I am confused as to what a galvanic isolation transformer (GIT) is and what/how it does.

Ours is an American boat - with 110V circuits and 'protected' by a galvanic isolation transformer.
The previous owner also installed a 240V Generator and added a few 240V circuits in Toodle-oo!

I am now plugged into 240V shore power and realize that I don't have a GIT specific to that circuit... - or do I? Since it operates on the earth, is the GIT working no matter what shore supply is applied? (I'm 98% sure that the earth of both circuits is connected!)

I have not noticed significant anode erosion in the past - I guess I'll find out how badly I screwed up in a couple of weeks when they haul the boat!

Bill Balme

Hi Bill,

I am no expert. My understanding:
Galvanic isolators (GIs) are not, to my knowledge, referred to as galvanic isolator transformers. They are not transformers: they just isolate. If I am correct in my former statement, I suspect you are conflating GIs with isolation transformers which are the gold standard in protecting the boat, but a whole separate order of size and complexity. It is possible that GIT may be a way of describing equipment on your side of the pond, but I have not heard it before.
A galvanic isolator (GI) is a fairly simple safety device (small in size and weight) that breaks the green ground wire of your shore power cord and the device is inserted in series. Its job is to block low voltage DC current (the kind that causes galvanic corrosion), but still allow AC current from a fault flow through thereby protecting the user from the kind of shock one might get say, from a defective toaster.

If you do have an isolation transformer (IT), unusual in cruising sailboats our size, but not unheard of (large box weighing 50-60 lbs, 20-30 kg and more complicated wiring), there are all sorts of possibilities: most are programable. Now I am operating above my pay scale, but ITs can be 1-1 (110 v to 110 v) or they can be 1-2 (110 transformed into 240 and vice versa 2-1). They also galvanically isolate, protect against some forms of shock, and ensure that the power on the boat is clean and pure and consistent even if the input power is not.
I do not believe that ITs change hertz, so one has that problem to deal with.
While on the subject, my understanding is that those of us with older boats might swap out our GIs and benefit from the advances made in design in recent years.

My best, Dick Stevenson, s/v Alchemy

Sorry Dick, Yes, mine is a GI...

So question is: I have one on my 110V shore power earth. My 240V supply does not have a GI on it's supply earth, but the earth of the 110V and 240V circuits are (I'm nearly positive) connected. So my question is: am I protected when connected to 240V shore power?

Bill Balme

In the way that you describe the connections, 110V goes through the galvanic isolator and 220 doesn't. In that configuration, the 220V is not protected. It would be easy to fix though. What amperage rating do you have for your galvanic isolator?


As an example, I have two 30A 110V inputs to my boat. They share a 60A galvanic isolator. Assuming that the 220V and 110V are never ever plugged in at the same time, you need to the have higher of the two currents protected. If both will be plugged in simultaneously, you need a galvanic isolator that is capable of carrying both input currents to ground.


In your recent post, you state that the grounds of these two systems are connected. If they are, the wire after the connection needs to be rated for the new possible current flow. If they connect before the isolator, they are both protected and if they combine after it, only one is protected.

Oh boy! Thanks Bill! Looks like you've just handed me yet another project!! I'm going to have to chase down the wiring to see where the grounds first connect. Thanks for the steer...

Bill Balme