Shed straight concentric supply query

[Furball]

Hi all,
I've recently moved into my own place where ive found 2 lengths of straight concentric feeding the shed about 30m away.
One doing the live and one doing the neutral with the outer sheath's earthed, all into a separate cu in the house.
Would anyone be able to tell me if this is up to regs ?

 

[JohnW2]

I've recently moved into my own place where ive found 2 lengths of straight concentric feeding the shed about 30m away. One doing the live and one doing the neutral with the outer sheath's earthed, all into a separate cu in the house. Would anyone be able to tell me if this is up to regs ?

If you really do mean straight con (i.e. with combined neutral and earth), then I don't believe that is allowed within installations. Split con is a different matter.

Kind Regards, John

 

[davelx]

If you really do mean straight con (i.e. with combined neutral and earth), then I don't believe that is allowed within installations. Split con is a different matter.

Kind Regards, John

Maybe that's why there are two lengths, one for L and one for N, with both outers acting as the earth? So that the N and E are not combined. Not sure what the regs say about that - can't find any obvious reference after a quick look. 523.201 might be relevant?

 

[JohnW2]

Maybe that's why there are two lengths, one for L and one for N, with both outers acting as the earth? So that the N and E are not combined

True, I hadn't thought about that possibility!

Not sure what the regs say about that - can't find any obvious reference after a quick look.

I can't think of any reg that would prohibit it - after all, it's no different from having two singles, which happen to have an earthed 'sheath' around them.

As for why anyone would do that, other than "that's what I happened to have in my van", I'm not sure!

Kind Regards, John

 

[EFLImpudence]

Yes, he does say two.

Seems alright, assuming all done correctly.

 

[JohnW2]

Yes, he does say two. Seems alright, assuming all done correctly.

Agreed, albeit it sounds very unusual. Other than "that's what I happened to have in my van", I can't think of a reason!

If it's buried (we're not told) I'm not sure that the outer would necessarily afford much 'mechanical protection' (e.g. as per SWA), but I suppose the fact that it's earthed makes it relatively safe.

Kind Regards, John

 

[EFLImpudence]

That's what I meant by done correctly.

I would question the "mechanical protection" of swa.
It merely, like the concentric, causes disconnection when pierced far enough to cause disconnection.

 

[JohnW2]

That's what I meant by done correctly. I would question the "mechanical protection" of swa. It merely, like the concentric, causes disconnection when pierced far enough to cause disconnection.

That's all very true, but I suspect that if someone was proposing to bury cable which had an outer (earthed) 'flimsy' covering made of braided copper or aluminium (rather than 'steel wire armour') that the suitability of that might well be questioned, mightn't it?

Kind Regards, John

 

[EFLImpudence]

As usual you have introduce something different.
Is it flimsy? It is not braided copper or aluminium.

I suppose it must be suitable for burying as the DNOs bury split concentric, don't they?

People think swa is tough but if your spade is in line with the steel wire winding, it offers no mechanical protection at all.

 

[Furball]

That's all very true, but I suspect that if someone was proposing to bury cable which had an outer (earthed) 'flimsy' covering made of braided copper or aluminium (rather than 'steel wire armour') that the suitability of that might well be questioned, mightn't it?

Kind Regards, John

I believe the previous owner worked for the electricity board so probably had a bit of cable laying about.

Its in a hard duct ( don't know if that's counts as mechanical protection ? ) it goes under the middle of the house then deep under ground to the bottom of the garden where it reappears in the shed.
I've got a patio going in the next few weeks and I didn't want to be digging all up again

 

[JohnW2]

As usual you have introduce something different. Is it flimsy? It is not braided copper or aluminium.

Agreed, but were you not saying that an earthed outer covering was 'enough' (for burying)? Or do you feel there there is also some minimum level of mechanical properties as well?

I suppose it must be suitable for burying as the DNOs bury split concentric, don't they?

I believe they do - so, as you imply, they must regard it as suitable for burying. However, I need not tell you that the views of DNOs and BS7671 as to what is acceptable have been known to differ!

People think swa is tough but if your spade is in line with the steel wire winding, it offers no mechanical protection at all.

I agree totally - but, again, I think that if someone came to this forum suggesting that they were going to bury a cable which had an outer earthed layer, but a layer which was mechanically 'inferior' to the SWA of SWA cable, at least some people would question the acceptability of that, wouldn't they?

Kind Regards, John

 

[JohnW2]

I believe the previous owner worked for the electricity board so probably had a bit of cable laying about. Its in a hard duct ( don't know if that's counts as mechanical protection ? ) it goes under the middle of the house then deep under ground to the bottom of the garden where it reappears in the shed. I've got a patio going in the next few weeks and I didn't want to be digging all up again

As has been said, even by me (after I initially didn't read your post carefully enough!), it would seem that what you have is probably acceptable (per regs), even if very unusual!

Kind Regards, John

 

[EFLImpudence]

Agreed, but were you not saying that an earthed outer covering was 'enough' (for burying)? Or do you feel there there is also some minimum level of mechanical properties as well?

I don't know. Is there a definition of a minimum level of mechanical protection?

I believe they do - so, as you imply, they must regard it as suitable for burying. However, I need not tell you that the views of DNOs and BS7671 as to what is acceptable have been known to differ!

It could be said that if the DNO consider it adequate, then if BS7671 disagree, they could be wrong.

I agree totally - but, again, I think that if someone came to this forum suggesting that they were going to bury a cable which had an outer earthed layer, but a layer which was mechanically 'inferior' to the SWA of SWA cable, at least some people would question the acceptability of that, wouldn't they?

They very well might. Whether they would be correct, I do not know.

I can't be bothered to investigate. You could.

 

[JohnW2]

I don't know. Is there a definition of a minimum level of mechanical protection?

That's what I asked you!

It could be said that if the DNO consider it adequate, then if BS7671 disagree, they could be wrong.

Do you mean that if there is a disagreement, then either (or potentially even both!) could be 'wrong'? If so, I agree. However, we're not talking about absolutes, so there is not a binary choice between 'right' and 'wrong' - merely (possibly) differing opinions as to what is 'acceptably safe'.

They very well might. Whether they would be correct, I do not know.

Nor do I - hence my questions. What about you? If someone came here and said that they intended to bury a cable with an earthed covering which was clearly mechanically inferior to the armour of SWA, would you tell them that it was OK?

Kind Regards, John

 

[EFLImpudence]

If someone came here and said that they intended to bury a cable with an earthed covering which was clearly mechanically inferior to the armour of SWA, would you tell them that it was OK?

I have said, I don't know.

If such a cable is suitable for burying in a wall (ready to be drilled through) then presumably it could be buried in the ground.

What are the requirements?
Presumably the outer sheath shall not quickly deteriorate.
That it should just cause an OPD to operate or prevent rodents chewing through it?
Neither such a cable nor swa prevents detrimental damage by spades, forks, or JCBs.

Investigate and let us know.