THE GALVANIC ISOLATOR EXPLAINED
NOTE: Recent boat wiring recommendations may specify that commercial galvanic isolators be constructed with indicating and testing electronics included. This article does not contemplate these devices but refers to the millions of units installed prior to these changes.
The purpose is to disconnect your wet metal parts from the dock supply in order to prevent electrolysis. The trouble is, however, you need them connected together so that if there is an electrical short on the boat, it doesn't make the boat alive at 120 volts or worse which can give you quite a kick when you step off an aluminum dock!!
The galvanic isolator relies on the fact that electrolysis voltages are quite low - usually less than one volt - whereas electrical failure voltages are quite high. Silicon diodes, which are used to conduct electricity in one direction but block it in the reverse direction, have a built in forward voltage drop of about 0.6 volts. It is not like a resistor voltage drop - no current has to flow to create the drop - so below 0.6 volts it is disconnected, above this it conducts with very little resistance to current flow.
Since we don't know the polarity of the fault voltage, and if it is an AC fault, it will be flowing both ways, two diodes are placed in parallel pointing opposite directions so there is always one available to conduct, but at low voltages, both are switched off and no electrolytic current can flow.
Since some electrolytic voltages are higher than 0.6 volts, a good galvanic isolator should have two diodes in series in each direction to give a 1.2 volt isolation. Some also add a capacitor to increase the ability to conduct AC current, however I personally think this is a mistake as it does allow low level AC currents to flow and cause electrolytic type activity, even if not true electrolysis. This activity can remove paint from the fitting and generate chlorine bubbles that damage surrounding antifouling paint.
The diodes have to have enough capacity to pop a shore power circuit breaker if there is a short on your boat. This can require a capacity of more than 100 amps. Galvanic isolator diodes are designed to carry this current for a very short time - long enough to blow the circuit breaker plus a safety margin - but they cannot carry it for very long without overheating. They should be able to stand the shore power current rating indefinitely.
DOES MY GALVANIC ISOLATOR NEED A CAPACITOR TO CONDUCT AC?
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.
WHAT DOES THE ACTUAL VOLTAGE INDICATE?
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.
Meets all ABYC specifications except the remote monitor display.