Extending a Ring Main


Postby mruddo71 » Tue Apr 08, 2008 11:12 am

Hi

This is my first post, so apologies if its a duplicate, its just I havent the time to read through forty pages of posts. Also apologies if my question makes me look like a noob, I am :-)

I read your article on how to extend a ringmain, (very good) which is what I am planning on doing in my bedroom which has just a single wall socket (thanks for that forward thinking!)

The socket is wall mounted and there is a cable going to it and a cable going from it. A Supply and a Return???

I intend to cut into these cables with two juntion boxes, (like it says to do on your article) then extend the ringmain around the bedroom to have two additional double sockets.

My question is, how do I know which is the Supply cable and which is the Return cable; If I get them the wrong way around will my new portion of ringmain be wired incorrectly and therefore pose a hazard?

Thanks for looking
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Postby ericmark » Tue Apr 08, 2008 8:49 pm

A ring main does not have supply and return in the way you have written. I have just had a quick look at projects and all in old wiring colours so must have been there a long time. To extend a ring main you must ensure it is a ring main in the first place and not a spur off a spur or a radial. I have come to offices where even an electrician has broken into what he thought was a ring main but was in fact a radial and put a loop of 2.5 twin and earth around the office braking the original 4mm radial. Which when winter arrived and fan heaters came out caused an overload.
Since you have not grasped the concept of ring mains I do not feel happy explaining how to check ring mains.
The instructions say nothing about volt drop or earth loop impedance or any of the other checks that should be made.
ericmark

Postby mruddo71 » Wed Apr 09, 2008 1:03 am

Hi Ericmark,

Thanks for replying so quickly!

Sorry my first post wasn’t right. To reiterate, I have a single socket on the wall in my bedroom. There is a cable going into it and one coming out of it. I assumed this meant it was sitting on the ring main???

The reason (which I forgot to mention in my first post, apologies) why I make this assumption, is that there is a 3rd cable coming out the bottom of the socket, which goes under the floorboards to another socket in a second bedroom. This would be a spur? So this made me think that the original two wires and socket are on the ring main! Yes/no? If not then I already have a spur with a spur off it!

BTW: I know you are only allowed one spur. This is why I would not cut into the 3rd cable under the floorboards, as I am trying to avoid spurs.

Please tell me if you think the following is ok to do:

1. Cut the cable coming out the original socket. (About 1000mm along from the socket)
2. Add 30amp junction boxes to both ends of the old cable.
3. Run 2.5mm cable from 1st junction box to 1st new socket.
4. Run cable from 1st new socket to 2nd new socket.
5. Run cable from 2nd new socket to 2nd junction box, completing the circuit.

Hope that all makes sense!

Thinking it over, I don’t know why I mentioned the supply and return thing? Something a qualified electrician once said to me :-)

So it’s a good thing that I posted on your forum!

Thanks for looking
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Postby ericmark » Wed Apr 09, 2008 9:18 am

If it is possible I would avoid adding a junction box. One would normally take one of the cables going into the original socket and try re-routing to new socket then from new socket then add new cables back to original socket.
If you need a junction box it should be assessable you may need trap door in floor boards which is why we try to avoid them. Better with crimped or soldered cable joint in epoxy resin lined shrink sleeve. i.e. proper cable joint. But go with what your happy with better a good joint in junction box than bad one with cable joint.

It is very easy to lose a ring main. Each time a socket is pulled out and pushed back there is a chance that a wire is dislodged. We would always check for continuity between each pair of wire in a ring main to ensure it is still a ring. And also do an insulation test and finally earth loop impedance test.

With 4% volt drop permitted and with 32 amp MCB in use you should get no more than 0.575Ω between Line Line and Neutral Neutral the earth earth reading will be 0.926Ω max. The new regs allow 0.72Ω and 1.16Ω but then all the other changes also need doing so no real help.

I will direct you to Projects and Part P not that you need Part P for a bedroom but it does have the forms that should be filled in after any electrical job in your case minor works.

Biggest problem is not your job but work before and once you work on the circuit you are responsible for ensuring it is still save. Unless you have lived in the house from new or last rewire you don't know what has already been done. It is for your family and your own safety I point out that it should be tested. As well as shocks you also have fire risk with overloaded circuits.
ericmark

Postby mruddo71 » Wed Apr 09, 2008 10:11 am

Hi Ericmark,

Thanks once again for replying!

Funny isn't it how everyone insists you got to use a junction box (“its the regs!â€
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Postby ericmark » Wed Apr 09, 2008 1:00 pm

I was a little nervous on telling you about crimps as in my opinion a good junction box connection is better than bad crimped connection. Which is why I said about doing what your happy with.
The government Part P regulations have stopped the commercial electricians who did the one or two private jobs from doing domestic work. Which has resulted in a increase in DIY and a reduction in house safety but bad work on QT does not hit the press so guys in White hall are still patting each other on the back. Before Part P to get an electrician to check your work was no problem but now their insurance will not allow them to do such work.
As to testing yourself the meters are about £700 for cheap set so not really an option. B&Q meters are in the main for extra low voltage and the leads supplied do not comply with GS38 regulations. i.e. dangerous on 230 volt.
So many times I have found things in a dangerous condition and one of the reasons they go unnoticed for years is lack of testing. And cumulative faults.

For example if in a previously correctly wired house you were to by accident connect line to earth then a fuse would blow. But if the house had a previous fault of earths not connected then it could electrocute someone. So I am sure you can see why testing is so important and with meters at £700 per set why the electricians charge so much. And with transport costs in time as well as fuel 15 minuets will need to generate wage for at least an hour.

As to an electrician who knows what they are doing C&G 2391 is required to test and inspect and it is not an easy exam anyone with that qualification should be a good electrician. There are others of course but some are only tick the box exams take it enough times and in the end you will pass.
ericmark

Postby mruddo71 » Fri Apr 11, 2008 10:34 am

Hi Ericmark

Thanks for all your advice.

Just wanted to let you know I went ahead and did the work myself.

Then my neighbour who is an electrician, with testing qualifications came over with his "proper" test equipment and checked it out.

He said I did a better job, than most of the stuff he sees 9-5 Mon - Fri.

Anyway thanks for all your help and advice.

Cheers
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Postby hutchi0 » Sat Jun 21, 2008 5:32 pm

This is all a very useful thread. It would seem that if one does need to check if a socket with two cables coming out of it is on a ring main, then one should do the following. I can do it with a meter that cost £6.75! from Maplin. No mains will go through the meter.

a) Turn off the mains
b) disconnect the socket.
Set your meter to the smallest ohm measuring range. See what reading you get when you connect the two leads together. On mine I get 0.5, which means half and ohm. ie. nearly a short circuit.

Check the ohms between the live connections that joined in the socket, when they are not joined. If there is significant resistance (lots of ohms), then you do not have a ring main.

Do the same check for the neutral and the earth.

Clearly this is not testing the circuitry. However it is good enough to tell you whether you have a ring main, rather than tapping into some wiring that is already dodgy.
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