Isolation Transformer Issues

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Bay Retriever

Senior Member
Oct 13, 2019
Vessel Name
Bay Retriever
Vessel Make
2004 Mainship 400
Has anyone had any issues with Mainship 400s, or others, that have Isolation Transformers affecting the GFCIs? With mine (2004 Mainship 400 original)the symptom is no ground when using a GFCI tester.


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Are you having issues with a GFIC socket on the boat or are you tripping a GFIC breaker on the dock?
Are you having issues with a GFIC socket on the boat or are you tripping a GFIC breaker on the dock?
I am having issues while hooked up to shore power. The GFCIs(one in head and one in master) do not operate/trip properly. My electrician has tested all sorts of things, including bypassing the Transformer. When the Transformer is bypasssed, both GFCIs operate properly.

So we are wondering if anyone else is having a problem. We are tentatively going to order a replacement Charles Marine Isolation Transformer, however they are very expensive $2,500 and heavy 225 lbs. The goal would be to test the system with the replacement Transformer and possibly return it if there is still a problem with the GFCIs ground while winder load. Another work around that has been mentioned is to keep the current transformer and add a Galvanic Isolator to handle the problem with the GFCIs not operating properly.

Honestly it’s way above my pay grade and the only reason I have been able to repeat some of these technical aspects is because it’s so freaking expensive and at the same time so very important to get right from a safety aspect. So any help is much appreciated.
We need details of how the isolation transformer is wires on both side, especially the shield, ground, and neutral to ground bonding.

BTW, Charles exited the transformer business quite a few years ago. So any product you might find is very old stock. And I don't know what Charles's warranty and support policy is on discontinued products like that.
To properly diagnose this we’d need detailed information on how it’s wired up and the switch gear and panels on the boat.
An isolation transformer should never stop a gfci from tripping. Obviously, something’s not right with the way it’s wired or it could be, although not likely, the transformer has an internal issue.
This issue has finally been resolved after my ABCY Technician finished testing and retesting everything within the boat’s electric system. In the end, the original Isolation Transformer was diagnosed as improperly grounded and needed attention to get the boats two GFCI’s to read as “grounded”.
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