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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
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  #1  
Old 12-20-2009
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Galvanic ground isolation

Hello,

I'm a student from belgium and I study at the nautical academy (last year now).
My boat is in antwerp and I use it as my student home (or dorm).

I've been reading around about galvanic corrosion trough the grounding cable from the shore power ( 230v 50Hz here).

I would like to stop this possible current flow, but standard galvanic isolators are to expensive for my studentbudget.

Therefore i would like to wire zenerdiodes in between, with a zenervoltage of about 3volts or so.
In this way 2 times (3 zdiodes in serie) in parallel:
The first 3 are installed in the other direction then the last.


z z
/ \ / \
to shore -- z -- z-----to boat
\ / \ /
z z
What do you guys think?
Will this hold if there is a serious power leak?

Last edited by bartvdv; 12-20-2009 at 07:28 AM.
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Old 12-20-2009
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The galvanic isolator does more than just stopping stray current through the ground connection on shore power. It also needs to withstand a lightning strike that discharges into the water. And it needs to be safe so you do not get stray currents from your shore power line going into the water. This could be fatal to nearby swimmers. I installed one last winter and mine is a very simple device. It is just two diodes connected in series such that the current flow goes from the boat to the shore power. Since diodes have a small resistance in the "open" direction they still stop the galvanic currents. I believe these diodes have 1.5 ohms each. Two in series is 3 ohms and this is enough to stop the glavanic current while allowing the grounding system to work properly. These diodes are very hefty so they do not get fried the first time a significant current goes to ground.

I spent about $300 USD (with our economy this has to be about 200 euro). Here is a link to the one I used Home | DEI Marine
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Last edited by nickmerc; 08-18-2011 at 05:39 AM.
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Old 12-20-2009
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The danger here is that a lightning strike will burn out the diodes leaving you with no ground on the AC system and you wont know it. The of-the-shelf products now have an indicator light that illuminates if the ground circuit is open so that you know that you have a problem. If you can add this feature to your setup you will be safe.
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Old 12-20-2009
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The November issue of Practical Boat Owner (UK) has an article about building a galvanic isolator for less than 10 pounds. I don't have the details handy as I'm out of town at the moment.
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Old 12-21-2009
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I've just ordered my two diodebridges, as seen in that PBO isue.
Just € 10 !
What a difference with those €200 isolators!!!

Thank you!
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Old 01-14-2010
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Hi,
Can you place a like to where you got these diodes.

Thanks.
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Old 01-15-2010
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Conrad Electronic - Europa's grootste webwinkel in elektronica en techniek

Belgium site....
I bet you're sorry you're not from €urope
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Old 01-30-2010
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I built my own with zener diodes. It's not a bridge just blocking in each AC direction. Key considerations: Each diode has to be able to handle the full load current of you AC source, that's what a ground is supposed to accomplish. Most electronic supply sources will have these. I doubled up to have back-up. Second you need to mount the dioeds in a manner that has the heat dissapation capability through the studs ahs the load current. I used a number of spaced flat aluminium bars. It's not a hard project and not costly.
One other issue is that you don't know if you have a blown diode unless you check it from time to time. There are multi-meters with a zener diode check function. If the diode's blown you've lost your ground protection.
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Old 01-30-2010
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My previous comments may have been a bit cryptic. Here's the longer version as best as I recall:
You need 2 high current bridge rectifiers each capable of handling full shore power current. They are square devices with a bolt hole in the center and four spade connector terminals on the back 1 labeled +, one - and two usually labeled AC.
About 30 sq in of aluminium 1/8 inch thick for a heat sink.
Wire to make connections.
Stainess steel bolts and nuts to mount the rectiifers to the aluminium.

You're only ging to use the + and - terminals that are diagonally opposie eachother on the bridge recitfire, so cut off the other two terminals.
Mount the rectifiers on the heat sink with the bolts spacing the recitfiers at lest 4" apart. It helps to put heatsink grease in the cavity when mounting the recitfiers if you have it.
Hook one ground wire end to the + and the other to the - . Hook the second bridge in parallel. That way if one recitfer blows then the other will carry the load
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Old 01-31-2010
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thats the exact same thing i did :-)
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