Underground T&E?

[JohnW2]

By virtue of what might perhaps be regarded as some degree of foresight many years ago, I have a run of about 25m of ~100mm plastic conduit encased in concrete buried (about 800mm depth, if I recall) in part of my garden. One end enters the cellar of the house and the other emerges in a garden building (dare I say 'shed'?!). Currently it only carries some small SELV 'signal' cables.

I am contemplating the possibility of running a power cable through it. I would have thought that concrete-encased conduit probably satisfies the qualification of 522.8.10 regarding "equivalent [to armour] protection against mechanical damage", in which case I could probably theoretically use ordinary T&E cable. What do people think? Admittedly, the price differential between T&E and SWA is fairly small (less than 2:1), but in these hard financial times, I guess that 'every penny counts'!

Kind Regards, John.

 

[ban-all-sheds]

I think 2 things:

1) You should use SWA.

2) You can't put any LV cable down that conduit/duct unless you remove the signal cables (assuming, as it is almost certain, that those are not insulated to Band II voltage levels).

 

[ricicle]

And if you did decide to go ahead and put a mains cable down there anyway alongside the 'signal' cables then the armour would provide some screening.
Also rodents can find their way down ducts to gnaw at T+E.

 

[Click4]

+1 armoured cable

BAS knows his stuff

 

[Spark123]

I think buried cables are meant to have an earthed sheath, hence +1 for SWA.

 

[JohnW2]

1) You should use SWA.

I assumed that at least some people would say that, but why? If the "equivalent protection" provision of 522.8.10 was ever going to be satisfied, I would have thought that what I described would satisfy it.

2) You can't put any LV cable down that conduit/duct unless you remove the signal cables (assuming, as it is almost certain, that those are not insulated to Band II voltage levels).

Yes, I thought of that one, but (unless I'm missing something) it sounds totally daft to me. If (as is the case) the LV cable is sufficiently insulated for me to safely handle it, or to be in contact with earthed materials, then what on earth is the perceived risk? Furthermore, although you are undoubtedly right that the current signal cables are not rated at Band II voltage levels, I'd be pretty surprised if, in reality, their insulation can't stand 230V (I think you may have moved me to conduct an experiment!).

However, regs (even daft ones) are regs, so if I decided that compliance with them was an issue, then I was contemplating replacing the signal cables with something that would keep even silly regulations happy - of which plenty of options are available, such as this

Kind Regards, John.

 

[JohnW2]

I think buried cables are meant to have an earthed sheath, hence +1 for SWA.

As I intimated, what 522.8.10 says is:

"Except where installed in a conduit or duct which provides equivalent protection against mechanical damage, a cable in the ground shall incorporate an earthed armour or metal sheat, or both ..."

As I said, I think I could argue that conduit encased in concrete does afford such 'equivalent protection' - with the possible exception of damage by rodents (which ricicle has mentioned).

Kind Regards, John

 

[JohnW2]

+1 armoured cable
BAS knows his stuff

Does that 'stuff' include (from 522.8.10):

"Except where installed in a conduit or duct which provides equivalent protection against mechanical damage, a cable in the ground shall incorporate an earthed armour or metal sheat, or both ..."

?
Kind Regards, John

 

[JohnW2]

And if you did decide to go ahead and put a mains cable down there anyway alongside the 'signal' cables then the armour would provide some screening.

True, but I don't think screening is an issue - or, at least, an issue which couldn't be dealt with; the signal circuits are already designed to be noise-immune.

Also rodents can find their way down ducts to gnaw at T+E.

I suppose that's a theoretical possibility. However, they's have to either knaw through concrete to get to the underground duct, or else get into the cellar or shed and somehow overcome the 'anti-rodent' cappings (incorporating steel mesh) at one of the ends. I think that, in reality, the wiring in my roofspace (not in SWA!) is at far more potential risk from gnawing creatures (particularly squirrels).

Kind Regards, John.

 

[electronicsuk]

I don't think anyone here is likely to recommend that you bury T+E in your duct. While I agree in theory that the concrete may well afford enough mechanical protection, I don't think that what you plan to do is really in the spirit of the regs - though I've often thought the same about singles or T+E in high impact conduit buried in a screed. Besides, can you really guarantee that mechanical protection throughout the entire length of the duct?

Suck up the cost and buy SWA. I think you already know that this is the correct thing to do, and it'll allow you to sleep soundly at night

 

[JohnW2]

I don't think anyone here is likely to recommend that you bury T+E in your duct. While I agree in theory that the concrete may well afford enough mechanical protection, I don't think that what you plan to do is really in the spirit of the regs ....

No, I suppose you're right that I didn't really expect anyone to recommend T&E. However, as for the spirit of the regs, if high impact ducting encased in concrete is not within the spirit, then what on earth did they have in mind when they included the "duct which provides equivalent protection" provision in 522.8.10; they could obviously have omitted that provision if they had wanted to.

Besides, can you really guarantee that mechanical protection throughout the entire length of the duct?

As much as one can 'guarantee' anything, I reckon. The entire underground run was encased in concrete, which is unlikely to have gone away (it's under a lawn, so will not have been interferred with in any way).

Returning to the spirit of the regs, what I do find rather incongrous is that we are generally allowed to bury T&E in plaster (as shallow as we like, if its RCD protected), yet we are quibbling about the mechanical protection of something buried 2.5 feet underground within hard plastic and concrete! Be honest (and particularly bearing in mind that this is a 'DIY' forum!) - which of the two do you think is more likely to suffer mechanical damage?

Suck up the cost and buy SWA. I think you already know that this is the correct thing to do, and it'll allow you to sleep soundly at night

Although I mentioned it, cost is not really the issue; as I said, the price differential is quite small. To be honest, my greater concern is getting SWA through - if you have experience of pulling a cable through a duct which already contains cables, you'll presumably know that it can be a nighmare. However, if I go ahead with this I dare say I will probably relent and try, even if I don't think anyone is going to convince me that it's really necessary!

Kind Regards, John.

 

[Stoday]

Way back in my early days in the electricity industry, we used paper insulated lead covered cables. When laid in the ground, these were armoured with steel wire or tape. Whan laid in ducts, they were unarmoured and had nothing covering the lead sheath.

Do not think for one minute that I'm suggesting that TW&E is OK for your application. I'm just saying that I would have no objection to an unarmoured cable providing it had a metal sheath. I doubt that anyone would object to your using MICC.

 

[electronicsuk]

Returning to the spirit of the regs, what I do find rather incongrous is that we are generally allowed to bury T&E in plaster (as shallow as we like, if its RCD protected), yet we are quibbling about the mechanical protection of something buried 2.5 feet underground within hard plastic and concrete! Be honest (and particularly bearing in mind that this is a 'DIY' forum!) - which of the two do you think is more likely to suffer mechanical damage?

I can't disagree with that, but nor you can expect anyone to openly recommend on this forum that you should carry out in installation in contravention of BS7671

Suck up the cost and buy SWA. I think you already know that this is the correct thing to do, and it'll allow you to sleep soundly at night

Although I mentioned it, cost is not really the issue; as I said, the price differential is quite small. To be honest, my greater concern is getting SWA through - if you have experience of pulling a cable through a duct which already contains cables, you'll presumably know that it can be a nighmare. However, if I go ahead with this I dare say I will probably relent and try, even if I don't think anyone is going to convince me that it's really necessary!

Kind Regards, John.

What size SWA are you looking to pull through? 100mm is a fairly generous diameter as far as the ducting goes, so unless you already have a lot of cables in there, I can't foresee a problem. Apply lubricant liberally, of course.

 

[ricicle]

What size SWA are you looking to pull through? 100mm is a fairly generous diameter as far as the ducting goes, so unless you already have a lot of cables in there, I can't foresee a problem. Apply lubricant liberally, of course.

If this is a domestic set-up then I can't see there being that many existing cables in the duct so if you are concerned then if it is not too much trouble disconnect and pull the old ones out and draw them all back in together.

 

[JohnW2]

Returning to the spirit of the regs, what I do find rather incongrous is that we are generally allowed to bury T&E in plaster (as shallow as we like, if its RCD protected), yet we are quibbling about the mechanical protection of something buried 2.5 feet underground within hard plastic and concrete! Be honest (and particularly bearing in mind that this is a 'DIY' forum!) - which of the two do you think is more likely to suffer mechanical damage?

I can't disagree with that, but nor you can expect anyone to openly recommend on this forum that you should carry out in installation in contravention of BS7671

Well, as you know, I personally don't think it does contravene BS7671; if high-impact plastic inside concrete does not satisfy the 'mechanical protection' required by 522.8.10, then I really don't know what would - and if nothing would, why on earth did they include that provision in BS7671.

However, returning to your main point, as I've said, I did not expect anyone to openly recommend T&E (even though I believe it is probably compliant) - but I was interested in hearing what their arguments would be. In that respect, I have to say that no-one has really yet provided what I would regard as a good reason to believe that what I'm talking about would not satisfy 522.8.10.

What size SWA are you looking to pull through? 100mm is a fairly generous diameter as far as the ducting goes, so unless you already have a lot of cables in there, I can't foresee a problem. Apply lubricant liberally, of course.

It would only be very modest sized SWA, but size it not the issue (it's a big duct with not much in it). Have you not experienced the problem when pull cords/tapes have got wrpped around existing cables? The additional flexibility of T&E might then be an advantage. However, as ricicle has just suggested, one solution would be to pull out the existsing cables and then pull them back in with the SWA.

Kind Regards, John.