Ok so just what is a Galvanic Isolator (GI), why do I need one or have one & why on Earth would I need to test one?
Why do I need one? A GI is a device that is inserted, in series, into the green grounding wire (safety ground) of your shore power feed to help minimize stray direct current / DC from flowing into your vessel. While blocking stray DC current it also has to allow for the passage of AC fault current. GI's are a simple hook up and installation. If plugging in at a marina a GI is the bare minimum level of protection a boater should have. I do prefer an isolation transformer (IT) to a GI but the conversation of a galvanic isolator vs. isolation transformer is a whole other discussion & topic.
To install a GI all one needs to do is to break the green wire after the shore power inlet but before the AC panel. Cut the green wire in two, crimp on two terminals and connect them to each stud. Very simple. The purpose of a GI is to block low voltage stray DC current while still allowing any fault current to pass through the green wire to ground and allow it to activate fault protection devices.
This blockage of low voltage stray DC current is achieved by using two diodes in each direction. Each diode drops approximately .6V or requires more than .6V to open and "Flow". Two of them in series results in approximately a 1.0V - 1.2V threshold for blocking stray DC current. They normally have two diodes in each direction so the green wire is not "check valved" and acts just like a wire would. The only difference is it acts as one that won't pass voltages below 1.0V - 1.2V. Simple and pretty effective at blocking stray DC current.
What is a diode? Think of a diode as an electrical check valve. It allows current to flow in only one direction but not in the other direction. One of the inherent traits of diodes is the voltage drop associated with them, which is usually around 0.6V. In a galvanic isolator application however they have used this often assumed bad trait of a diode to an advantage. By wiring two diodes in series you now have a device that can block any stray current below 1.0V - 1.2V from flowing into or out of your vessel.. This essentially blocks any DC currents caused by dissimilar metals being connected in the marina when you "plug in".