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RandallT · 26-09-2022, 04:29

Greetings,
Without triggering a new debate on whether DC grounding should or should not be connected to AC bonding/grounding systems, I have a question about a couple of boats I've been working on recently that have a curious installation of galvanic isolators.

The first boat, a 2002 47' Beneteau, has 2 GI's, wired in series, behind the chart table amidships. One end connected to DC - and the other end to AC safety ground. Shore power comes in at the stern and the green wire first connects to a ground bus for all the through hull bonding system. There is also a black conductor connecting this bus to engine/battery- system. So there'a a hard connection between AC and DC grounds as well as a soft connection through the GI's.

Second boat is a 30' power boat, 2000-ish, twin mercruiser V8's. There are 2 GI's mounted behind the electrical panel amidships, again wired in series and connecting the DC ground to the AC ground. Shore power comes in at the stern and only connects to the bonding system and the AC distribution panels. No connection to DC ground system, (other than through the GI's.)

So,.... in no article or standard or practice or install guide for galvanic isolators have I seen any reference to wiring 2 GI's, in series, between AC and DC grounds. All show a single GI installed near shore power entrance, in line with the AC shore ground green wire.

I thought I'd throw this out to the collective intelligence and see if anyone had any thoughts on the rationale for this. Or is this just another example of boat owners watching too many hairbrained YouTube DIY videos, LOL.

Thanks for listening....
Randall

goboatingnow · 26-09-2022, 04:45

That’s very wierd all right makes no sense

RandallT · 26-09-2022, 06:00

Quote:

Originally Posted by goboatingnow

That’s very wierd all right makes no sense

Exactly our response when we encountered the first one. Thought it was just a freak thing. But then to work on another one this way within the same month, well, that's what prompted my post!

Rumrace · 26-09-2022, 06:46

I had a SeaRay Twin and it had Mercathode wired that way on each engine. The Shore power is some distance away beside the port cabin entry.
My understanding is producing negative ions from either voltage. I’ve seen the tiny ground wire corrode quickly in Ford Trucks where the Unit was widely used.
Sure works on cars, motor homes and my Rays underwater gear only ever needed cleaning and new wax

YANDINA · 27-09-2022, 06:29

Two Galvanic Isolators connected in series can add protection in "dirty" docks where AC voltage on the ground wire can exceed the 1.2 volts isolation of an isolator. I've measured AC voltages as high as 3.6 on a dock. Each time the AC voltage exceeds the isolation DC current flows through with the AC and eats up zincs.

With two isolators in series you get 2.5 volts of isolation. On Aluminum boats where the natural voltage of the aluminum can use up as much as 0.9 volts of isolation, a dual isolator is essential.

Yandina sells a 50 amp 2.5 volt galvanic isolator that meets ABYC specifications for everything, except of course for the double isolation voltage, at a price less than most single isolators.

goboatingnow · 27-09-2022, 07:40

Quote:

Originally Posted by YANDINA

Two Galvanic Isolators connected in series can add protection in "dirty" docks where AC voltage on the ground wire can exceed the 1.2 volts isolation of an isolator. I've measured AC voltages as high as 3.6 on a dock. Each time the AC voltage exceeds the isolation DC current flows through with the AC and eats up zincs.

With two isolators in series you get 2.5 volts of isolation. On Aluminum boats where the natural voltage of the aluminum can use up as much as 0.9 volts of isolation, a dual isolator is essential.

Yandina sells a 50 amp 2.5 volt galvanic isolator that meets ABYC specifications for everything, except of course for the double isolation voltage, at a price less than most single isolators.

Yep I just finished building mine. Essentially 4 diodes In series.

Total cost including enclosure , $20!

RandallT · 01-10-2022, 07:50

Thanks, Ann! That clears that puzzle up nicely.

boatpoker · 01-10-2022, 08:39

Quote:

Originally Posted by YANDINA

Yandina sells a 50 amp 2.5 volt galvanic isolator that meets ABYC specifications for everything, except of course for the double isolation voltage, at a price less than most single isolators.

Does it have a capacitor ?