The galvanic isolator consists internally of two diodes which are
connected in anti-parallel fashion. When they are connected in this way,
the diodes allow current in both directions but only above a certain
threshold voltage. The voltage at which diodes conduct is about 1.4 Vdc.
The isolator is installed directly behind your boat's 230V connection.
The forward voltage from the galvanic isolator is higher than the
potential difference between metals. As a result, this voltage will not
allow conduction and as such, the galvanic isolator will prevent any
electrolytic current. However, if there is a (higher) error voltage in
the AC circuit, the diodes will allow current through and the residual-
current device will break the circuit.