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Old 05-09-2010, 09:52   #1
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Galvanic Isolator

My surveyor suggested I install a galvanic isolator (even though zincs last 6 months).

Does anyone have a recommendation as to the best unit/brand available?

Thanks.
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Old 05-09-2010, 09:56   #2
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I thought it was a statutory requirment in the US?.
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Old 05-09-2010, 09:59   #3
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They are very simple devices. It would be hard to make a bad one. I would go with any decent marine manufacturer.

There is no law requiring them in the US.
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Old 05-09-2010, 10:50   #4
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You may want to review the advantages/disadvantages of an isolation transformer
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Old 05-09-2010, 11:06   #5
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We installed an isolation transformer in April and I can share a downside factor which no one told us before hand.

Besides the obvious ones -- weight, bulk and expense. Which are big disadvantages.

Ours is a Victron 3600W job, which is the size and weight (and cost) of a decent battery charger.

I thought the capacity should be all right, because our shore power system (and the dock's) is only rated for 16 amps (at 230 volts). Hah! Little did I know that 16 amps is not always 16 amps. The previous system would happily absorb a short-term load of 20 or 24 amps, and I think it could handle 20 on a longer term basis too. At any rate, I was able to run all of the equipment on board without particularly thinking about it.

The Victron is absolutely strictly limited to 16 amps, and if you exceed that for a millisecond, it shuts down. That means I have to carefully plan what equipment is used simultaneously. And even with careful planning, it trips almost once a day, when I'm on shore power. A pain in the buttocks.

And the upshot? My zincs are being eaten up just as voraciously as they were before, so I have not realized any benefit yet despite all the expense and hassle.

Just one sailor's experience, for whatever little it may be worth.
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Old 05-09-2010, 20:04   #6
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I recommend and install the fail safe units by Dairyland Electrical Industries. These folks have been making GIs for many years to support the dairy and pipeline industries. They are solidly built pieces of equipment that do not require monitoring because they are designed to fail safe. Here is their website: Home | DEI Marine

Hope this helps.
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Old 06-09-2010, 13:34   #7
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And the upshot? My zincs are being eaten up just as voraciously as they were before, so I have not realized any benefit yet despite all the expense and hassle.
Is your transformer wired as an isolation system or as a polarization system? If the latter, than a galvanic isolator is still required. If the former, than there is no way galvanic current can flow via the shore power ground IF it's wired properly. If you have a bonding system on your boat, even with an isolation system, stray current from other sources besides your boat can still flow into and out of your underwater fittings and eat your zincs.

Eric
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Old 06-09-2010, 13:46   #8
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Is your transformer wired as an isolation system or as a polarization system? If the latter, than a galvanic isolator is still required. If the former, than there is no way galvanic current can flow via the shore power ground IF it's wired properly. If you have a bonding system on your boat, even with an isolation system, stray current from other sources besides your boat can still flow into and out of your underwater fittings and eat your zincs.

Eric
No, as an isolation system, of course.

Yes, I know that theoretically stray current (not galvanic, I think) cannot flow through shore power.

Yes, the boat has a good bonding system. I'm not sure how that facilitates stray current from other sources but I'm all ears.

In any case yes the zincs are being eaten up.
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Old 06-09-2010, 14:00   #9
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And the upshot? My zincs are being eaten up just as voraciously as they were before, so I have not realized any benefit yet despite all the expense and hassle
I presume as part of the isolation transformer, you seperated the incoming protective earth wire from the boats one. Also where did you terminate that incoming protective earth wire,

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Old 06-09-2010, 14:33   #10
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No, as an isolation system, of course.
Why do you say "of course"? Both systems are widely used with the polarization system actually being the safer way to do it.

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Yes, I know that theoretically stray current (not galvanic, I think) cannot flow through shore power.
Galvanic isolators are used primarily to prevent galvanic currents which are generally under 1 volt and will eat your zincs rather slowly. If your shore power ground is truely islolated from your boat ground and bonding system and your zincs are being eaten quickly, this is most likely from stray currents. These are currents already flowing in the water from other boats and/or shore power problems. These currents will always seek the path of least resistance. If your boat is in the path of these currents, they can flow into one or more of your underwater fittings and out through others including your zincs which can be eaten away rather rapidly.

Eric
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Old 14-09-2010, 21:54   #11
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Better to have the isolator and not need it than to need it and not have it. When you plug in that grounded shore cord you are greatly increasing your chances of galvanic corrosion.
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Old 15-09-2010, 01:04   #12
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I presume as part of the isolation transformer, you seperated the incoming protective earth wire from the boats one. Also where did you terminate that incoming protective earth wire,

Dave
Well, naturally, that's the whole point. The ground (earth) wire from shore goes nowhere except into the isolation transformer. The boat's ground (earth) system is connected to the other side of the isolation transformer.
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Old 15-09-2010, 01:07   #13
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Why do you say "of course"? Both systems are widely used with the polarization system actually being the safer way to do it.
Well, you learn something every day. Never heard of this. It's not in Calder's book and it's not mentioned in the Victron instructions. I'll have to investigate.




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Originally Posted by fairbank56 View Post
Galvanic isolators are used primarily to prevent galvanic currents which are generally under 1 volt and will eat your zincs rather slowly. If your shore power ground is truely islolated from your boat ground and bonding system and your zincs are being eaten quickly, this is most likely from stray currents. These are currents already flowing in the water from other boats and/or shore power problems. These currents will always seek the path of least resistance. If your boat is in the path of these currents, they can flow into one or more of your underwater fittings and out through others including your zincs which can be eaten away rather rapidly.

Eric
I always thought that the main problem was stray currents using your ground connection to the shore. I hadn't really thought of this -- current simply going in one side of the boat and out the other. It would seem like a much weaker phenomenon. But it would explain my case, wouldn't it.
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Old 15-09-2010, 02:20   #14
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Dockhead,

If you have a isolation traffo, there is no situation where you can get impressed corrosion.

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These currents will always seek the path of least resistance. If your boat is in the path of these currents, they can flow into one or more of your underwater fittings and out through others including your zincs which can be eaten away rather rapidly.
Sorry this is nonsense, electrically. if so removing the bondng would cure it. But its not whats happening here.

If your zincs are been eaten away quickly with an traffo then there is a ground fault somewhere. The primary reason for such corrosion , is ground fault currents in the protective earth wire , teh very thing a traffo is designed to remove.

The other thing is that it could be DC stray current corrosion from your batteries, Are they normally switched off, what about bilge feeds etc, anything that might be feeding DC current into the bilge water?
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Old 15-09-2010, 02:45   #15
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Dockhead,

If you have a isolation traffo, there is no situation where you can get impressed corrosion.



Sorry this is nonsense, electrically. if so removing the bondng would cure it. But its not whats happening here.

If your zincs are been eaten away quickly with an traffo then there is a ground fault somewhere. The primary reason for such corrosion , is ground fault currents in the protective earth wire , teh very thing a traffo is designed to remove.

The other thing is that it could be DC stray current corrosion from your batteries, Are they normally switched off, what about bilge feeds etc, anything that might be feeding DC current into the bilge water?
Conflicting advice here. One says stray currents can flow through the boat even if there is no ground connection to the shore at all. Another says this can't happen; there must be a ground fault or DC stray current on board.

I don't really know what to believe. In any case, however, I do understand that an onboard stray DC current can wreak havoc. I have three electric bilge pumps on board, and since my boat lives on a marina berth with shore power when I'm not sailing, it is connected to a battery charger all the time. I wouldn't notice a stray DC current. When I'm gone I basically shut down the DC electrical system by isolating the house batteries, but a few things, including the bilge pumps, are wired directly.

I'll have to crack the Calder books again and find the procedure for testing for this. I suppose if my bilge pumps are leaking current I would be able to find a potential difference with a multimeter.

Still, that wouldn't explain the intense consumption of the prop anode, which is far away from the bilge pumps. I would think that bilge pump leaky current would attack the hull anode which is much closer.

I guess I'm just as confused as ever, but thanks everyone for trying.
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