Yet another Q about supplying an outbuilding.

[ban-all-sheds]

I plan (hope) to be erecting a new outbuilding next year, and I will have the luxury of being able to carry out groundworks, part of which will be the laying of two plastic drainage pipes between the house and the outbuilding. one to carry electricity and the other telephone and network cables. Even though the buried electric cable will be in a tough pipe, I'll probably still use SWA cable, partly because I've already got some.

Q1. I've read elsewhere about the problem with terminating SWA cable - if I simply (carefully) remove the armour and sheathing so that I can (a) get to the cores, and (b) connect an earth wire to the armour (using a jubilee clip occurs to me), if I then make the connections to the cable using a junction box and I do that either inside the house/outbuilding, or inside an IP65 box, am I going to have problems?

Q2. (Actually I'm 99.9% certain I know the answer to this). The outbuilding will have an RCD protected CU supplying its circuits, so I want to feed it not from one of the MCBs in my house CU, but instead directly from the service splitter. So the cable run from the house to the outbuilding would have no load or leakage protection, and part of the run would be a length of T/E inside the house to where I join it to the SWA cable. I'm thinking that this is a Bad Idea, and that I should actually have the supply go from the splitter to an RCD switch and then off through the house and into the wild blue yonder. Yesno? And does anybody know where I can get a "standalone" RCD switch (40/50/60A, I guess), i.e. one which does the same as the master switch in a CU but not rail-mounted and without all the CU gubbins?

Q3. Earthing. As I'm also connecting telephone and network cables between the two buildings (15-20m apart) I'm pretty keen to have them at the same earth potential, so the outbuilding earth circuits will be wired back, via the supply cable, to the house earthing point. Would it be a good idea, when the outbuilding goes up, to install an earth rod, and connect to that as well (not instead of) the house earth? This would therefore mean that the single house/outbuilding earth circuit would be physically connected to the earth at two different locations.

 

[breezer]

1) The thing is with an swa cable you must put a proper gland on it to terminate it. so if you then put this gland into a metal box you can then connect your earth to the metal box (providing the box has an earth terminal). it saves you having to strip the swa more than necessary also stops the swa corroding / being exposed

2) NO. you can not legaly open the supply company "spliter" and a standalone rcd would do you no good on its own since as you said you would have no load protection. (not to mention the size of the cable you would have to use because the only fuse would be the main incoming 100A one) It would cost you more money (having an rcd at each end) and is not safe. you would be beter off bringing the swa into the consumers unit with gland attached and using an mcb in your consumers unit.

3) i would not bother with the earth rod, you could look at it as "insurance" but an earth rod has to have its resitance tested to qualify as an earth rod, and dont forget that as it will be connected to the out building supply cable it will also "back feed" into the house.

I have seen swa cable runs longer than you intend and the only earth is the armour of the cable with no ill effects what so ever.

Also in most swa gland kits you get an earth tag, so that you can still use the gland if it is to be mounted in a plastic box

 

[ban-all-sheds]

1) Yeah - you're right - I should use the proper components - hope I can find some!

2) The things you learn, and the things you do in ignorance, eh? When I rewired my house, and it came time to connect the new CUs to the supply, I gaily opened up the splitter which was not sealed, or marked to indicate that I shouldn't, and wired them in. (Well, maybe not "gaily" - I was more than a little focussed in my attention to what I was doing!).

Re the supply to the outbuilding, whatever I do I'll need a CU for it to give me control over the individual circuits out there. But as I said I really knew, and as you say, I need the cable running TO that CU to be protected as well, which is why I wanted to obtain an RCD switch with a 40-50A-ish overload - an example of which I have now found: http://www.tlc-direct.co.uk/Products/CMP1slash40SH.html, and it's not very much money.

I'd rather not run the supply off one of my existing MCB's as I don't have a spare.

My second CU is not RCD protected - I installed that because I had heard that RCDs could be tripped by nearby lightning strikes, and I didn't want to risk losing the supply to the fridge freezer, so I put it in to serve the socket where the F/F plugged in. But the F/F will be moving soon, and in 15 years I've never had the RCD trip in that way - I'm not going to put in a new run of non-RCD to where the new fridge will be, so I guess that Plan B would be to replace that little 2-way CU with a new, small, RCD one.

Actually, Plan C ought to be to replace both CUs, as the RCD one that I put in 15 years ago (when they were known as ELCB) is a 100mA device...

As for bringing the SWA into the CU - impossible, as its outside diameter is about 25mm.

 

[breezer]

plan c is good as for outside diameter of of swa i didn't literaly mean bring swa INTO c/u i meant put a galnd on it first so you will only bring in the cores

 

[ban-all-sheds]

plan c is good as for outside diameter of of swa i didn't literaly mean bring swa INTO c/u i meant put a galnd on it first so you will only bring in the cores

Yeah - sorry - I knew that - what I meant was it's a stonking great cable - I haven't measured but I guess the individual conductors are 20-30sq. mm (5-6mm diameter). And anyway, isn't a gland designed to fit into a hole in a flat surface, terminate the armour to an earth tag, and present the cores for connection to something? i.e. wouldn't a gland for a 25mm cable need a hole of, say, around 30mm diameter, plus a fair bit of space on the inside for the nuts, washers and tag?

I've never seen one in the flesh, but looking at a picture (e.g. http://www.towerman.co.uk/images/electrical_accessories/swa_gland.jpg) I've got a pretty good idea about how they work, and I'm not sure a CU would have the space.

And it's probably academic - I don' think the cable I have is long enough to reach the CU, so I'll terminate it where it enters the house, and use 6mm T/E from there to the CU