Galvanic Isolator or Isolating Transformer, which is best and why?
All shore power appliances on your boat are always connected through the earth wire on the shore power cable to appliances on all the other boats in the marina. The potential dangers to valuable underwater fittings is well known, so there must be an important reason for it. The earth pin on a three pin plug is fatter and longer than the other two, and so it is fair to assume that it is carries the most current or is in some way the most important. Actually, the earth pin plug does absolutely nothing, or hopefully, does absolutely nothing ever. The reason that it is fatter is to prevent it being pushed into the wrong hole and it is longer so that it makes contact before and after the other two.
So why do we have to have it? And why do some appliances not have it? The answer is to provide an easy route to earth in the event of a fault developing in an appliance. On any metal cased appliance if the the live comes into contact with the casing, which is earthed, it will conduct a current to earth through the wire rather than through you. Appliances that are double insulated, usually by having plastic casings, are not fitted with earths as there is no way that the live supply can come into contact.
So it is quite safe to disconnect the earth wire for 99.9999% of the time. That is provided it can be re-instated when necessary. The purpose of a GALVANIC ISOLATOR is to disconnect the earth wire for all voltages below 2 volts. A quick look at a galvanic scale of metals shows that the maximum potential difference or voltage that can be generated by dissimilar metals is about 1.8 volts between magnesium and gold, an unlikely combination agreed! But it is taken as the threshold. Your vessel is insulated from all the other vessels for galvanic voltages. But at all voltages above 2 volts the earth connection is re-instated, thereby maintaining a protective route to earth in the event of a fault.
An ISOLATING TRANSFORMER is completely different although the effective galvanic protection is the same. In this device there is never any continuous earth wire connection. It is a large transformer that effectively generates the vessels 240 volt AC supply within the vessel, therefore the earth connection need only be routed back to the transformer within the vessel for the vessels own ground fault protection to work.
A residual current device (RCD) doesn’t actually need connection to earth for it to work. It compares the supply current flowing through the live to the return current flowing through the neutral as they should always be the same. If there is any imbalance it is assumed that the current is returning to earth another way, i.e. possibly through you, so its breaks the supply.
So which is best? Isolating transformers are big heavy and expensive, whereas galvanic isolators are small, light and cheap. Case closed! You may think that, but galvanic isolators have their limitations. They will not protect from voltages above 2 volts. Marina pontoon electrical supplies work in very difficult environments which can frequently have faults. These can often result in the earth wire carrying a small voltage. This voltage is not enough to cause a problem itself, and will not be sensed by an RCD as it only monitors the live and neutral wires. It will however cause the galvanic isolator to go into continuity thereby defeating its purpose.
The other limitation is that galvanic isolators can fail. They are usually designed like all electrical devices to ‘fail safe’. That is to fail into a safe condition for the earth leakage protection function of the earth wire. So they fail into continuity which is not ‘safe’ for the vessel. As many of the cheaper devices are simply two terminal, in and out, there is no means by which the failure of the device can be monitored. As there should be no current flowing in the earth wire there is nothing to reliably power a monitoring system. You could have one that has been giving no effective protection for years without knowing it.
So the old adage is that you get what you pay for. Isolating transformers are clearly the best if you are prepared to pay for it. Particularly wise on a steel or aluminium hulled vessel. A galvanic isolator can do much the same job but look for a system with a proper monitoring function with a display at the chart table if possible.