The galvanic isolator uses semi-conductors to isolate the boat from low voltage DC currents carried on board on the AC ground wire. A galvanic isolator is a relatively small lightweight device that contains diodes and a capacitor arranged in such as way that they will not allow currents to pass when the voltage is less than about 1.2 volts. As the currents that cause corrosion are almost always less than 1 volt the protection is achieved, while the diodes still allow higher voltage current to pass in the event of a fault. While not exactly cheap these galvanic isolators are less expensive than their alternative, the isolation transformer.
Isolation transformers are big, heavy and expensive. They achieve isolation by providing an electromagnetic connection between the boat and the dock. There is no wire directly from the dock on board the boat, shore power goes to the primary windings of the transformer when it generates an electromagnetic field that is picked up by the secondary windings and serves the boat. No actual wire connects the primary and secondary windings, it is all done my magnetism. The shore ground wire stops at the transformer primary. A new ground wire is made at the secondary windings.
So these two very different ways both achieve the same result. How can we compare and contrast them and say which is the best?
Size weight and cost:
Transformers are big heavy and expensive so on that basis they lose.
Because galvanic isolators contain semiconductors they are more prone to damage from power surges and lighting strikes. The transformer is more resilient to surges and lightning, it could still be damaged by a direct hit but is less likely to be damaged by a power surge due to lightning down the line. Modern galvanic isolators are almost always of the "fail safe" kind. This doesn't mean they can't fail, it just means that if they do they will fail in the closed position and will maintain the safety ground even if they do not maintain isolation. If a transformer is damaged in a lightning strike it probably won't function at all. So on the question of reliability the transformer wins.
Transformers have other uses, and galvanic isolators do not, so the "other uses" usually ends up being the deciding factor. Transformers can easily accommodate boats that travel the world and need to plug into different voltages. Transformers can also be used to adjust or condition power, for example many docks in the US have 208 volt power supplies and transformers can be used to boost it to 240 volts for use on board. There are other specialized applications for transformers that I wont go into here.
In summary the decision to use an isolation transformer or a galvanic isolator boils down to availability of space, weight carrying ability, expense and whether the other uses of the transformer are considered desirable for the boat in question.
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