Galvanic Isolator

[IanC]

What is one, and why do I need one. How much is one, and if I had one where and how would I fit it.

I have a twin engined petrol boat with all the usual 12volt electrics + shore-power etc.

IanC

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[Talbot]

as I understand it, the galvanic isolator sits on the earth line from your RCD and stops any DC current from leaking through the earth line (which would set up a circuit through the anodes and using them up at a prodigous rate). However the GI would allow any AC current, thus allowing the RCD to work. Again, as I understand it, these are particularly important in a marina, and especially if you or your neighbours boat is made of Ali.

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[Kevin]

Hi

the galvanic isolator provents corrosion by halting the low voltage DC galvanic current that happens when a boat is in water and connected to shore power- thats my understanding anyway

they are fitted between the entry of the shore power and the internal a/c control panel in the boat and are a must IMHO another option is to fit an isolating transformer again between the shorepower input and the a/c control panel.

can get them for about £60 or so

hope this helps

kevin

 

[Tomsk]

Ooooh... this could explain a lot of my problems! I'll put money on that I don't have one. Anyone know what they look like? /forums/images/icons/crazy.gif

<hr width=100% size=1>Tomsk -

What the hell... it's only money!

 

[Mollari]

About the size of a 70amp diode, very useful, I have one (inherited with boat) a GRP boat and my anodes are 2 years old and look about the usual 6 months old for hamble river, and in the hamble I've seen anodes erode away in 8 months.

if you find where to get one let me know as I could always use a back-up

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[Talbot]

<<Anyone know what they look like?>>

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[gonfishing]

Try Here

For a full and detailed explanation

<A target="_blank" HREF=http://www.adverc.co.uk/technical/technical-galv.asp>http://www.adverc.co.uk/technical/technical-galv.asp</A>

Hope this helps

Julian

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[Tomsk]

Bu&&er me! I haven't got room for anything that big!

<hr width=100% size=1>Tomsk -

What the hell... it's only money!

 

[alanporter]

Do you need one if the AC ground is not connected to the boat's DC ground point?
Separating them would prevent galvanic corrosion, wouldn't it ?

<hr width=100% size=1>Alan Porter

 

[Kevin]

If that is the case it does mean that it would prevent galvanic corrosion but it would then raise the safety issue that if a short happened between the A/C and D/C circuit or DC grounding there would not be enough 'fault' current to trip the A/C breaker it would mean the whole boat could become 240 volt live a potential disaster. as far as i undersdtand it would also make the water around the boat charged and kill off fish and any passing swimmers!

Isolating transformers also prevent this corrosion problem.

kevin

 

[Tomsk]

Interesting description and solution <A target="_blank" HREF=http://www.adverc.co.uk/products/protection/galvanic.asp>HERE</A>.

<hr width=100% size=1>Tomsk -

What the hell... it's only money!

 

[alanporter]

Thanks for the response. However, if the shore power and DC circuits are totally separated, woudn't this be like just leading an extension cable on board ? There wouldn't be any risk of a short between the two circuits. I have had this system in my boat for the past 18 years without a problem but, as I am not an electrician, I wonder if I have just been lucky.

<hr width=100% size=1>Alan Porter

 

[MainlySteam]

The ac earth conductor on the boat's shorepower (and same for onboard invertor/generator) installation must be grounded to the surrounding seawater on the boat (via the hull if metal, or via metal components if boat is plastic/wood - typically via the anode, including the engine etc if a bonded boat /forums/images/icons/frown.gif) for the safety reasons that were given in the previous post.

John

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[halcyon]

From memory they are a couple of diodes back to back. The volt drop of the diode stops small current flows due to small voltage variations, once the voltage exceeds the volt drop across the diode currents flows, ie fault condition.


Brian

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[Kevin]

Hi

Im sure i have the understanding of things right but i am definitly not an electrician, its just info i learnt before undertaking work on my boat a number of years back, so i probably should tactfully withdraw from this post and let any experts out there respond.

Tomsk thats a good clear article on the subject def worth the look.

kevin

 

[Kevin]

Like you say i think its the RCD that states that all A/C on boats have to be grounded to the sea water

kevin

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[halcyon]

The RCD only measures current in live to current in neutral, if you get more than it's set differnce it trips, were it goes goes does not matter.
In the old days of ELCB's that measured leakage to earth, it was more relevant.

Brian

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[VicS]

I believe the arrangement of diodes in a galvanic isolator is the same as that in a full wave bridge rectifier.
There was an article in a while back in PBO on making one although, if memory serves me correctly, there was subsequently some criticism of the component ratings. Four diodes are connected with two in series facing one way and in parallel with two in series facing the other way. Current can therefore flow either way but only when about 1.4 volts is exceeded. Thus the small voltages associated with electrolysis and galvanic corrosion are blocked but currents driven by the mains voltage will pass and operate the trip.
It is important that the diode current ratings exceed any possible fault current and of course that they can withstand mains voltages.

Correction: Having gone away and thought about it I realise that the opening statement is rubbish. There are four diodes but the way they are connected is different to the way they are connected in a bridge rectifier.

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple><P ID="edit"><FONT SIZE=-1>Edited by VicS on 08/05/2004 08:55 (server time).</FONT></P>

 

[halcyon]

Was just about to post a quick correction, my mental shorthand overtook me last night.
Yes you have two in series, that are connected to another pair in reverse parrallel, ie anode to cathode. Allowing current to flow in either direction with a 1.4 volt drop.

Brian

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[VicS]

I'm glad I got that right!
I am, I think, wrong about there having been a article in PBO though. At least I can't now find one. I think I must have followed a link in a thread on one of these forums to Yandina.com where they describe making a galvanic isolator and suggest using two 35amp bridge rectifiers to get a unit with a 70amp rating. That, I guess, is what sowed the seeds of the idea that a galvanic isolator was basically the same as a bridge rectifier.

Lots of other marine DIY ideas on that site too if anyone is interested.

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>