Thanks for the detailed explanation but it went in one eye and out the other
(here are some pics of the set up) I'll get the son in law to read this, he should understand it better than I ever will.
You can just see the black spot on the bottom front bolt on the right of the cabinet which goes through to a steel wall where the inverter touched and sparked, it's just about impossible to get the wiring wrong and the batteries are all earthed and interconnected with Subaru battery wiring set ups so pretty sure they are strong enough.
This thing I just don't understand.
Battery set up, power & earth comes through the plywood wall on the left from the feeder battery and as you can see both battery's are individually earthed from the earths that were on the Subi cable setups
