Galvanic Isolator -- When to Use

dread · Posted: Fri Jul 20, 2012 6:27 pm Post subject: Galvanic Isolator -- When to Use

When I am attached to shore power, do I always want the galvanic isolator on? When I am at sea, I assume that it doesn't function, just as the charger doesn't, right?

Captains Cat · Posted: Fri Jul 20, 2012 7:26 pm Post subject:

I think you can't "turn it off". It's in the shore power circuit.

Charlie

Will-C

Charlie,
Our boat came with one I wonder how to check and see if it is working. I never saw a switch for it. Question

D.D.

ferret30 · Posted: Fri Jul 20, 2012 10:16 pm Post subject:

Many of them are passive, just a box with 4 diodes inside that won't let current through until the voltage differential is over some amount (i.e. ground fault).

There's a fail safe type that will short the input to the output if the diodes fail. The non-fail safe types are supposed to have a remote display that indicates if it's operational.

None of them should have any effect while not on shore power, since they are only connected to the ground between the inlet and the AC panel, and even under normal conditions while on shore power, there shouldn't be any current through the ground. That's why there's no switches.

Captains Cat

journey on · Posted: Sat Jul 21, 2012 12:36 pm Post subject:

For those of you with C-Dory 25's, each one ( as far as I know ) come with a galvanic isolator. This is a passive box, that in Journey On sits in back of the AC Panel. As ferrit30 said, it's there to block stray currents fed through the green wire, whilst allowing ground faults through. I think it's just another example of the general level of quality in a C-Dory. I've had more expensive boats without that feature, which I installed myself. And several cheaper boats that didn't.

Remember, a C-Dory is basically a trailerable boat with an outboard which can be lifted out of the water so that there isn't a sure path for stray DC currents. I always wondered how they operated. The unit in Journey On is large enough that I thought it contained a transformer that allowed AC (60 Hz) to go through, but blocked DC. A diode bridge would pass AC in a fault but would have to be large enough to withstand the fault current. If a diode blew, there's no fault protection. They do come in different sizes and prices. C-Dory doesn't use the cheapest one.

Anyone have a schematic or take one apart?

Boris

ferret30 · Posted: Sat Jul 21, 2012 12:59 pm Post subject:

Captains Cat · Posted: Sat Jul 21, 2012 1:25 pm Post subject:

dread · Posted: Sat Jul 21, 2012 2:39 pm Post subject: Galvanic Isolator

On my 4-month old (for me) 2006, the 110v switch panel has a switch for the charger and a switch for the isolator. I turned the charger switch on yesterday, but the little green "on" light did not come on. I looked at the charger itself, and its indicators showed that it was functioning. I have never turned the isolator switch on, but I wonder when I should. Your comments suggest that, since I keep it in the water, I should always turn it on to prevent damage. Right?

ferret30 · Posted: Sat Jul 21, 2012 3:12 pm Post subject:

I have no idea why the isolator would be switched. It should really be wired near the inlet, between the inlet ground and the panel ground bus. What model is your panel and isolator?

Maybe the isolator switch on the panel turns on the 'monitoring' panel and self test functionality? Non-fail safe isolators (I think according to ABYC) are supposed to have an easily viewable status indicator, and/or self test feature.

The really cheap isolators do not have status indicators or fail safe, and are just the 4 diodes. If anything goes wrong with those, your DC system could be floating and you could potentially send AC into the water (via a shorted charger, for example).

Captains Cat · Posted: Sat Jul 21, 2012 4:00 pm Post subject:

According to the only owners manual I can find on this site, the galvanic isolator isn't switched.

http://www.c-brats.com//files/cdory_25_manual.pdf

The wiring diagram is an April '06 version, revised in August '07. It's back around page 64. Got a feeling that it depends on who happened to wire the boat.. Rolling Eyes

Charlie

journey on · Posted: Sat Jul 21, 2012 11:17 pm Post subject:

Captain's Cat/Charlie;

Thanks for the schematic(s). After looking through some, but not all, there appears to several ways to isolate the green/chassis ground. Several had series diodes, 2 in each direction as ferrit30 mentioned above, several had transformers and West Marine had a capacitor. Others which were called galvanic isolators were really complex with active circuits. You pays your money and takes your choice.

However, all were passive, i.e., not switched. Accept that. I like the ones which had no solid state devices to fail.

I found the Guest isolator which is on Journey On, but no schematic. I missed it.

Boris