Shorepower - kind of asked before but want more info...

[ontheplane]

Hi there,

Have bought a portable socket with RCD - which will do as a temp measure - but have decided that i'd like to wire up a decent kit in the boat.

I don't want to hook up when the boat is left, this is purely overnight at a marina and only to run a kettle, microwave, heaters in the spring/autumn battery chargers, and be able to have lights on and not worry about the battery.

So all I want is some sockets dotted around the boat, a tidy inlet etc.

So as I see it, I need the following - will someone please correct me if I have it wrong.

1) Inlet mounted on side of boat
runs to
2) RCD mounted in a consumer unit with the appropriate circuit breakers
runs to
3) Outlets dotted around the boat.

Now as I understand it, I'd need a galvanic isolator if I was being hooked up permanently, but for just an odd overnighter i'd be ok

Also, there seems two schools of thought - on the earth - some say it should be earthed to the boat, and the shore earth - some say they should remain TOTALLY separate.

The totally separate argument gets my (untrained) vote as it seems daft to me to mix the Mains with the boats 12v, it would seem far more sensible to keep the two totally separate. It seems to me that if a fault occurs, the earthing on the shore side should trip the rcd. The only potential for problem with this I can see, is if the earth on the marina was faulty - but would earthing it to the boats earth help?

Thanks in advance

 

[Elessar]

The earths need to be common, 12v (-ve) and 240 v (earth). Someone will know the clause in the wiring regs but this is how it should be.

You do need a galvanic isolator - if another boat has a big enough fault your anodes can disappear like alka selza. Even if it isn't that bad you may forget to disconnect the cable.

 

[Refueler]

[ QUOTE ]
Someone will know the clause in the wiring regs but this is how it should be.

[/ QUOTE ]

?? What regs ? Inland Waterways maybe - but not coastal / seagoing. Like gas - you can do as you please. And of course now someone will mention Insurance Co. - They don't require unless someone is daft enough to point it out to them.

For me - I am always on 240 when moored at back of my house - I most certainly do NOT combine 12v and 240v earthing. Second I do not have a Galvanic Isolator.

The only reason I could see for any changes to a set-up like mine is the increased electrical useage on the average mobo compared to raggie like myself etc.

There really is a lot of discussion on this that never seems to find a real solution. Even the books cannot agree.

I suggest "Alastair Garrods Electrics Afloat" - a Practical Boat Owner Book that describes very well all about the above and more. Forget the 12V handbook / Doctor - I found it not so good.


Keep it simple.

 

[gjgm]

I looked into this a couple of years ago. I cant remember the make, but there was a plug and play unit that did rcd,battery charger and (maybe) isolator, so no head scratching as to how to wire it all up. Was about £200, I think, which was a bit more than components, but probably a much easier route.
Changed boat, so never actually installed one myself /forums/images/graemlins/grin.gif

 

[VicS]

What is called a "garage consumer unit" can make a convenient unit for a modest shorepower system. An RCD, a 6A mcb and a 16A mcb in one box all for £25 or so maybe less. Depends on what you need, not suitable for a whole ring main but could be used for a couple of sockets, but limited of course to 16A max load.


Regarding the earthing debate. Connection of the shorepower earth to the DC negative etc is I understand required by the ABYC and CE standards but ISO 13297 does not require it if the boat is protected by an RCD.

IF you do not connect the shorepower earth to the ships earth/DC negative then you should not need a galvanic isolator. If you do connect them you will almost certainly need one if you leave the boat connected to a shorepower supply but not really if you only occassionally connect.

A GI protects you from currents originating from galvanic sources and is limited to around 1.4 volts in most cases. It will not protect you from obscure faults in the wiring of nearby boats as any currents from that kind of source are driven by, presumably 12 volts. or even 240 volts! I also understand that equipment with switch mode power supply units may render a GI useless unless it also incorporates a capacitor to conduct AC .

Don't rely on the marina RCD. Include your own in your shorepower system.

GJGM is probably referring to the Dolphin combined shorepower and battery charging systems. Excellent idea but at a price. £600 and upwards from Merlin Powerstore!

 

[gjgm]

ooh.. there was something cheaper.. I think !
I think a garage unit rcd is only single isolation, not double?

 

[colvic]

Well said and in our case spot on.

 

[VicS]

[ QUOTE ]
I think a garage unit rcd is only single isolation, not double

[/ QUOTE ] Checked on several available from Screwfix. All double pole !

Dont know but suspect regs now require them to be DP. Something to bear in mind and check though.

 

[ontheplane]

Thank you VicS

So as I suspected, if I keep the Earths (and in fact the whole thing) totally separate from the 12v I won't need a galvanic isolator because there is no connection between the 240 mains and my boat.

Point taken on the RCD on board - I may even have a garage consumer unit, and rcd's in each socket, although that might be overkill.

Thanks for the good advice.

 

[Elessar]

BS 7671. I believe boats were covered and that marinas are now too in the latest edition. As I said I'm sure someone will have the exact clause.

your house isn't floating in a conductive fluid to electrically connect it to the house next door so you wouldn't need a galvanic isolator.

You're welcome to do it your way but I'd advise the original questioner strongly against it as it sounds to me like your boat could well be sort of nightmare we all need to isolate ourselves from using a (very very simple) galvanic isolator.

 

[ontheplane]

Elessar,

Can you explain why??

If I have a simple circuit, and nothing on the boat is connected to the mains - i.e. it's totally separate - how will that affect any other boat in the marina or anywhere else??

If this was an issue - then why do they sell these :- http://cgi.ebay.co.uk/PORTABLE-TENT-CAMP...A1%7C240%3A1318

and others like them??

I want to do it right - BUT I cannot for the life of me see what is wrong with what VicS has suggested and a simple circuit as long as it's properly earthed, has RCD's where required and C/B in the appropriate places.

Thanks

 

[VicS]

There are two schools of thought over the earthing thing. As you must have realised.

Until recently I argued that the earths should be interconnected, based on the belief that that was what all the recommendations and standards required even though I could never really understand the dangers if there was an RCD in the system

Then some kind forumite provided me with a link to ISO 13297. There lo and behold I find

"4.2 The protective conductor shall be connected to the craft's d.c. negative ground (earth) as close as
practicable to the battery (d.c.) negative terminal.
NOTE If an RCD (whole-craft residual current device) or an isolation transformer is installed in the main supply circuit of the
a.c. system (see 8.2), the negative ground terminal of the d.c. system need not be connected to the a.c. shore ground
(protective conductor)."

While I was insisting that they should be connected half the world argued that they should not be. Now as a result of finding ISO13297 I have half changed my mind and the other half of the world argues that they must be connected.

I am still on the fence over it a bit. The snags with galvanic corrosion and galvanic isolators make me want to say dont connect them, but if someone can explain in decent detail exactly how a hazard can arise if they are not, even when an RCD is fitted, then I'll climb down the other side of the fence again.

That is a challenge to Elessar. Demonstrate the danger of not connecting the two to me and I am with you.

 

[ontheplane]

That's what I want too.

I want someone to give me a reason to connect the two together - because nothing I've heard so far has suggested it's a good idea (galvanic corrosion etc etc) and I've heard plenty to persuade me to keep them separate.

 

[Elessar]

I don't know the reason why I can only guess. My guess is as follows.

The reason for an earth on mains kit is that, in case of a wiring fault (ie the wire falls off and touches the case) the resistance to earth is very low, a large current will flow and blow the fuse. If the case wasn't earthed it would become and remain live. More than about 65V can produce enough current through your body to kill you so you have a potentially lethal situation.

If you have a portable piece of kit it is clearly separate from your boat wiring, it is temporary and can be inspected mechanically every time it is installed (ie only a numpty would plug it in if the cable was damaged)

With installed wiring there is the potential for a hidden fault to develop. If the mains cable were to fray and touch any bit of metalwork in the boat (well not quite any, but anything bonded), the whole boat would become live. If you bond the boat to earth the fuse will blow as the current will flow to earth in a fault condition.

Admittedly the chance of this being lethal is far far less with the advent of RCDs, but when did you last measure how long your RCD takes to trip? I think they are specced to something like 30mS so a stopwatch won't do the job. And anyway you have wiring before your RCD that probably goes right past the engine bay and lots of exposed metal.

 

[ontheplane]

Thank you, that's brilliant - it's a good reason....

So I can now make a proper decision - I may still go for what I was thinking, but the point of the cable chaffing, and making a circuit with a metal part of the boat is a very reasonable one, and I think I may run all my cabling in a plastic core / tube to prevent there being any chance of it happenning.

Thanks again, it's the first time the wiring them up together has made any sense at all...

 

[VicS]

[ QUOTE ]
it's the first time the wiring them up together has made any sense at all

[/ QUOTE ] I'm glad you are happy now. I am certainly not convinced.

All mains wiring should be run in conduits or fastened to trays so that there is no chance of it coming in contact with any metal parts of the boat and chafing.

An RCD is an accepted method of providing personal protection against electrocution due to a fault in a portable appliance but you would be well advised to use doubly insulated equipment (no earth connection to those anyway) wherever possible.

Rather than guess at reasons for interconnecting the earths I suggest that a system wired in accordance with ISO 13297 will be as safe as mains power on a boat is likely to be without resorting to an isolation transformer.

 

[ontheplane]

Nope - not convinced that connecting the mains earth to the boat earth is right, but I am happy that I at least know the theory behind the idea, and can cover it off with safe wiring practices.

 

[castlevar]

Hi
I built my boat over 10 years ago i wired it to B M E A code of practice for electrical and electronic installation in small craft.
I found it very helpfull even though I am good with all things elictrical and serviced an apprenticeship on electrical and mechinical engineering in harland and wolff belfast.
Just to prove that i am not to old to learn something new. But better to buy or borrow a copy to much info repeat on this site.