What is a galvanic isolator and why should my shore power system have one?

A galvanic isolator is a device used to block low voltage DC currents coming on board your boat on the shore power ground wire.  These currents could cause corrosion to your underwater metals; through hulls, propeller, shaft etc.

Boats in a marina plugged into shore power all act as a giant battery.  They are all connected together by the green shore power ground wire, which is (or should be) connected to their DC grounds, engine block, and bonded underwater metals.  If the boats are in salt water then that forms an electrolyte and the dissimilar metals connected together act as a battery, causing corrosion.

The galvanic isolator has two pairs of diodes set up so that a voltage of about 1.2 volts is required to cause them to conduct.  As most DC voltages caused by galvanic action will be less than this, they are blocked. Good quality isolators also contain a capacitor, which only conducts AC current, as a backup.

Normally no AC current is carried on the shore power ground wire, but it has to be able to carry the full load of the circuit in the event of a fault.  Therefore it is important to have a good quality unit that will not overheat when required to carry the rated load.  Some heat will be generated by the voltage drop and the unit must be able to withstand this.

As the galvanic isolator fulfills such a key function in the AC circuit it is only prudent to use the best quality unit available.