Spilt concentric cable duel use

[Rockomuir]

Morning all, started a job yesterday fit a bathroom fan etc.
When I took off consumer unit cover to do dead test I came across a split concentric cable supplying an outbuilding/shed in another Mcb. It was the type with the single insulated neutral and separate earth. What threw me was that some of the neutral cores had been separated and used as two way switch for an outside light. Theswitch was located next to fuseboard and the other next to the c/unit it’s in shed. All the cables had been sleeved for identification etc
Not having much experience of this cable I immediately thought this is not right, you cannot do that.
just wondered if anyone could shed some light on it, or is it fine but just something I have not seen before

 

[littlespark]

Can we get a photo?

Concentric is usually a live central core, with neutral as strands around that, and earth as strands around that on the outside.

There is also one with the neutral and earth strands in the same layer, but the neutrals are insulated.

Concentric isn’t normally used as installation wiring…. Just supplies for DNOs.
(someone may correct me there)

 

[Rockomuir]

Can we get a photo?

Concentric is usually a live central core, with neutral as strands around that, and earth as strands around that on the outside.

There is also one with the neutral and earth strands in the same layer, but the neutrals are insulated.

Concentric isn’t normally used as installation wiring…. Just supplies for DNOs.
(someone may correct me there)

Not the best picture, but you can see the neutral cores go into to of main switch then the 3 terminals they have separated for the 2 way. The person who lives here previously worked for Dno so might explain why this cable was used as probably got it for free

 

[littlespark]

It’s not really a multi core cable.

the number of neutral strands will give an equivelant CSA of the line conductor, so taking 3 out will reduce that.

C2 on EICR if I was feeling generous. C1 if that type of cable shouldn’t be used in that situation in the first place.
(I’m on holiday, regs book at home, so can’t check)

 

[timhoward]

Interesting!
My first thought is the reduced csa of the neutral - it's unlikely to be an issue but I'd want to check - I'd measure one of the cores with micrometer and work it out. My guess is you have at least 10mm^2 there. Whatever it is, you'd have to consider the csa of the Live to be too, and check the upstream OCPD is adequate.
(the connections in the terminal block look poor, middle one biting insulation?)

 

[timhoward]

@littlespark cross-post, and we agree on that bit!
But as long as cable is adequately sized, marked accordingly and appropriate for the conditions I'm struggling to immediately see why it's a C2.
Just because it's not common (understatement!) doesn't instantly make it dangerous in my view.

 

[brianmoooore]

The insulation on the neutrals in split con. is more fragile than the insulation on a normal conductor of the same csa. Possibly a problem with 230V between adjacent conductors?

 

[westward10]

The biggest size the cable can be with seven neutral cores is 16.0 and assuming it is 16.0 each neutral core is 1.70.
There is no issue with the neutral cores being used as a line conductor as the cable is rated at 600/1000v meaning 600v between live conductors and earth, 1000v between live conductors.
The question here is the reduced csa and whether it is installed to manufacturers requirements.

 

[brianmoooore]

1000V between live conductors, yes, but the neutrals would be classified as a whole, not as individual conductors. There's a plastic spacer core separating the neutrals from the earth and the much thicker and robust insulation surrounding the live central conductor separating the live and neutrals.

 

[Rockomuir]

It is 16mm as It had 1x16/16/16 on cable sheath. If you were doing an eicr what code would you give? I looked in regs last night but didn’t help as could not find anything to help make a decision if I was going to give c2 or c3.
thankfully only had to fit a fan but defo an interesting one

 

[timhoward]

Personally (distance permitting) I'd just reunite the Neutrals, and change the light to Quinetic!

 

[brianmoooore]

Personally (distance permitting) I'd just reunite the Neutrals,

And hopefully connect them properly into the main switch.
A perfect example of how NOT to do it. Tightly twisted together like that, they will almost certainly become loose if the wiring in the CU is disturbed.
I always lightly twist the individual neutrals of split conc. together , and then either bind them with insulating tape or a length of heatshrink, but the stripped ends that go into the device terminal are always parallel with each other, and gently twisted around a bit as the clamp is tightened.

 

[Lucien Nunes]

I think @brianmoooore has it; the cable voltage rating is probably only valid when all the neutral strands are paralleled, because the insulation between any two is not equivalent to that between one and earth or line. Even very thin insulation will in practice stand 230V or more but whether it has been tested or certified for use with voltage between the neutral strands is a different thing.

 

[Mikegh]

And hopefully connect them properly into the main switch.
A perfect example of how NOT to do it. Tightly twisted together like that, they will almost certainly become loose if the wiring in the CU is disturbed.
I always lightly twist the individual neutrals of split conc. together , and then either bind them with insulating tape or a length of heatshrink, but the stripped ends that go into the device terminal are always parallel with each other, and gently twisted around a bit as the clamp is tightened.

Dont twist the solids together or twist the stranded conductor