Which is the best choice? A Galvanic Isolator or an Isolation Transformer?

Until relatively recently this was quite a simple question. With a simple set of answers.

It came down to this.

Galvanic isolator.

Advantages

Cheap
Very simple to install

Disadvantages

Unit can fail without symptoms thus requiring regular checking. The failure could either endanger life by failing open circuit or remove the galvanic protection by failing short circuit. In fairness none of these failures are likely unless there is a large fault which blows the shorepower main circuit breaker. But the possibility does exist. The solution is to test the galvanic isolator after any problem with the AC system has been rectified, plus periodic checking.

Isolation Transformer

Advantages

The possibility of symptomless failure is almost zero. A failure will usually result in failure of the AC supply.

Disadvantages

More Expensive
Heavy
More complicated to install

And that was it. The choice was made on those points alone.

The goalposts have now been moved and the decision is no longer this straightforward.

In order to understand why, it is necessary to look at how each device breaks the connection between the shorepower earth and the hull of the boat.

Here is the internal schematic of a standard galvanic isolator.

It goes in-line with the earth lead between the shorepower plug on the quayside and the AC electrical system on the boat. Each diode in the isolator will drop around 0.6 volts before it starts to conduct. So 2 in series will require 1.2 volts before any conduction takes place. There are two diodes facing one way, and two facing the other way to enable AC fault currents to flow in both directions (thus tripping the circuit breaker or RCD).

This means that the galvanic currents (which are usually between 0.4 and 0.8 volts) which cause galvanic corrosion are blocked from flowing. They would have to exceed 1.2 volts in order for a current to flow. They never exceed this level due to the metals involved and the water. This is the protection method to prevent galvanic corrosion. This is how they work.

Now let's look at an isolation transformer. This is the correct way of wiring up such a device. You may see diagrams of them wired up in a different way. They are wrong.

Note that there is simply no connection between the shorepower earth and the earth on the boat. This is what protects the vessel from galvanic corrosion.

Note also that the Neutral and Earth are bonded on the output side of the transformer. This recreates exactly the same sitution as we normally receive from the national grid and is what allows fuses and RCDs to operate in the correct manner.

Finally note that there is a "safety screen" between the primary winding and the secondary winding which is connected to shorepower earth. This is so that any fault current in the primary (for instance from insulation breakdown) is returned to shorepower earth to trip the circuit breaker or RCD rather than electrifying the boat hull.

In effect, the transformer recreates a totally isolated AC supply just like having your own mini AC power station. Totally isolated from anythng else.

So that explains how each device protects against galvanic currents which cause galvanic corrosion. Now we need to move on to explain what has changed that now makes isolation transformers a better choice.