240V shorepower

[Peter]

I've just completed rewiring a simple 240V circuit in my yacht, via a RCD distribution panel. I'am about to connect the shore power to the panel and one question is raised. What do I do with the earth, a straight connection between the yacht circuit earth and the shorepower lead earth, or do I have to add an additional earth bond to something in the boat i.e to engine or hull (steel hull). Any anwsers appreciated.

Peter

 

[Guest]

You need to connect the mains earth firmly to the DC ground (cable current rating exceeding the supply and boat breaker rating) and hull ground (already connected I presume?).

If any wiring on the boat should be good quality, this is it (tinned wire).

There must not be any form of switch, breaker or fuse between the earth on the beach and DC ground (except the deck plug, if fitted).

The earth is your first line of defence - it permits a leak to (say) the hull to get back to the power source safely. The RCD is your second line of defence, which should hopefully trip if there is an imbalance (such as when you are being electrocuted), but there are some circumstances in which it won't help if the earth is not connected as well.

Lastly, you need to consider fitting a galvanic isolator to protect your boat from corrosion. Unfortunately, by tieing AC and DC grounds together, you create the ideal circumstances to get boat-to-boat corrosion "via the green wire". It is very unwise to avoid this problem by not connecting the grounds together however. An isolator is over 100 quid, but I would not otherwise use AC at all. This is fitted as close to the input as possible, and there must not be any connections to ground between it and the beach.

If you get really paranoid about corrosion, you can buy a very expensive and heavy isolation transformer (total protection, whereas an isolator is a 95% of the time solution). For this I would get some professional advice and/or fitting.

regards

 

[KevL]

Just a quick thought on the subject of isolation transformers. My boat has an Isolation transformer feeding the 110V fridge, cooker and water heater. The shore power comes in through an RCD but if there is an isolation transformer between me getting electricuted and the RCD it isn't going to trip is it. I think I'd better check how the earth for the 110V supply is set up don't you. Oh no I can feel more expense coming on :-(

Kev

 

[Bergman]

I would favour the idea of an isolation xformer over connecting mains earth to boat earth.

It will stop you being electrocuted by touching a live wire.

The RCD will not work by detecting inbalance between live and neutral in the normal way but will trip on over current, just like a fuse.

Whats more you do not have to worry about corrosion as much.

 

[KevL]

I agree that there is far less chance of getting electrocuted if you use an isolation transformer because to complete the current circuit you need be touching both the live and the neutral wires. But please remember RCD DO NOT DO CURRENT LIMITING ! that is done by the MCBs at the side of it in most consumer units. Now on my boat I've got 2 MCBs and from memory they are rated at 16A continuous so to get them to trip I need more than 16A flowing. If anyone gets 16A+ flowing through them then they are toast.

Kev

 

[Guest]

You need to put the isolation transformer on the supply side of the RCD/MCB if possible (however I presume from your reply that you have both 110v and 240V on your boat? - if not then re-wire it). If you have to have a separate supply, then you should really install another RCD after the transformer.

The "neutral" of the downstream side of the isolation transformer becomes the new earth (tieing it to ground defines this side of the winding as the earth/neutral point). Thus you must bring the earth on the boat side to the transformer separately, and this point must be tied to DC ground as before.

The earth on the supply side just protects the wiring from the supply to the transformer, but must also be done properly.

 

[jfkal]

And there was something else......
You need a ground isolator. This is a set of four high amperage diodes two in series in one direction and two in series in the other direction. Those will block low DC current which otherwise will eat up your throughhulls, prop, shaft and so on if you keep shore power on 24 hrs. Once a short to ground occurs those diodes will become conductive and allow enough current to trip the fuse.