Galvanic isolators

[duncan99210]

If there's just one cable running to a keel bolt, the chances are its for lightening protection from the mast, not a connection to the boat DC -ve.

 

[Plevier]

It has both and they are shown on the factory wiring diagram, it's not someone's additions.

If there's just one cable running to a keel bolt, the chances are its for lightening protection from the mast, not a connection to the boat DC -ve.

 

[Plevier]

Ok you're making the general point that the isolation is important. Fully agree and have checked it with a meter and will make sure it is never bridged. Actually familiar with the setup - my co-owners previous boat had to have the SD out for a repair and the yard didn't put back all the insulating washers. Luckily he checked it and they had to pay to sort it.

it was the DC neg connection to the keel that rang an alarm bell ... but only a quiet one,

Your situation is comparable in that in both cases the saildrive is isolated from the DC negative, just by different methods. In your case by insulating it from the engine. In Nigel's case by insulating the whole engine and saildrive from DC neg.

The primary cause of Nigel's problem was a fault on the wiring which then connected the block to DC neg. In your case a fault that connects your SD to the block or DC neg could have the same result. Hence the warning in your engine manual.

 

[Plevier]

I think the exposed lead may well be as effective as copper. However I'm not convinced that either is desirable!

Yes, I think a copper plate would be an option. Either that, or as you say, not linking the systems at all ( I don't, my shore power is also very simple). But I just think that linking AC earth to DC negative without providing a good path to earth (sea) means going half way, but with no real benefit.

 

[KellysEye]

>Galvanic corrosion is caused by dissimilar metals, ie metals well separated in the galvanic series, immersed in the same pool of electrolyte, namely seawater, being connected together electrically. This could be anodes on one boat and the steel hull of another, or the steel piling around the marina, connected by the earth conductor of the shore power system.

The idea that galvanic corrosion can be caused by steel, aluminium boats or a steel piling is a myth. All metal boats have epoxy paint (zinc rich on steel) then antifoul so no metal parts are in the water. Steel piling goes rusty quickly and becomes electrically inactive.

>Can you please explain how a mix of salt and fresh water is the cause. Surely it's just diluted seawater?

I am surprised that people don't know that mixing fresh and salt water it produces electricity. There was a science programme on TV years ago that showed them being mixed and a small light bulb lit up. It is already being used commercially. See here: http://news.mongabay.com/2005/1219-wetsus.html

>Can you expalin the purpose of the GI in the 12 volt battery circuit. A GI will not block current originating from a 12 volt source

The galvanic isolator must be rated for the size of your shore power circuit AC supply and the 12 volt system. It does block a 12 volt current you can check that online.

>Calcium reacts with water and will not form a covering on the anodes. Perhaps you meant to say calcium carbonate but Ca2+ ions are not attracted toward the anode in normal circumstances. It's the anions eg Cl- which are attracted to the anode. If you have calcium carbonate building up on the anodes something very odd, which requires investigation, is happening. Electrolysis perhaps.

I have never see a boat hauled out, including ours, that doesn't need it's anodes cleaned. Calcium and Calcium Carbonate are both found in salt and fresh water both have a small electrolysis element when dissolved which is why anodes get pitted.

 

[JumbleDuck]

I am surprised that people don't know that mixing fresh and salt water it produces electricity..

It doesn't. It is possible to extract energy if you have a supplies of fresh and salt water, but you don't just mix them - you either put them of opposite side of ion-exchange membranes (reverse electrodialysis) and get electricity out or on each side of a semi-permeable membrane to build up pressure on the salt side as fresh water moves through (osmosis). Just mixing fresh and salt water will, as VicS says, give you diluted salt water (or polluted fresh water), with no electricity whatsoever.

 

[vyv_cox]

I have never see a boat hauled out, including ours, that doesn't need it's anodes cleaned. Calcium and Calcium Carbonate are both found in salt and fresh water both have a small electrolysis element when dissolved which is why anodes get pitted.

The white deposit on anodes is zinc hydroxide, known as 'white rust' http://www.azom.com/article.aspx?ArticleID=1220. It's exactly the same deposit that occurs on galvanised chain left wet in the locker. It occurs quite rapidly on anodes in fresh water, inhibiting the protection of metal attached to it.

I don't know what 'a small electrolysis element' is supposed to mean. Anodes get pitted because they corrode, along with the vast majority of other metals.

 

[salar]

I understand how galvanic isolators work but I don't understand why a shorepower earth has to be connected to a boat -DC. Can someone please help me by explaining why that is necessary, if the shorepower has a RCD in the supply and presumably a working earth connection?

 

[BabaYaga]

I understand how galvanic isolators work but I don't understand why a shorepower earth has to be connected to a boat -DC. Can someone please help me by explaining why that is necessary, if the shorepower has a RCD in the supply and presumably a working earth connection?

I could well be wrong, but my understanding is that the requirement of the ISO standard is for the shore power protective earth line to be connected to boat earth (the sea) so that there is an alternative path to earth if the lands side earth line is faulty. Boat earth could be DC negative, provided that this is connected to the sea, e g by way of the engine - shaft - prop.

 

[JumbleDuck]

I understand how galvanic isolators work but I don't understand why a shorepower earth has to be connected to a boat -DC.

The idea seems to be that if the shorepower fails in such a way as to make the +12V line live at 240V, you have earthing protection. I suppose it may have happened somewhere, sometime.

 

[VicS]

>Galvanic corrosion is caused by dissimilar metals, ie metals well separated in the galvanic series, immersed in the same pool of electrolyte, namely seawater, being connected together electrically. This could be anodes on one boat and the steel hull of another, or the steel piling around the marina, connected by the earth conductor of the shore power system

The idea that galvanic corrosion can be caused by steel, aluminium boats or a steel piling is a myth. All metal boats have epoxy paint (zinc rich on steel) then antifoul so no metal parts are in the water. Steel piling goes rusty quickly and becomes electrically inactive.

You may be lucky that the rusting steel hull next to you has an intact epoxy coating below the waterline, Aluminium boats can suffer from galvanic corrosion but AFAIK not cause it, Anyway I never mentioned aluminium. Rust does not form an impermeable layer. That is why steel continues to rust and why it can continue to cause galvanic problems

>Can you please explain how a mix of salt and fresh water is the cause. Surely it's just diluted seawater?

I am surprised that people don't know that mixing fresh and salt water it produces electricity. There was a science programme on TV years ago that showed them being mixed and a small light bulb lit up. It is already being used commercially. See here: http://news.mongabay.com/2005/1219-wetsus.html

The techniques discussed in your link have already been explained by JD. The other involving the light bulb was probably a process in which fresh and salt water are alternately admitted to a cell equipped with special electrodes.
None of these are relevant. No membranes and no cells with special electrodes!

>Can you expalin the purpose of the GI in the 12 volt battery circuit. A GI will not block current originating from a 12 volt source

The galvanic isolator must be rated for the size of your shore power circuit AC supply and the 12 volt system. It does block a 12 volt current you can check that online.

A galvanic isolator must be rated to withstand to very high fault current that can flow briefly before the fuse blows or circuit breaker trips.
They usualy block only about 1.2 volts, some 2.4 volts. Where do you get them to block 12 volts?

>Calcium reacts with water and will not form a covering on the anodes. Perhaps you meant to say calcium carbonate but Ca2+ ions are not attracted toward the anode in normal circumstances. It's the anions eg Cl- which are attracted to the anode. If you have calcium carbonate building up on the anodes something very odd, which requires investigation, is happening. Electrolysis perhaps.

I have never see a boat hauled out, including ours, that doesn't need it's anodes cleaned. Calcium and Calcium Carbonate are both found in salt and fresh water both have a small electrolysis element when dissolved which is why anodes get pitted.

Vyv has told you what the deposit on anodes is and why they become pitted. Calcium exists in solution as calcium ions, Ca²⁺. Calcium carbonate is insoluble. Calcium ions migrate towards the cathodic surfaces. Over protection can lead to calcium carbonate deposition, but not on the anodes.

 

[salar]

The idea seems to be that if the shorepower fails in such a way as to make the +12V line live at 240V, you have earthing protection. I suppose it may have happened somewhere, sometime.

I'm not sure how that could happen as the shore power is not connected to 12v except through a charger (in my case). Would it be acceptable to link the earth cable to something hanging over the side? It would only need to be there when shore power is connected. That would mean the boat metalwork was not linked to earth and not so vulnerable to galvanic action.

 

[KREW2]

I connect to shore power when I go into a marina.
As far as I know I have no G.I. but I do have a sterling type stepped charger which keeps the batteries topped up.
I do have a couple of 13 amp sockets which are connected via an RCD which I never seem to use.
As I only tend to run 12 volt things, mainly lighting, phone charger and sometimes the eberspacher do I need to fit a G.I.

 

[VicS]

I connect to shore power when I go into a marina.
As far as I know I have no G.I. but I do have a sterling type stepped charger which keeps the batteries topped up.
I do have a couple of 13 amp sockets which are connected via an RCD which I never seem to use.
As I only tend to run 12 volt things, mainly lighting, phone charger and sometimes the eberspacher do I need to fit a G.I.

If you only occasionally connect to shorepower, probably no.

If no connection exits between the shore power earth and your anodes or other underwater appendages, no.


OTOH if you are connected to shorepower for pronged periods, even if not actually using it, and it is connected to the anodes and other underwater bits and pieces, yes.