electrolysis problems with earths on boats!

[ericmark]

Has anyone been involved in wiring a boat and the electrolysis problems involved with importing an earth. I am considering an isolation transformer and not importing an earth at all. But although all items can run from a 3Kw transformer hysteresis losses when plugged in 24/7 are a concern and also overload problems where multi-items are required at the same time. Consideration has also been given to running a DC charging unit and an inverter along side the straight isolated supply so basically two isolation transformers with will increase the hysteresis effect when both units are on but allow either unit to be switched of on no or little load. I know with generators units are available which sense load and only turn on the generator when required is there similar for isolation transformers. Also on washing day for a few hours only what would be the result in connecting an earth and using direct power for limited time only.
Although I have been an electrician all my life I have never come across this problem before. The people using the boat are skilled an electrician and his wife a plumber so having a system needing user input is not a problem but also I can’t do anything which is dangerous both in eroding the hull or giving them shocks.
This must be regular problem and I will guess there is a standard way around the problem.
Eric
I have been told that sometimes diodes are used in the earth cable as the 0.6 threshold voltage is enough to stop the difference between copper and iron hulls from causing a problem but Fe = -0.44 and Cu = +0.34 which when I was at school is 0.78 volts so maybe they double up on diodes? Also a problem if diodes blow so may need semi-conductors fuses as well? And one would hope no Aluminium hulls as at -1.66 would give 1.22 volts and opposite polarity to copper earth rods or hulls. Also have to think about the standard cathodic protection as Zn = -0.76 so really 1.1 volt differential. Since people would tend to step straight onto the boat rather than use any gang plank and to grab onto the metal super structure I am a little nervous on using diode method it is of course a narrow boat so could even step from boat to boat in the marina.
Added to original post as I realise once answered less people look with view to helping and this is all new to me.
I now wonder where I have earth iron waste pipes in a house if I caused any problems with erosion? Never thought about it until this came up.
Eric

 

[holmslaw]

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[JohnD]

this is a specialist job, but IIRC you are not allowed to import an earth to a moored boat supplied from the shore.

 

[ericmark]

Thank you what is a IIRC what I did note in BS7671:2008 708.3

"Unless you have an isolating transformer fitted on board to isolate the electrical system on your craft from the shore supply system, corrosion through electrolysis could damage your craft or surrounding craft."

So it is a real danger and it seems from what we can find out up to date it will take out the cathodic protection anodes first but this is still a big problem as the boat has to be taken out of the water to weld on new anodes so very expensive to renew.
Eric

 

[THRIPSTER]

have a look at this link - it is from Wiring Matters - an IET publication - it may help.


http://www2.theiet.org/Publish/Wire...ing_matters_marinas_and_similar_locations.pdf