The galvanic isolator prevents electrolytic corrosion. It blocks low-voltage DC currents entering the boat through the shore cable of the shore supply. These currents can cause corrosion of metal parts below the waterline, such as the hull, propeller and propeller shaft, and others.
The galvanic isolator has two diodes inside it that are connected in an antiparallel manner. When connected in this way, the diodes allow current to pass in both directions, but only above a certain voltage threshold. The voltage at which the diodes are conductive is about 1.4 Vdc.

The isolator is installed directly behind the 230V connection of your boat. The direct voltage coming 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 erroneous voltage in the AC circuit, the diodes will allow the current to pass and the residual current device will break the circuit.

Victron VDI-16 Galvanic Isolator Manual – GDI000016000

See Manual Galvanic Isolator Victron VDI-16 – GDI000016000