Q:

Shall I connect inverter with boat GND?

A friend of mine is building a boat. On this boat, he has 12V batteries with a 230V/13A transformer.
Recently somebody suggested he should connect the null with the ground. With DC, this is common practice. But how would somebody suggest doing the same with AC?! Polarity switches continuously on AC.
I suspect he either misunderstood the advice or the advisor didn't understand it himself either.
What might the advisor have wanted to tell? There's no reason to assume malicious intent.

A:

I think the "adviser" may have been talking about connecting the socket's earth to the inverter's neutral / cold / null point:

simulate this circuit – Schematic created using CircuitLab
With this arrangement, if there is a short or other fault causing current to flow through the earth connection, it will either cause enough current to flow to trip / blow the MCB or Fuse, or cause a mismatch in the live / neutral currents which would trip an RCD (GFCI).  You could also add an ELCB into the mix as well, and that would function normally.

