Extending ring mains

Maybe it was a radial at some point, then was taken back to the CU and made into a ring?

Are there any markings on the cable?

Sparks...

No markings on the cable. What I have noticed is that some of the existing cable doesn't appear to be copper - it is a silvery colour.

Cheers, Grant

... and can you see both ends of the ring coming back to the C.U.?

It's tinned copper.

There is a limit (can't remember what it is!) to the permitted number of outlets on a single ring - but you probably won't exceed that unless the ring has already been extended.

My understanding is that any such work now comes under the provisions of Part P of the Building Regs - and that, legally, the work needs to be certified by a qualified electrician - or by Building Control if they are capable! Many competent DIY-ers choose to ignore this - particularly if there's no obvious evidence as to *when* the work was carried out.

But if you're not confident *and* competent, get an electrician.

Probably tinned standed cable, as used yonks ago. You can probably use the

If it really is a radial (which is unlikely), your choices are to use bigger cable, or drop the MCB to 20A, which is no hardship if the circuit just covers a few reception or bedrooms. Don't even think about it for a kitchen, though.

However, if you're unlucky the test may show that the old cable isn't up to the earth loop impedence test. In that case, you probably must drop to 20A, or even rewire.

You'll need an electrician to do the testing. It is probably well beyond your experience, and the test equipment costs many hundreds of pounds.

Christian.

Effectively, it doesn't matter when it was done, as long as there isn't an exact record existing of what's there.

I'm in Scotland - I think the regs are different here aren't they? Even stricter?

Cheers, Grant

Many thanks for your reply. I think it's time for a professional!

Cheers, Grant

I don't really know what I'm looking for at the CU so I think, based on other advice in this thread, I should really be getting in a qualified electrician.

Cheers, Grant

Grant, if it has 7 cores then it may be the old 7/029 standard. 7 cores,

I'm pretty sure that it has six cores, but I'll check when I get home tonight. However, based on other advice in the thread regarding full circuit & earth loop impedence tests I think I will hand this job over to a professional.

Cheers, Grant

Flipping the 32A MCB off shows that all sockets in the house (dining room, sitting room, hall, 3 bedrooms & kitchen) are all on the same circuit.

Cheers, Grant

Ouch. You should really look into splitting the kitchen off it, unless it is a tiny kitchenette.

Christian.

Note that there is no harm in running the cables before they come. That's the long, expensive laborious bit, and I would say that there was an 80% chance of the cabling being useful.

You only need the electrician for the skilled bit involving expensive equipment.

Christian.

No, it's a "proper" kitchen. :-(

Grant

Yes, I was planning on doing that. I am now having nighmares about my house being ripped apart for a rewire (especially after having had the floors done). Not enough forward planning I suppose :-(

Out of interest, maybe you could enlighten me on what a failed earth loop impedence test would signify and what dangers does it put me at risk from?

Cheers, Grant

When you have an earth to live fault, in the worst case, the current has to flow from your incoming supply, through the consumer unit, along the circuit cables to the furthest point. It then has to flow back down the earth conductor back to your incoming supply. The incoming supply will also have some resistance along its live and earth connection.

The earth loop impedence is the total of all the resistances of the path that the current has to flow down.

Now, a socket circuit must disconnect rapidly in the event of a fault. Using MCBs, to trip quickly enough, you must be able to guarantee that the current flowing will be 5 times the circuit rating. Therefore, a 32A circuit needs

Now, the old cables you have found, I believe, had a smaller earth conductor than modern cables. The horrendously large ring, including your entire house has more of it, too. It is quite possible that the resistance of the cable will be too much, either as is, or with your proposed extension.

The danger of not fixing this potential problem is that in the event of an earth fault, it could take tens of seconds to disconnect the power, which is a fire and electrocution risk.

In reality, such circuits are normally RCD protected, so provided the RCD is present and working, then the danger is not real, although it is not permitted to design circuits that rely on this, with the exception of TT earthed (earth rod) systems, where it is simply not possible to comply.

Christian.

Christian, thanks very much for that explanation. The 1.44 ohms figure has jogged my memory - last year an engineer came to fix my washing machine and he carried out a test on the circuit. He told me that the desired figure for a sucessful test was 1.44 ohms but the figure he got from my circuit was 3-point-something (I have a note of it at home). he told me not to worry because some old houses have values above 30 ohms! So it looks like that circuit is already dangerous and I need a rewire.

Thanks again, Grant

Make sure the sockets are RCD protected. Although the circuit still won't be up to scratch and would still fail an inspection, this removes the main potential danger.

Christian.