Shore Power

[BobA]

Could anyone advise if it is preferable to make/build your own shorepower installation to power a couple of 240 sockets and a battery charger or to splash out on a ready made system such as advertised by Merlin power store for 249 squid.
If so would normal domestic RCB and sockets suffice - i won't need a galvonic isolator as the boat is normally out of the water on its trailer and if it is in a marina for a couple of days there is nothing that could be eroded in the water.
or have i got it all wrong?

Bob

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[Trevethan]

I made mine -- bought a small domestic consumer unit, space for 4 breakers.

I used armoured bable between the inlet and consumer unit and to the sockets.

The total cost from my local electrical fellow for a system with 7 twin outless and an 70 foot cable run was about £180, but that inclused fancy brass sockets and a timer for the immersion on the calorifier.

I made up a pair of shore power cables 15 metres each (one is usually long enough but sometimes you need run cables a long way...) using heacy duty arctic cable which cost another £30 or so.

I guess it could be cheaper is I used plastic sockets and non armoured cable, but for the sake of safety the armoured cable is worth it, although its a pig to work with.

Does the Merlin kit come with a galvanic isolator?



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[Stemar]

There are no reasons not to do it yourself if you know what you're doing with 240V in your house. A few points to consider:

1. USE AN RCD!!! Mains plus water is a lethal combination, and you can't rely on the one on the pontoon.

2. Don't use twin & earth domestic cable. The big conductors will work harden with vibration and couls easily cause a fire. I think the one to use is arctic multistrand 1.5 or 2.5 sq mm depending on load. Unless you plan on using a lot of electric heating, 1.5 should be fine.

3. Use a galvanic isolator. This will help to keep stray currents from eating your prop - or someone else's steel or ally boat.

4. Run all cable in conduit or trunking that's only used for mains (12v in a separate conduit, preferably a foot away - especially data cables) & keep it out of the bilges. Don't let it wander round loose! The stick-on stuff with a removable cover from B&Q is fine.

5. With domestic wiring, you'll always find you need more sockets, so put plenty in - you don't want long leads to your equiment trailing across cabins

Good luck.

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[broadcaster]

Hi,

A good source of electrical kit is www.tlc-direct.co.uk, I would use 2.5mm cable just to be on the safe side as you never know if you will need the power later. You will probably get the whole lot for less than £100.00.

Andy



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[Talbot]

You only need a galvanic isolator if you have combined the earth from 240, with the 12v negative (which is supposed to be a good idea electrically), however once you have combined these, there is the possibility of dc current flow from one boat to another - hence corrosion - hence need for a galvanic isolator. If you dont connect the earth and negative, you dont need the galvanic isolator!

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[MainlySteam]

That is completely incorrect.

A galvanic isolator (or isolating transformer) breaks the earth path from the ac earth on the boat (which the DC negative may or may not be connected to) from its immersed metals forming part of the AC ground (all of them if it is a bonded boat, heaven forbid) to those similalrly connected to ground and immersed on another boat or shore structure through the earth conductor of the shore cable, thus preventing galvanic action.

Whether the dc negative is grounded to the ac earth is of no relevance whatsoever.

John

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[Talbot]

Whilst I am happy to acknowledge that I may be wrong /forums/images/icons/smile.gif PBO's top 100 questions answered page 13 is quite clear that it is the combination of DC/AC bonding that creates the galvanic circuit with the shore, for which the isolator is designed.

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[Benbow]

Mainlysteam is quite correct. However, I think the confusion arises because most dc systems have their neg bonded to immersed metal. Look at it this way; if you had a boat with no dc system at all you would need a galvanic isolator if you connected your ac neg to your hull/engine etc, but not if it was left entirely floating. So although its actually nothing to do with the dc system, in practice if you bond your ac and dc negs you are usually creating the requrement for an isolator.

Hope that makes sense.



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[Talbot]

I always like to learn something each day, such a pity that PBO didnt bother with the detail!

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[MainlySteam]

And can I add that all codes that I know of require the ac on board to be grounded to the boat's underwater metal (usually the anode only in an unbonded boat or, of course, all underwater metals by default in a bonded one). So if you have a correct ac installation then the galvanic isolator is necessary.

Mark, I assume that the PBO you refer to is a recent edition, which probably has not got out to the colonies yet (we have just got April ed. in the shops now) - could you tell me which one and I will look out for it. Thanks.

John

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[halcyon]

Do you have the reason for bonding ac earth to dc bonding ?
In the early days with ELCB's you needed a bonding to earth ( water ) to give a trip path. But with RCD's there is no connection to earth, the unit only checking amps in live to amps in neutral. Were the current goes does not matter. only that there is a miss-match of the required value. If it goes via the electric kettle or the dc light fitment it will trip the RCD if large enough, so why connect ?

Brian

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[MainlySteam]

I have wondered the same thing and the only reason that has been given to me by professional electrical people in shipyards is in case of a AC to DC short in the boat - the AC in such a fault then going to ground rather than livening DC equipment with exposed chassis. Those same electrical engineers, however, promote not grounding the DC negative to the AC earth because of the advantages in protection against stray currents causing corrosion. Against that, ABYC still require it I believe.

It would seem to me that in a competently built boat that such a short would be extremely unlikely and obviously not grounding the DC negative is preferable on a metal boat (I have recently been involved with a new build which the negative is grounded, but the link is continually monitored for DC leakage current with an automatic break in the case of). I think all the classification codes require 2 pole breakers on the DC board for vessels not having the DC grounded and for a small vessel that has cost implications and makes the DC board much bigger.

My own boat, which is steel, has the DC negative not grounded to AC earth (all the engine sensors, etc are DC isolated also as is now common on marine diesels) and uses 2 pole breakers on the DC board.

John

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[halcyon]

Seems were both confused, I follow your thinking.

I'll do some digging to try and find the requirements under UK / EEC / RCD regs, need it for our web site.

Brian

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