Galvanic isolator

[pjohnson]

Hello all,

I was browsing through ebay and came across a Galvanic Isolator, "every boat must have one". I may be relatively new to yachting but I have never heard of problems that galvanic currents can cause to your metalwork on-board, Engine etc. Can any of you boffins enlighten me on this as it scared the, out of me as we leave our boat connected to shore power quite often.

Thank you in advance.

 

[Colvic Watson]

I ain't no boffin but if you have bonded the shore power to your -ve supply on board then you need one, otherwise your anodes are potentially being quickly eaten away by someone else's dodgy 240v installation. If, like me, you just run a cable on-board to power a de-humidifier or heater etc (using an RCD of course) then in theory there is no electrical connection between the shore power and your anodes and hence no need for (or way of connecting) a galvanic isolator.

 

[pvb]

Brief explanation...

I don't claim to be a boffin either, but I'll try to give you a brief explanation. Boats with fixed shorepower installations should have the AC earth connected to the DC ground, and hence to the anodes. This connection, often called "earth bonding", is for safety reasons. In the event of a fault in the marina's earth wiring (and some marina wiring leaves a lot to be desired!) a fault in the equipment in the boat could cause that equipment to become live at mains voltage. If the AC earth on the boat equipment is also connected to the DC ground, there should be enough of a circuit through the water back to earth to allow the RCD to trip and cut off the mains. For this reason, earth bonding is now specified by the EU Recreational Craft Directive, by the BMEA, by the ABYC, and by the US CoastGuard.

Unfortunately, earth bonding introduces the possibility of rapid galvanic corrosion of underwater metals, because all the earth-bonded boats are interconnected through the marina's earth cable. The solution to this is to add a galvanic isolator, which fits in the AC earth cable immediately inside the boat. The galvanic isolator blocks the tiny electrolytic currents which cause corrosion, but allows AC fault currents to flow unimpeded, thus preserving safety.

Galvanic isolators can be obtained from many boat electrical suppliers. There's lots of information on earth bonding and galvanic isolators on the internet - so don't just take the word of strangers on a forum, see what the experts say. For starters, you might like to check out the Merlin Equipment page on galvanic isolators, which also has a link to an information sheet (Merlin's James Hortop is a contributor to PBO on boat electrical subjects). Or the Adverc page on galvanic isolators. Or the Reading College course notes on Boat Electrics - look for "Shore Line Earthing" about halfway down the page.

 

[pjohnson]

Thank you for your explanations, I will check how the shore power is connected as soon as I can. I am not certain but I think the shorepower connection justs acts as a socket for the extension cable and is not connected directly to the boats system. A stirling charger is connected to the shorepower and then to the batteries however, and left connected for long periods.

Thanks again.

 

[jkim1]

It is better to have a galvanic isolator fitted in the earth line which should be bonded to the boats earth. The term galvanic isolator is rather strange as I was brought up to think that galvanic has something to do with magnetism. ie a magnetically coupled isolation. I could not understand what this had to do with boats unless you had to purchase a 1:1 transformer and run you own separate earthing system completely independent of the mains earth.
Galvanic isolators cost about £50 which is rather a lot of money for what is a very simple system, I think its a word like cross bonding in central heating systems which very few people have heard about but they think its expensive.
A galvanic isolator is basically a full wave bridge rectifier a 35 amp 600volt one will do with the negative and positive outputs shorted out ( to each other) the earth from the mains is connected after the rccd to one of the ac inputs and the boats earth is connected to the other. that gives a difference of 1,4 volts between the boats earth and the mains earth which is enough to protect the boat against corrosion whilst still allowing the rccd to operate. It should be mounted on a heatsink and in a box. It took me an age to find this on the net, but there are thousands that want to sell you one for upwards of £50. I would have thought that most electricians could make you one for considerably less than that, or you could make it yourself. Now what all that has to do with galvanic, I would be greatful if someone would explain that to me.