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There are two ways for a galvanic isolator to fail Either the diodes are shorted, or they are blown open. You can test them with a digital volt meter that can read positive and negative voltages. At any time, with the voltmeter on the DC range, put it across the shore power side to the boat side of the isolator. There should always be some residual electrolytic voltage (unless you are hauled out) so the meter should read something less than one volt. If it always reads zero, the diodes are shorted out. If it reads greater than about 1.2 volts then the diodes are open circuit. Switch to AC volts and check again since if AC is flowing, the DC meter setting may not show any activity.
Link to an easy to read introduction to electrolysis and how to troubleshoot your shore connection if you are having problems.
Different metals exposed to seawater in the whole system (boat, dock, neighbor etc) will cause a multitude of voltage potentials. If no current flows, the voltage will be higher. If you allow more current to flow, the voltage will be lower just like a flashlight battery will read a lower voltage when the lamp is on - so you could argue that higher voltages are better than lower ones as they indicate that electrolytic currents are not flowing.
On our steel boat, with a good paint job to insulate the steel from the seawater, and no current flowing from the zincs to the hull, the boat is about 0.8 volts above water voltage. As the paint job deteriorates, or the zincs are used up, the voltage will fall.
In a complex system, the actual voltage you would read across a galvanic isolator gives very little indication of what is happening with regard to electrolysis.
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