DOES MY GALVANIC ISOLATOR NEED A CAPACITOR TO CONDUCT AC?
First a link to an easy to read introduction to electrolysis and how to troubleshoot your shore connection if you are having problems.
NO. The galvanic isolator is designed to carry 130% of the maximum shore power AC current rating you are using.
However very small AC leakage currents can reduce the level of DC protection available from a galvanic isolator.
There is no easy answer to the capacitor question. The use of a capacitor is sufficiently controversial that the ABYC continue to list it as optional in their specifications.
The theory is that if AC is leaking to ground through your galvanic isolator, then the isolating diodes are going into conduction for a percentage of each half cycle and while they are in conduction, they are no longer suppressing any superimposed DC current. The DC potential would be added to one half cycle and subtracted from the other. If the total voltage when they are added exceeds about 1.2 volts then there is a path for just a portion of the DC voltage you are trying to block.
So by adding a capacitor you are trying to provide a path for the AC current while still blocking the DC. The size of the capacitor boils down to what you consider acceptable leakage current from the AC equipment on your boat and you have to size the capacitor so the voltage across it at this current is typically small compared to the DC voltages you are trying to avoid. If you are protecting from DC voltages typically in the 0.9 volts range maximum, then you have to keep the AC voltage across the capacitor to less than 0.3 volts.
What is an acceptable level of leakage? If you want to protect against a leakage of 100 ma, then the equivalent resistance required of the capacitor is 0.3/0.1 = 3 ohms. It is really not that simple because we are dealing with peak voltages, nor RMS so you really need less than this. To get 3 ohms will require a capacitor of 10,000 microfarad. This has to be an AC capacitor which rules out using electrolytic capacitors. The voltage rating only needs to be 1.5 volts or more because the diodes will limit any voltages to this level.
Perhaps 100 ma of leakage is too unrealistic to conduct to ground. Would 10 ma be a more acceptable figure? Now you're down to a 1,000 uF capacitor which is somewhat more reasonable. But AC capacitors are typically in the 10uF range maximum which would protect only 1 milliamp of leakage.
Therein lies the dilemma and the original question. What is acceptable leakage? My philophosy is you shouldn't be trying to cover up a problem by conducting it to ground. I maintain you should check every item on your AC supply and if any show any leakage at all they need to be repaired. That is a safer alternative to just allowing the leakage to flow to ground.
You can easily determine if you have any AC leakage - take an AC voltmeter and put it across your galvanic isolator. The reading should be less than about 0.25 volts. If it is not, start turning off breakers until you track down the culprit and repair it.
Keep in mind that some surge protectors and power line conditiners can also include filter networks that may included capacitors connected to ground. These will introduce a low level of AC current in the ground lead and may contribute to the problem. Again they can be easily checked using the technique in the previous paragraph.
It is not uncommon, especially on older boats, that you are unable to eliminate AC leakage. In this case, the addition of a capacitor in parallel with the isolator is the best solution. We make an add-on Galvanic Capacitor for this purpose. It can bypass up to 5 amps of AC leakage while maintaining the DC Galvanic Isolation. Download the owner's manual from Galvanic Capacitor Owner's Manual for further information.