joe points out you should be able to grab either line and stand in a puddle of water without getting a shock or tripping the GFCI. There is no path for the current back to the other terminal. If you add a ground or connect to a ground now there is a second path likely to whichever side is deemed the neutral. That connection is to the power source and bypasses the GFCI. So if the ground conducts any current the flow through the GFCI is not balanced and it will trip. The ground lead does not have to connect to the GFCI, maybe it does but there is no need. The two UL standards, which may seem a bit strange at first, make sense in that they accommodate what may be already in place that the inverter is connecting to. In a power failure backup mode to a wired building there will be a ground to neutral bond already at the distribution panel if the inverter is connecting there. With no utility involved ( off grid ) the wiring codes may still require a ground and bonding in the inverter. On a boat I do not know what to recommend. If the loads ( appliances ) are all three prong, all the outlets are wired with ground and the inverter supports a bonded ground and GFCI maybe that is the way to hook it up. If the inverter auto switches from battery to shore power then you are wired normally when on shore power. If the AC is stand alone or a three pole switch completely disconnects when going to shore power wiring in a ground and GFCI is not going to hurt. Doesn't help much either. So if the inverter is floating and has no ground you cannot get a ground fault shock. If there is a ground wired in you could get a shock but the GFCI should trip. Maybe having the GFCI in is a good thing in that if there was a defect in the load you would never know it without a ground. With it the GFCI would keep tripping letting you know something is wrong. Not that there is ever any wiring problems on old sailboats .... Michael Brown Windburn C&C 30-1 From: Tortuga <tortugas...@gmail.com> "Similarly with an inverter connected to a battery that is not grounded a GFCI should never trip. Theoretically in a floating system the power has to come out of the Hot lead and return in the Neutral. There will never be a ground fault to trip a GFCI. There is no ground." I'm not an electrician, but my limited understanding is that a Ground Fault Circuit Interrupter monitors the hot and neutral leads and interrupts the circuit almost instantly if it detects a difference between them. The ground lead does not come into it. The Xantrex Freedom HF 1000 that I mentioned in an earlier post is *UL458* -listed. "The two UL standards differ in how they handle AC system grounding: *UL1741*-listed inverters must allow for the neutral-to-ground bond to *only* occur at the main AC service panel. *UL458*-listed inverters have internal neutral-to-ground switching relays to allow for this bond to occur at the inverter if in off-grid mode, OR at the utility power service if it is connected to a utility hookup." Perhaps I'm missing something. Derek Kennedy SV Tortuga, 30 mk1 Ballantyne's Cove, NS
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