electrolysis and earths on boats

Postby ericmark » Sat Nov 29, 2008 12:28 pm

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.
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.


Simply Build It

Postby collectors » Tue Dec 09, 2008 8:01 pm

Hi, very Grey area with eclectics. A friend asked me to have a look around for some answers & its seems like there is more arguments on the correct approach than i thought possible. One of the main problems is different materials boats are built with & who has got a better earth & who has no earth at all. This will make a difference on whose boat is going end up as a sacrificial anode & who will end up with the leak.
You might think the NICEIC has some quirks. (I think they steer clear of boats for this reason). lol
There is some good reading, (plus some confusion) if you Google trhe following sites. On the 1st one, have a look at the mains electric links.


Good luck.

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Postby ericmark » Wed Dec 10, 2008 6:34 am

Since my first post it has come to light they only has a 16 amp supply. So using a 3Kw isolation transformer which seems to be the norm as price jumps once you go over 3Kw ideal would be 3.5Kw but if the battery charger which also contains an isolation transformer is also feed direct from shore supply that will allow a full 4.5Kw to be used (Inverter supplied sockets and isolation transformer supplied sockets) my son seems to favour a change over switch which would reduce max to 13A.
I lived in a caravan with 10A supply for years but I did not try to run a washing machine in the caravan. Mind you I don't think the washing machine will fit through doorway anyway!
Personally I think a canal holiday may be great but would not want to live on one but its what his girl friend wants and he wants the girl friend so no real option.

Postby collectors » Thu Dec 11, 2008 4:06 pm

The only thing with a 3kw i/tranys is the have a habit of tripping with the serge.

They could maybe have a compromise & go for a Dutch barge. :wink:
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Postby ericmark » Thu Dec 11, 2008 6:23 pm

I would think since nearly all barges would need protection that a type C or D MCB would be provided. I am told Dutch barges are too wide for our canal system! Not sure how they would help anyway unless made of a non conducting material? Even the tall ships had copper plated hulls for extra speed. And with such a long narrow boat if made of anything other than steel then they could easy be damaged. Even the steel hulls can be damaged should a canal bank burst so can't see any plastic hull being an option.
I think I will have to get into habit of hopping on and off one leg on shore and one on the boat could be a little shocking!
Although I would fancy the broads and becoming a maritime mobile! But a little too far to travel from North Wales.

Postby sparx » Thu Dec 11, 2008 9:59 pm

have wired a couple of fishing boats, many years ago, one GRP the other Steel hull.
Both had anodes fitted, (GRP one to look after prop, rudder etc).
Both wired earth free, with metal hulled one having earth monitoring by 2 lamps in series so if one leg starts to make contact with hull that lamp dims and the other glows brighter as early warning, also needs both legs to go down to trip system.
I remember neither had connection to anodes.
Unfortunately neither had shore hook-up facilities but I guess a double wound isolating transformer would do it.
sorry not much help but it was a long while back and regs did not cover vessels, unlike todays!
Seem to remember using double pole MCB's for each circuit, Regards

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Postby ericmark » Thu Dec 11, 2008 10:48 pm

Thank you Sparx the regulations still do not cover boat only the shore side of the supply. I had considered some monitoring device but I am worried the device it’s self could cause a problem. I had considered one of those testers that have a series of lights 6 to 400 volt connected between the two earths? But I thought there is likely a propriety device to monitor this. I remember the bulbs on the old DC gantry cranes in the steel works but I had intended to earth one leg of the isolation transformer to hull so that would not work.
On board a normal earth would be used but with an earth leakage unit it is just can’t directly connect on shore earth with on board earth.
It all goes against the grain but I can’t see a way around it.

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