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. 

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How to test a galvanic isolator that doesn't have a monitoring system..

Galvanic isolators are a bit difficult to test.  Current ABYC standards require that galvanic isolators be self testing.  For testing older isolators without this feature there are a number of methods that can be used, the one that follows is one that I found most useful.

Unplug the boat from shore power before starting the test.  
Disconnect one lead of the isolator so that you are testing it only.  Get a digital multimeter set to the diode test function.  Put one lead on one side of the isolator and the other lead on the other side.

As the capacitor starts to conduct current  the reading should rise to approximately 0.9 volts.  Remove the test leads, short the two wires of the isolator together to discharge the capacitor and repeat the test with the test leads reversed. You should get the same answer.

Interpreting the readings:

• If the reading is instantly 0.9 volts then the capacitor is defective or there is no capacitor.
• If a voltage of 0.45 volts is observed one of the diodes is shorted.
• If there is a reading of 0 volts then both diodes could be shorted.
• If there is a reading in excess of 0.9 volts then one or both diodes are open (not conducting) in which case you should stop the test before the capacitor reaches 2.0 volts or you will damage it.

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