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Old 08-03-2007, 09:36 PM   #15
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Originally Posted by Auzzee View Post
So, providing the vessel is connected to a normal, reliable grid, is the primary benefit compatability, or protection?

David.
Without a doubt, the primary benefit is protection against galvanic corrosion. Doesn't come cheap though. Despite that, I have decided to go all the way and invest in a complet, although simple, AC system.

Aye

Stephen
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Old 08-17-2007, 11:07 AM   #16
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Hi Stephen,

Please will you (and any other like minded souls) check the following for me. http://www.marinedirect.com.au/catal...0/product57929 This link is to a Galvanic corrosion and electrolysis protection device which costs AU$345. Is it in your opinion worth the money and do these devices genuinely offer a high degree of protection?

Thanks

David.
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Old 08-19-2007, 07:51 PM   #17
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Is it in your opinion worth the money and do these devices genuinely offer a high degree of protection?
Hi David,

Thanks for posting this. I have had a good look at what little there is to read about this "electrolysis blocker". My take on this is that there is no decription of what this piece of kit actually is but I believe it to be a galvanic isolator, sometimes know as a zinc saver.

A galvanic isolator and an isolating transformer are two different bits of kit. I will try to explain this using information from a Swedish book called, "Elsystem för Båtar (Electrical Systems for Boats) by Magnus Sterky.

A galvanic isolator is designed to hinder the low voltage currents which cause anodic corrosion and the high rate of consumption of zinc anodes.

In its simplest form a galvanic isolator is two diodes connected in parallel but in opposite directions so that current can flow in either direction. The galvanic isolater is "plumbed in" between the protective earth of the grid (shore power) and the boat's earth. The diodes allow current from a short circuit to flow which will then blow the circuit breakers. On the other hand, the pure galvanic potential differences of very few volts will be blocked by the unit.

Galvanic isolaters are good pieces of kit in the right places. The AYBC recommends their usage. However, and here is the crux, they will not be as effective some parts of the world as in others. The reason for this is that the ground (by this I mean the rocks, silt, sand and soil we stand on) is more or less conductive. Here in Scandinavia we have a problem as mountainous areas composed of crystilline rock types are poorer conducters than say a clay valley. In other words, the greater the amount of crystaline strata the greater the resistance. Here, in Sweden, the resistance is about 100 times greater than that of continental Europe. I can immagine that similar differences can be the case between say, British Colombia / Washington State and the Gulf of Mexico.

Now, to continue, when electricity is "delivered" it comes in three phases, although we are probably only connecting one phase to the boat. The other two phases are connected to other users. As electrical consumption on each phase will not be identical a phase inbalance is created. This creates a problem as the high voltage, three-phase current is transformed down to 220 volt and to create an earth the earth wire is connected to the neautral at the transformer. Now, to take it further, at the marina we also have an earth but this earth is a little distance from the earthed transformer and - here comes the important bit - because of the different rates of power consuption on the different phases, which results in the phase imbalance, the middle point of the neautral at the transformer will actually vary in voltage. As you will remember, the earth wire is connected to the neutral at the transformer which will therefore result in a flow of current to earth at the transformer. When this happens there will also be a voltage difference between the earth at the transformer and the earth at the marina but, as we have connected to the earth at the marina there will also be a voltage difference between our boat and earth. This will give rise to corrosion problems onboard.

Now, back to the mountains. As the crystaline areas are better insulators than the clay soils then a greater current will be required to overcome the resistance than in clay soils. This means that the small currents necessary to create galvanic problems when connected to a common ground will need to be much higher to have the same effect. The galvanic isolator works on very small currents and thus is unsuitable for areas such as this where a larger current is required to create galavic corrosion when connected to a common earth.

An isoating transformer seperates completely the boat electics from shore power and, as the current used on board is generated in the output side of the transformer, there can be no voltage inbalance.

The bottom line here is that galvanic isolators are not recommended in Scandinavia, not because they do not work but just because they do not work HERE! Isolating transformers are recommended HERE!

I know this does not answer your question David but I think you have to work out which are your potential cruising areas. If you will be hovering arround the Far East then I am sure a galvanic isolator will be a good piece of kit to have but if you plan on visiting the fjords of South Island or Chile then an isolating transformer would be a better bet.

Sorry about the complicated and difficult explination. I am no expert on the subject and am trying to understand it myself.

Aye

Stephen
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Old 08-19-2007, 09:48 PM   #18
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Hi Stephen,

Thank you very much. Your explanation is far more easily understood than anything else I have tried to comprehend. It provides me with the means to make an informed decision.

Thanks again,

David.
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Old 08-19-2007, 11:10 PM   #19
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Hi David ,

This site is extremely informative - although specifically directed to the US boating industry.

http://www.uscgboating.org/safety/boatbuil.../electrical.htm

Richard
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