Shed straight concentric supply query

[JohnW2]

Do you intend to ignore the fact that as well as suitability as a cpc, it also says that the cable must be armoured or have a metal sheath? .... Ali-tube and SY cables are/have neither, and therefore no matter in what qualitative or quantitative ways their screens or braids are suitable as a cpc, they may not be directly buried.

I'm not 'ignoring it', but you appear to be talking on the basis of some definition of 'metal sheath' with which I am not familiar. Would you be able to share it with us?

Kind Regards, John

 

[ban-all-sheds]

No.

Again who knows, but I would doubt that such a description means more than it says, hence with no requirement for mechanical properties.

That might also be incorrect.

AIUI there is/are BS EN standard(s) for MI cables, so if there are no "metal sheathed" cables apart from MI, we might know what "metal sheathed" meant re mechanical properties.

What about the 'ali-tube' cable we're always hearing about here (but which I have never even seen)?

That is not a metal sheathed cable.

... and how does SY fit into this spectrum, I wonder?

That can be your assignment.

Are there any official definition(s) or standard(s) for SY cable? Is it classed in anything de jure or de facto as "metal sheathed"?

Spoiler: My [I][B]guess[/B][/I]

No.

 

[EFLImpudence]

That is not a metal sheathed cable.

Why not?

 

[JohnW2]

Why not?

One can but presume that he has found some definition of "sheathed" but, although I have asked, he has yet to share it with us.

Up until now, and unless/until he can convince me otherwise, I have taken it to simply refer to 'a covering'.

Kind Regards, John

 

[EFLImpudence]

You appear to have shifted this discussion from a very general one (about what cables can be buried) to one that relates specifically so some particular cable.

Only in as much as that is one cable which could be buried if it meets the criteria.

What particular cable do you have in mind?

Have you not noticed? I haven been asking about ali-tube.

SY, for example has an insulated CPC, within another 'insulating' sheath - so two layers of plastic between the CPC core and the outer "wire braid" which one earths.

Ok.

I think confusion has arisen because 522.8.10 states
"a cable buried in the ground shall incorporate an earthed armour or metal sheath or both, suitable for use as a protective conductor"
therefore armour is not actually required.

It is dependent on the definition of sheath and suitable for use.

The same cable buried in a wall has to have an earthed metallic covering.

 

[JohnW2]

I think confusion has arisen because 522.8.10 states "a cable buried in the ground shall incorporate an earthed armour or metal sheath or both, suitable for use as a protective conductor"
therefore armour is not actually required.
It is dependent on the definition of sheath and suitable for use.

Indeed - and I suppose that's largely what we have been discussing.

The same cable buried in a wall has to have an earthed metallic covering.

Again, indeed - and that's one of the reasons why, rightly or wrongly, I have been regarding a 'metal sheath' and a 'metal covering' as being the same thing.

Even if BAS comes up with a definition of "sheath"/"sheathed", it cannot be a BS7671 definition - so I'm not sure that we would then be much the wiser.

Kind Regards, John

 

[SimonH2]

... why, rightly or wrongly, I have been regarding a 'metal sheath' and a 'metal covering' as being the same thing.

Indeed, so have I.
Looking up dictionary definitions of sheath, I find A protective covering around an electric cable and a close-fitting covering to protect something. On that basis, in the absence of some official technical definition to the contrary, any cable with a continuous tube or foil round it would count.
And then you can argue about whether a pattern of closely spaced wires would also count. I think we can all agree that the intent is that if someone finds a buried cable with some tool, then when they penetrate the cable the supply to it will get automatically disconnected. We could all come up with hypothetical situations where a small probe could penetrate the armour of an SWA without making a good electrical contact - but in the real world that's not likely. With Ali tube I'd say it's equally unrealistic - needing an insulated probe.
With SY it's somewhat more debatable - a thin point could well miss the wires, but something like a spade could not.

Also, when considering prospective fault currents, are we using the covering as the only CPC, or are we using a 3 core cable with one core as CPC ? Because with the latter, then in the event of a penetration there's two paths back to the supply - one direct via the covering, the other via the covering to the load end and then back through the other CPC. So the worst case will be somewhere between the midpoint and the load end - the actual point depending on the ratio of (effective/equivalent) CSAs

 

[JohnW2]

Indeed, so have I. Looking up dictionary definitions of sheath, I find A protective covering around an electric cable and a close-fitting covering to protect something. On that basis, in the absence of some official technical definition to the contrary, any cable with a continuous tube or foil round it would count.

Yes, that what I have been 'assuming'.

And then you can argue about whether a pattern of closely spaced wires would also count. I think we can all agree that the intent is that if someone finds a buried cable with some tool, then when they penetrate the cable the supply to it will get automatically disconnected. We could all come up with hypothetical situations where a small probe could penetrate the armour of an SWA without making a good electrical contact - but in the real world that's not likely. With Ali tube I'd say it's equally unrealistic - needing an insulated probe. With SY it's somewhat more debatable - a thin point could well miss the wires, but something like a spade could not.

Yes, all agreed. As you imply, it's probably with 'braided' coverings that there is the most scope for debate, but I would personally think that most (unless very 'open' in their weave) probably 'count'/

Also, when considering prospective fault currents, are we using the covering as the only CPC, or are we using a 3 core cable with one core as CPC ? Because with the latter, then in the event of a penetration there's two paths back to the supply - one direct via the covering, the other via the covering to the load end and then back through the other CPC. So the worst case will be somewhere between the midpoint and the load end - the actual point depending on the ratio of (effective/equivalent) CSAs

That's an argument that EFLI has been presenting. As so often, the reg is not totally clear, but, for what it's worth, when it requires the metal sheath to be "adequate as a CPC", I have taken that to mean that it would be adequate on its own. Indeed, the reg does not actually say anything about there having to be any CPC core, so (just with SWA armour), I suppose that the covering could be the only CPC, couldn't it?

Kind Regards, John

 

[SimonH2]

As you imply, it's probably with 'braided' coverings that there is the most scope for debate, but I would personally think that most (unless very 'open' in their weave) probably 'count'/

SY is fairly "open" in that respect, but I think you'd be doing well to stick (say) a fork tine through it while missing every strand. Though I suppose there's the argument that each strand is quite fine - so could "fuse" without tripping the protection on the circuit.

... I suppose that the covering could be the only CPC, couldn't it?

As "well discussed" in the past, it could be if it was adequate to meet the requirements.

 

[JohnW2]

As "well discussed" in the past, it could be if it was adequate to meet the requirements.

Indeed, but my point was that I am (rightly or wrongly) reading this reg to say that the armour/sheath/whatever has to be such that it would be adequate if it were the only CPC - but goodness only knows whether that was the intended meaning!

Kind Regards, John

 

[SimonH2]

... I am (rightly or wrongly) reading this reg to say that the armour/sheath/whatever has to be such that it would be adequate if it were the only CPC - but goodness only knows whether that was the intended meaning!

Ah, another worm crawling out of the can. If it says "adequate as a CPC" then it's debatable whether that should be read as "adequate as the CPC on it's own". As you've pointed out, without being able to read the minds of those who wrote the reg, it's hard to know what they intended.

 

[JohnW2]

Ah, another worm crawling out of the can. If it says "adequate as a CPC" then it's debatable whether that should be read as "adequate as the CPC on it's own". As you've pointed out, without being able to read the minds of those who wrote the reg, it's hard to know what they intended.

Exactly.

However, my interpretation was the only one I could think of which made much sense. If it meant (as you have {and EFLI has} implicitly suggested as an alternative) "adequate as a CPC when used in parallel with another ['unknown'] CPC", then that would be meaningless, since whether or not it was "adequate" would depend entirely on what the 'other' CPC was. Indeed, if the 'other' CPC were adequate in itself, then no additional 'adequacy' would have to come from the sheath/whatever!

Can you think of any other (meaningful) way of interpreting it?

Kind Regards, John

 

[ban-all-sheds]

Why not?

Which part of a cable is the sheath?

Does BS 8436 describe the foil layer as a sheath?

Do any makers of BS 8436 describe them as metal sheathed?

Can you find any support for the idea that because BS 8436 cable is metal sheathed it is suitable for direct burial?

 

[ban-all-sheds]

Even if BAS comes up with a definition of "sheath"/"sheathed", it cannot be a BS7671 definition - so I'm not sure that we would then be much the wiser.

I think you'll find that BS 7671 implicitly inherits definitions from other standards.

 

[JohnW2]

I think you'll find that BS 7671 implicitly inherits definitions from other standards.

Maybe. Perhaps you could let us know what definition of "sheath" or "sheathed" it has inherited. ... or are you perhaps just guessing, then arguing on the basis of your guess?

Kind Regards, John