unusual cable - can anyone tell me what it is please

28 Jun 2005
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United Kingdom
Hi I found this cable in a property, it looks like armoured on the outside, ie black sheath but it does not appear to have any armour as far as I can tell so far. Also it appears to have a 25mm approx centre core surrounded by separately insulated stranded neutral conductor which I assume add up to the same as a 25mm red conductor.



As for the earth I cannot be sure where that figures into this cable, ie a red centre core surrounded by the neutral sort of sounds neat but obviously the earth would muck that neatness up. If I could cut the cable I could easily see its configuration/layout but i don't want to do that if it is ok and can stay.

With a 25mm cable I would assume a 16mm earth but its difficult to be sure and could be a 10mm earth. also it looks like it may be sleeved.

Please look at my picture and tell me what ever you can about it.

Any help appreciated.
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Wow, now that was a quick answer, thanks, holmslaw, but that leads me to some questions now.

1. Can I assume the neutral is equivalent to the live conductor?

2. Assuming the live is a 25mm, what will the earth be?

3. Is this cable acceptable under the 17th edition?

4. Is it armoured?

5. Why is it different, ie is it meant for a different use etc?

6. Can you still buy it?

UPDATE: Ok googled it and know more about it now that you told me what it's called, but answers still welcome in case I missed any points on this stuff.
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The earth complies with tqable 56g of the 16th regs and which ever table number it is of the 17th (I forget).

For example, a 16mm split has a 10mm earth, a 25mm split a 16mm earth.

You cannot use it as an armoured cable - treat it as TWIN AND EARTH!

It cannot be burried in a wall to circumvent the 17th's RCD requirements.

This is split concentric copper cable it was originally manufactured under licence from the secretary of State to electricity Boards as lateral mains cable but was also used in underground supplies.

The basic philosophy is protection against electric shock due to the earth and neutral concentric conductors surrounding the live inner core. Any object penetrating the outer sheath would make contact with these cores befre reaching the live core thereby causing an immediate short circiut fault.

It is not armoured cable but can be used underground without additional protection, under current regulations

Single phase split cable was mainly 16mm up to the 1980s when the 15th Edition came out. This introduced the upgrading of the REC cut out fuse to 100A requiring 25mm tails and supplies so 25mm split became available that.

The neutral conductors are the same size as the live but the neutral insulation is very thin and easily damaged, there were no special terminations but it was considered good practice to twist or bunch the neutrals and apply at least one layer of tape over them.

This cable should not be confused with straight concentric which is still only licensed to electricity companies. This has an aluminium inner live core and bare copper as the concentric outer conductor. It is combined neutral/earth CNE cable for REC power supplies only in PME systems. As a CNE cable it is prohibited in consumers installations under any circumstances in the UK.
Not sure I agree with the above fully.

You cannot use it directly buried, or ducted if you wish to comply to bs7671.

This type of cable should be treated the same way as you treat twin and earth cable - it cannot be used in place of armoured, and cannot be used to satisfy the regs regarding cables buried in the buildings fabric. The easiest thing to do is to simply regard it as twin and earth.

Split Concentric has equal sized Live and Neutral (as you would expect!), and the earth is sized to BS7671's table 54g (or whatever the 17th table is called), so you know it will always comply, unlike 16mm T+E with a 6mm earth.
I would suggest that if it were buried at a minimum 450mm, and marker tape used it could be argued that it complies (or is safe) as this is the spec used on my side of the meter, where fuse sizes are far higher than on an installation.
I guess what they mean is that the cable will not come to any harm by being buried.

But that's not the same as meaning that it complies with BS 7671 if directly buried. It doesn't matter what the maker says about the tolerance of his cable to the environmental conditions, nor what installations which are not required to comply with BS 7671 can have - the point is that if you want to comply with BS 7671 you may not use it directly buried.

Just read 522.8.10 - there's no doubt.

[EDIT]Typo corrected[/EDIT]
Just read 522.1.10 - there's no doubt.
I take it that you mean 522.8.10 - but I agree with you that it appears to leave no doubt. Concentric cable obviously does not have earthed armour and I don't think anyone could really argue that the outer conductors constituted a 'earthed metal sheath' (indeed, with split con, it would be incomplete).

Kind Regards, John.

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