Concentric wiring

swifters · Apr 23, 2012

Could someone please explain to me a bit about concentric wiring? If you can't utilise an RCD as TLC direct claim below then how do you meet requirements of 17th edition??? Are they just talking about a combined neutral and earth concentric wiring? I imagine you could still use a RCD with concentric wiring that has a line, neutral and external concentric copper conductor encasing both line and neutral?

"Earthed concentric wiring may only be used under very special conditions, which usually involve the use of a private transformer supply or a private generating plant. Since there is no separate path for earth currents, it follows that residual current devices (RCDs) will not be effective and therefore must not be used. The cross-sectional area of the sheath (neutral and earth conductor) of a cable used in such a system must never be less than 4 mm² copper, or 16 mm² aluminium or less than the inner core for a single core cable. All multicore copper mineral insulated cables comply with this requirement, even a I mm² two core cable having the necessary sheath cross-sectional area. However, only single core cables of 6 mm² and below may be used. The combined protective and neutral conductors (sheaths) of such cables must not serve more than one final circuit.
Wherever a joint becomes necessary in the PEN conductor, the contact through the normal sealing pot and gland is insufficient; an extra earth tail must be used as shown in {Fig 5.19}. If it becomes necessary to separate the neutral and protective conductors at any point in an installation, they must not be connected together again beyond that point."

Knobhead · Apr 23, 2012

With split concentric treat it just as you would T+E. RCD protect it.

You can’t use combined concentric other than in a DNO supply, so it’s out of the normal realm.

Edd · Apr 23, 2012

RCD's wont become unballanced, as the N and E are combined. Consentric cables are really for DNO side only as after that they seperate to give us our seprate N and E. consentric as far as im aware is only a single core with outer protected sheath that is The N and E compined. Its not a seprate L and N. That would just be a kind of SWA

Sparkystu79 · Apr 23, 2012

Split concentric cable has a separate N & E. Straight concentric has a combined N & E outer core and normally has a solid aluminium line conductor.

Guest55 · Apr 23, 2012

swifters said:
Could someone please explain to me a bit about concentric wiring? If you can't utilise an RCD as TLC direct claim below then how do you meet requirements of 17th edition??? Are they just talking about a combined neutral and earth concentric wiring? I imagine you could still use a RCD with concentric wiring that has a line, neutral and external concentric copper conductor encasing both line and neutral?

Well the issues are this..
Split concentric , where the earth and neutral conductors are separate , the earth strands only protect the live , not the neutral.
Which it means it offers less protection than swa cable.
Secondly , the insulation on the neutral strands is very thin , considerably less protection than on the live ( you'll notice this if you ever strip some wires back )
Its the second issue that causes debate as to how it can be used.

Knobhead · Apr 23, 2012

Split concentric

View attachment 11997

View attachment 11998

swifters · Apr 23, 2012

Thanks for all the replies, the site I saw today looked like it had a line and neutral (aprox 6mm each) encased in what looked like a 15mm copper pipe with an earth clamp connecting to the outer copper.

Knobhead · Apr 23, 2012

Are you thinking of Pyro cable?

View attachment 11999

swifters · Apr 23, 2012

yes, it looks like that except it has no insulation outside but does have red and black insulation inside.

Silly Sausage · Apr 23, 2012

Tony said:
Are you thinking of Pyro cable?

View attachment 11999

You look at that picture and the small gap between the conductors, it never fails to amaze me how robust they are.

Guest55 · Apr 23, 2012

Archy Styrigg said:
You look at that picture and the small gap between the conductors, it never fails to amaze me how robust they are.

Amazing stuff is pyro.
Can remember college tutor giving a demo of pyro in one of the test rigs.
Heated up part of an energised pyro with gas blow torch , the cable was feeding a baton holder with a lit up bulb.
Left heat on till it was glowing red , bulb didnt even flicker the whole time.
Class was stood watching in amazement lol.
Good times before H&S got a grip.

turin · Apr 24, 2012

Well the issues are this..
Split concentric , where the earth and neutral conductors are separate , the earth strands only protect the live , not the neutral.
Which it means it offers less protection than swa cable.
Secondly , the insulation on the neutral strands is very thin , considerably less protection than on the live ( you'll notice this if you ever strip some wires back )
Its the second issue that causes debate as to how it can be used.

Are you suggesting that the insulation on the neutral conductor is not satisfactory?

Like every wiring system, there are applications where specific cables can be used. I see no reason why split concentric cable cannot be used by non-DNO for sub mains installations. It is never suggested that it is an armoured cable and should not be treated as such.

I personally use it for the majority of domestic sub-main installations, where appropriate. The reason I do is because I find it much more flexible and therfore easier to work with than SWA.

Silly Sausage · Apr 24, 2012

Tony said:
Split concentric

View attachment 11997

View attachment 11998

That would almost be decent job if they'd taken a bit more effort.
It would be quite funny if that was a roll of heat shrink in the corner.

Engineer54 · Apr 25, 2012

turin said:
Are you suggesting that the insulation on the neutral conductor is not satisfactory?

Like every wiring system, there are applications where specific cables can be used. I see no reason why split concentric cable cannot be used by non-DNO for sub mains installations. It is never suggested that it is an armoured cable and should not be treated as such.

I personally use it for the majority of domestic sub-main installations, where appropriate. The reason I do is because I find it much more flexible and therfore easier to work with than SWA.

How do you terminate these split concentric sub-main cables?? The reason i ask and why most electricians do not use them, is because they can be a bit of a pig to terminate without them looking like a DIY'ers pig ear!! lol!! As most CU's and other domestic enclosures don't really have the space to terminate them anywhere near as easily as with SWA cables.....