Shed straight concentric supply query

[JohnW2]

I have said, I don't know.

... and nor really do I.

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.

That would seem logical - provided that, as you go on to say, the outer sheath was made of a material not likely to deteriorate quickly when underground.

Neither such a cable nor swa prevents detrimental damage by spades, forks, or JCBs.

Indeed, and I have already agreed with that.

Investigate and let us know.

That's the second time you've suggested that, but I'm not sure what type of 'investigation' you have in mind. As far as regulations are concerned, I don't think one has to look any further than 522.8.10. That seems to indicate very clearly that any cable with an earthed metal sheath may be directly buried, provided only that that sheath was "suitable for use as a protective conductor" (does that just mean adequate CSA, or what, I wonder?). [other regs obviously require that the cable be suitable for the environment concerned, but without saying anything specific].

However, I still wonder what responses somewhat would get if they came to this forum and said that the wanted to directly bury something other than SWA!

Kind Regards, John

 

[EFLImpudence]

That's the second time you've suggested that, but I'm not sure what type of 'investigation' you have in mind.

I was thinking about perhaps the manufacturers details or specific physical characteristics.
Everyone says that black cable is better for exterior use. Does that come from experience or is it the properties of the material or folk lore?
Having said that you have stated, and I agree, that T&E is fine for years.

As far as regulations are concerned, I don't think one has to look any further than 522.8.10. That seems to indicate very clearly that any cable with an earthed metal sheath may be directly buried,

Agreed.

provided only that that sheath was "suitable for use as a protective conductor" (does that just mean adequate CSA, or what, I wonder?).

I would think able to cause ADS - in effect a minimum c.s.a. but wouldn't that rule out foil?

[other regs obviously require that the cable be suitable for the environment concerned, but without saying anything specific].

I'm sure that would relate to the manufacturer's specifications.

However, I still wonder what responses somewhat would get if they came to this forum and said that the wanted to directly bury something other than SWA!

There might be howls of unjustified protest - like T&E outside.

 

[JohnW2]

I was thinking about perhaps the manufacturers details or specific physical characteristics.

Fair enough - that might help to answer the question about what was suitable for burying, but not the question we're really discussing - i.e. what is 'allowed' (by regs).

Everyone says that black cable is better for exterior use. Does that come from experience or is it the properties of the material or folk lore?

I don't think it's folk lore - but, rather, to do with susceptibility to UV.

Having said that you have stated, and I agree, that T&E is fine for years.

It is, at least in my experience. Similarly (again in my experience, although I didn't 'do it'!), so does directly buried T+E seem to be OK for years

I would think able to cause ADS - in effect a minimum c.s.a. but wouldn't that rule out foil?

Dunno - I would have to ascertain the CSA of the foil concerned. Considering , say, an ~12mm diameter earthed covering, it would only have to have a thickness of about 0.08mm 0.04mm (80 40 microns) to have a CSA of 1.5mm², or a thickness of about 0.135mm 0.067mm for a CSA of 2.5mm² or a thickness of about 0.27mm 0.107mm for a CSA of 4mm².

There might be howls of unjustified protest - like T&E outside.

I don't doubt that there would be at least some howls of protest. Whether or not they might be justified is what we are discussing.

Kind Regards, John
Edit: arithmetic errors corrected - even 'thinner' answers than I got before!

 

[Notch7]

Sorry for being dumb, but what is a concentric supply?

SWA has an outer sheath does that make it concentric?

I guess Coaxial cable is a concentric supply.........

 

[EFLImpudence]

it would only have to have a thickness of about 0.08mm (80 microns) to have a CSA of 1.5mm², or a thickness of about 0.135mm for a CSA of 2.5mm² or a thickness of about 0.27mm for a CSA of 4mm².

At first glance, that seems "quite thick". Is that the case?

The bit you were piercing would only be a fraction of the total c.s.a.

 

[JohnW2]

Sorry for being dumb, but what is a concentric supply? SWA has an outer sheath does that make it concentric? I guess Coaxial cable is a concentric supply.........

yes, technically speaking, they are all 'concentric'.

However, in present context, "Straight concentric" refers to the situation in which there is a single conductor (used for combined neutral and earth by DNOs) 'wound around' the central conductor, and 'Split concentric' refers to a situation in which both 'earth' and (insulated) 'neutral' conductors are wound around the central conductor.

Click here to see piccies and explanation in our wiki.

Kind Regards, John

 

[JohnW2]

At first glance, that seems "quite thick". Is that the case?

I guess that's a matter of opinion. As I said, it would have to be about 80 40 microns thick to give 1.5mm². Having just measured, a hair from my head is also about 40 microns and a thin sheet of paper about 100 microns. On that basis, do you regard 80 40 microns as "quite thick"?

The bit you were piercing would only be a fraction of the total c.s.a.

Very true, but I thought we were talking about the adequacy of the CSA (of the entire sheath) for ADS? The extent of contact with the CPC does not really affect that.

Kind Regards, John
Edit: arithmetic corrected

 

[EFLImpudence]

I guess that's a matter of opinion. As I said, it would have to be about 80 microns thick to give 1.5mm². Having just measured, a hair from my head is bout 40 microns and a thin sheet of paper about 100 microns. On that basis, do you regard 80 microns as "quite thick"?

Oh. fair enough. I did say at first glance. Nearly a tenth of a millimetre sounded quite thick for a sheet of foil.

I thought we were talking about the adequacy of the CSA (of the entire sheath) for ADS? The extent of contact with the CPC does not really affect that.

Bearing in mind I don't know what is required, I am merely trying to answer your questions regarding subjects I don't know.

I was thinking "adequate for ADS" as meaning to cause the OPD to operate when pierced without vapourising first; not to act as a CPC which it isn't designed to do.

 

[JohnW2]

Oh. fair enough. I did say at first glance. Nearly a tenth of a millimetre sounded quite thick for a sheet of foil.

OK. As I implied, "nearly a tenth of a millimetre" (well, actually less than a twelfth) is actually only about twice the thickness of a (my) human hair - so would think of it as 'pretty thin'.

I was thinking "adequate for ADS" as meaning to cause the OPD to operate when pierced without vapourising first; not to act as a CPC which it isn't designed to do.

Fair enough, but one can really only consider how adequate the CSA is for use as a CPC. When talking about the adequacy of a CPC (i.e. Zs), we do not (cannot, really) consider the area of contact with something causing a fault (which will often be a very small area of contact).

If one has something which is, theoretically, adequate as a CPC, then it should not 'vapourise', although I suppose it's always possible that material (either in the CPC or the live something coming into contact with it to create a fault) will vaporise locally, thereby self-clearing the fault.

Kind Regards, John
Edit: arithmetic corrected

 

[EFLImpudence]

Fair enough, but one can really only consider how adequate the CSA is for use as a CPC. When talking about the adequacy of a CPC (i.e. Zs), we do not (cannot, really) consider the area of contact with something causing a fault (which will often be a very small area of contact).

That is true.

 

[ban-all-sheds]

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

Is the "outer sheath" copper?

 

[JohnW2]

Is the "outer sheath" copper?

If it's straight con, the 'outer sheath" will presumably consist of multiple conductors, probably copper (but conceivably aluminium) wrapped/twisted around the central core, won't it?

Kind Regards, John

 

[ban-all-sheds]

If it's single core armoured cable the 'outer sheath" will presumably consist of multiple conductors wrapped/twisted around the central core, won't it?

 

[JohnW2]

If it's single core armoured cable the 'outer sheath" will presumably consist of multiple conductors wrapped/twisted around the central core, won't it?

It will, but to the best of my knowledge single-core-armoured is not referred to as "straight concentric". Since the OP used that term (which is unfamiliar to the general public, and probably even a significant number of electricians), I presumed that he probably knew that that was what it was - but I (or he) may, of course, be wrong.

Kind Regards, John

 

[ban-all-sheds]

I just thought it might be worth making sure that it really is straight concentric, and not single core SWA.