GFCI outlet with N/G bonded(just kidding) MPP Solar all-in-one inverter on battery power questions.

[meetyg]

So it seems that the inverter messes with the neutral while inverting. When running on shore power (pass through) is the only time the N-B bond is there. The simple fix would be a relay across the inverter input, using the normally closed contacts to make a N-G bond on the output side. If you don't have power coming from the shore, the relay contacts will make a bond.

That's the brute force method of relay logic that worked for my 40 year traffic signal maintenance career. I think using the program #38 and inverter contacts might be more elegant.

That's how I solved the N-G bonding issue on my AIO: Normally Closed relay/contactor, powered by grid (open).
Works fine. I do have some worries about the lifetime of the contactor (coil powered most of the time, except for power outages). But I got some spares, just in case one would fail.

It does take a few seconds for the inverter to reconnect to grid when grid power is back, meanwhile the contactor is open. This means that the output will float during those few seconds, but I'm fine with that. I could have used a time-delay contactor, but didn't know this existed at the time.
I might upgrade in the future.

 

[BobShrunkle]

Well unfortunately I have zero experience with relays. I wouldn't know where to start unless I could find some documentation or instructions.
Not sure about program 38 either without seeing some examples and so far I haven't had any luck.

 

[BobShrunkle]

I just temporarily bonded the output side of the inverter N-G and everything seemed to function as it should.
On battery power, grid side disconnected.
GFCI test button worked
Circuit checker showed correct and test button worked
119v L-N
119v L-G
0v N-G
Continuity(obviously) N-G
Continuity between input and output ground
No continuity between input N and output N

 

[TacomaJoe]

I just temporarily bonded the output side of the inverter N-G and everything seemed to function as it should.
On battery power, grid side disconnected.
GFCI test button worked
Circuit checker showed correct and test button worked
119v L-N
119v L-G
0v N-G
Continuity(obviously) N-G
Continuity between input and output ground
No continuity between input N and output N

This shows the need for the N-G bond when on battery power. If you left that bond in there and then plugged in the shore cord, you would have two bonds which is a no-no. The relay solution makes it an automatic operation.

I did poke around and saw a couple bonding boxes, so they're out there. Also saw a grounding box that was pure bunk (a wooden box with dirt/minerals in it). I'm a DIY'r so that would be my direction.

 

[BobShrunkle]

I finally got a response from MPP Solar....

Output N-G is automatically bonded in inverter (battery) mode and this is meant to address any RCD application at the output side which could trip if it senses a differential between N-G. This is known as the N-G bond feature.

1. LINE MODE -- N-G output will remain open, letting the AC mains panel decide (which should be closed anyway).
2. BATTERY MODE -- N-G output will bond (closed).

Why the RCD trip if there’s no leakage current?

Best Regards
Lucas Lee
_______________________________________________________________________________________________________________________

My response.

Measurements from the inverter with what you are saying has an internal N-G bond on battery power.
Output:
119v L-N
105v L-G
5v N-G
No continuity on N-G output side when not powered
GFCI test button works on outlet
Circuit tester shows correct circuit but circuit tester won't trip GFCI when on battery power. (It will when on grid power)

VS

Temporarily bonded the output side of the inverter N-G and everything seemed to function as it should.

On battery power, Grid side disconnected.
GFCI test button worked
Circuit checker showed correct and test button worked
119v L-N (Correct)
119v L-G (Correct)
0v N-G (Correct)
Continuity N-G (Correct)
Continuity between input and output ground
No continuity between input N and output N