Is armour of buried SWA an extraneous-c-p?

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I would not have thought that one should assume that the outer sheath of buried SWA inevitably remains intact. That being the case, if the SWA enters a premises, its armour presumably has to be considered 'liable to introduce a potential', and therefore constitutes an extraneous-conductive part.

The armour will obviously be bonded to the installation's MET to some extent by virtue of the 'earthing' system of the installation. However, if the connection between the SWA and CU (or whatever) is in T&E cable, the connection to the MET will be via the T&E's CPC - at worst only 1.5mm² (and unlikely to be more than 4mm² in a domestic setting). Since these connections are inadequate as main equipotential bonding conductors, should additional separate MPB conductors be installed in such situations? (something I haven't heard of being done routinely)

Kind Regards, John
 
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HNY.

I would not have thought that one should assume that the outer sheath of buried SWA inevitably remains intact.
Why not? Presumably not for ever but ...
Continuity test ?
No sheath ?

That being the case, if the SWA enters a premises, its armour presumably has to be considered 'liable to introduce a potential', and therefore constitutes an extraneous-conductive part.
No sheath ? Are plastic boxes better, after all?

The armour will obviously be bonded to the installation's MET to some extent by virtue of the 'earthing' system of the installation.
Oxymoron ?

However, if the connection between the SWA and CU (or whatever) is in T&E cable, the connection to the MET will be via the T&E's CPC - at worst only 1.5mm² (and unlikely to be more than 4mm² in a domestic setting). Since these connections are inadequate as main equipotential bonding conductors, should additional separate MPB conductors be installed in such situations? (something I haven't heard of being done routinely)
To what would you connect it?

Would you remove some of the presumably still intact sheath so that it could become a simultaneously accessible conductive part where it enters the premises?

I can see what you are saying, if completely rotted and so in need of replacement, but surely, with intact sheath, the only touchable part would be a glanded metal box which will not be in contact with the ground.
 
The armour is simply a circuit protective conductor, the exposed ends are an exposed conductive part as they are parts of the electrical installation.

If the armour CSA (in copper equivalent terms) is inadequate for MPBs and it is intended that MPBs are to be connected to it then additional conductors will need to be run in, it is only a real issue with TN-CS as the sizing is derived from supply neutral size (along with DNO reqs).
 
..and to you.
I would not have thought that one should assume that the outer sheath of buried SWA inevitably remains intact.
Why not? Presumably not for ever but ... Continuity test ? No sheath ?
Rodents, moles, other underground creatures, sharp stones + ground movement (maybe due to plant/tree root growth), other 'environmental' phenomenon and, of course, the possible helping hand of human beings!
That being the case, if the SWA enters a premises, its armour presumably has to be considered 'liable to introduce a potential', and therefore constitutes an extraneous-conductive part.
No sheath ? Are plastic boxes better, after all?
Incomplete/compromised sheath - c.f. a buried plastic box with a crack in it. Once there is any breach of the sheath/box, electrolyte-ridden water will eventually do its work.
The armour will obviously be bonded to the installation's MET to some extent by virtue of the 'earthing' system of the installation.
Oxymoron ?
I was trying exceptionally hard to choose wording that wouldn't stimulate '5 pages' :) The CPCs are primarily there to achieve the functionality of 'earthing'. However, they will inevitably also 'bond', albeit via a conductor of inadequate CSA (per regs) for main protective/ equipotential bonding.
However, if the connection between the SWA and CU (or whatever) is in T&E cable, the connection to the MET will be via the T&E's CPC - at worst only 1.5mm² (and unlikely to be more than 4mm² in a domestic setting). Since these connections are inadequate as main equipotential bonding conductors, should additional separate MPB conductors be installed in such situations? (something I haven't heard of being done routinely)
To what would you connect it? Would you remove some of the presumably still intact sheath so that it could become a simultaneously accessible conductive part where it enters the premises? I can see what you are saying, if completely rotted and so in need of replacement, but surely, with intact sheath, the only touchable part would be a glanded metal box which will not be in contact with the ground.
It's not literally the CSA armour which is the issue as far as the house is concerned, but, rather, what that armour is (deliberately) connected to - namely the earthing system of the installation, particularly any parts of the earthing system (and their exposed-conductive-parts) 'electrically closer' to the connection with the SWA armour than to their connection with the MET....

... in practice, I see no real hazard since, even if you accept my argument that the armour should be treated as an extraneous-c-p, the 'bonding' (if I dare call it that!) to the MET via the installation's earthing system should ensure equipotentiality within the premises under nearly all circumstances. However, regs-wise, we would not normally be allowed to rely on a conductor which could be as small as 1.5mm² to achieve that equipotentiality. I suspect that if you had just a fairly short length of water supply pipe within a premises before it changed to plastic for the rest of the house's pipework, you would still feel obliged to main bond it - but I wouldn't expect you to try arguing that, because there was not much of the extraneous-c-p (or things connected to it) to touch, a 1.5mm² main bonding conductor would be adequate, would you?

Kind Regards, John
 
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The armour is simply a circuit protective conductor, the exposed ends are an exposed conductive part as they are parts of the electrical installation. ... If the armour CSA (in copper equivalent terms) is inadequate for MPBs and it is intended that MPBs are to be connected to it then additional conductors will need to be run in, it is only a real issue with TN-CS as the sizing is derived from supply neutral size (along with DNO reqs).
I'm afraid that you have completely misunderstood my point. I'm not talking about the adequacy of the CSA armour as a main bonding conductor for something at the other end (what I'm saying applies equally even if, as is sometimes the case, the armour is connected to nothing at the other end). I also agree that the primary intent of the armour is as a CPC, hence any exposed parts of the armour (or things in continuity thereto) are, in that sense, exposed-conductive parts - but I'm arguing that they can (and maybe should) also be regarded as (potential) extraneous conductive parts ...

...considier a simplified, and extreme, situation. Imagine that there was a run of buried SWA with nothing connected to the armour at the far end, and (to simplify things further) nothing connected to any of the cores at either end. Also imagine that the outer sheath of the buried SWA is damaged, so that at least some of the armour is in contact with the ground/earth. The SWA enters the house and its armour is terminated in, say, an accessible metal box (cores not connected to anything). Do you not consider that the metal box (and anything in electrical continuity with it) then coinstitutes an extraneous-c-p and that it therefore requires main bonding? If you do agree that it requires main bonding, would you really feel that use of a 1.5mm², 2.5mm² or 4mm² main bonding conductor would be compliant with the regs?

Kind Regards, John
 
My thoughts go straight to the SWA of a sub main supplying an outbuilding that is configured as TT with it's own local earth rod. This is a situation where the "earth" of the host building must NOT be exported to the outbuilding.

If the SWA is bonded to the MET of the outbuilding then the "earth" from the host building is exported.
 
You have some good points. A friend had the job of "Pressure testing" cables between lamp posts to ensure the outer plastic was not damaged. These were re-tested after any road works to see if the cable had be caught.

I have also had it where the SWA of a cable has lost continuity I would assume the outer has been caught and the SWA has rusted through. Could not dig up cable so made building TT.

The third was where the 4mm earth wire run alongside the SWA melted when the DNO lost it neutral/earth connection due to road works but this is due to the earth also being connected to an earth mat in the shed it supplied where the owner had radio equipment installed so it was an unusual set up.

The last does raise the question as to cable sizes used to connect an extraneous-conductive part. If we look to industry we see huge resistors connecting the DNO earth to site earth to ensure with situations like that nothing will burn out.

But for domestic I think we need a risk assessment. How likely is it that the DNO lose their neutral/earth alone. I have only seen it once in the UK. And in real terms the DNO was at fault for not fitting multiple earths had the PME system been used rather than just TN-C-S then there would not have been a problem.

I have questioned the shroud used on glands as they can be removed without a tool. So where exporting as a TT supply clearly there needs to be a break somewhere between DNO earth and the earth electrode. In the case of a shed should this be at house or at shed? Location of the RCD with dictate if at the house then break could be house side but more likely in the shed so the SWA has to be insulated at shed in a manor where it will require a tool or key to assess the SWA.

OK those may be the rules but is it being rather pedantic to fit some sort of box over the incoming gland?

The only time I really worry is with supplies to radio shacks. Since the RAE was dropped and replaced with the three part exams I worry if the novice really has the training required to avoid danger?

I have also questioned the idea of putting diodes in the earth cable? I know they are designed for the job but never seen a schedule for testing the diodes. They must fail from time to time. Until my son had a boat I did not even know they existed nothing was raised about diodes in the earth cable when doing the 2391. And what will using an insulation tester do to the diodes?

OK on industrial one does have to consider things like this but with domestic is it really an issue?
 
due to the earth also being connected to an earth mat
or it could be connected to a metallic service pipe with an impedance low enough for high currents to flow to ground.

How likely is it that the DNO lose their neutral/earth alone.
At my last house in 30 years there were 3 significant incidents of network cable faults bouncing our neutral.

The fitting of diodes is intended to create a DC potential between "earthed" ( bonded ) metal and true ground where that metal is in contact with wet ground. Forcing the metal to have a negative potential relative to ground reduces the corrosion of the metal when earth fault currents are generated.
 
I'm afraid that you have completely misunderstood my point. I'm not talking about the adequacy of the CSA armour as a main bonding conductor for something at the other end (what I'm saying applies equally even if, as is sometimes the case, the armour is connected to nothing at the other end). I also agree that the primary intent of the armour is as a CPC, hence any exposed parts of the armour (or things in continuity thereto) are, in that sense, exposed-conductive parts - but I'm arguing that they can (and maybe should) also be regarded as (potential) extraneous conductive parts ...
Simply put, if it is part of the electrical system then it cannot be an extraneous conductive part. See definitions in part 2 of the regs.
...considier a simplified, and extreme, situation. Imagine that there was a run of buried SWA with nothing connected to the armour at the far end, and (to simplify things further) nothing connected to any of the cores at either end. Also imagine that the outer sheath of the buried SWA is damaged, so that at least some of the armour is in contact with the ground/earth. The SWA enters the house and its armour is terminated in, say, an accessible metal box (cores not connected to anything). Do you not consider that the metal box (and anything in electrical continuity with it) then coinstitutes an extraneous-c-p and that it therefore requires main bonding? If you do agree that it requires main bonding, would you really feel that use of a 1.5mm², 2.5mm² or 4mm² main bonding conductor would be compliant with the regs?
You could argue this one is an extraneous conductive part as it isn't part of the electrical system hence should technically be main bonded, but probably easier to remove the cable.
The bonding size being dependant on supply type etc. The minimum size for bonding is 4mm which may be adequate for TNS depending on the size of the earthing conductor. 4mm is also very likely ok for TT
 
My thoughts go straight to the SWA of a sub main supplying an outbuilding that is configured as TT with it's own local earth rod. This is a situation where the "earth" of the host building must NOT be exported to the outbuilding.
I agree that if one decided that one wanted to TT the outbuilding, the whole point would presumably be that one didn't want to export the host building's earth to that outbuilding (since that would defeat one's objective). However, one would then 'earth' the SWA armour at the host end in order to give the required protection to the cable - hence my question.
If the SWA is bonded to the MET of the outbuilding then the "earth" from the host building is exported.
I've never heard of anyone doing it that way around - as above, if the SWA armour is connected only at one end, it is invariably at the host end - which makes sense. If it were (only) connected to the outbuilding's TT electrode, it is most unlikely that there would be a low enough Zs to achieve the required disconnection times (or, indeed, any ADS at all!) in the case of an L-armour fault.

Kind Regards, John
 
John

By host builiding I meant the building from where the supply is coming from. The SWA would be connected to the MET of that building and not the MET of the outbuilding.

I have seen places where a SWA cable was earthed at the destination end. This was a communication site which had a very low impedance grounding system. I assume this was to avoid the SWA bringing in an "earth" that could be at a different potential to the site's grounding arrangements.
 
I do not think you can consider the consequences of swa armour which is not connected to earth at either end as that would defeat the purpose of having the armour.

As, has been said, because it is part of the electrical system it therefore is/can not be considered extraneous and so is earthed to requirements, the same as conduit, trays or trunking etc.

I suppose if you suspect that the armour is in contact with the ground it could/should be tested and the appropriate action taken to remedy or remove any danger. The simplest of which may be to ensure that no part of the armour and its connected parts are touchable metal.

I do not see bonding at the point of entry to be sensible.
This would be akin to you bonding your earthing conductor at its point of entry (ignoring the fact that you have other better earthing through pipes).
 
Simply put, if it is part of the electrical system then it cannot be an extraneous conductive part. See definitions in part 2 of the regs.
That's obviously true, but to concentrate just on that definition is to completely ignore the whole point of main equipotential bonding ....
You could argue this one is an extraneous conductive part as it isn't part of the electrical system hence should technically be main bonded, but probably easier to remove the cable.
That is, indeed, precisely what I was arguing and I wasn't talking about a situation in which the cores were not going to be used - so 'removing the cable' would not be an option. If you agree that ('it can be argued that') without the cores connected the SWA armour qualifies as an extraneous-c-p which 'is liable to introduce a potential' and therefore should be main bonded (to ensure equipotentiality), then why does that situation change when one starts using the cores? Sure, the armour then technically ceases to be an extraneous-c-p (per BS7671 definition), but it's 'liability to introduce a potential' will not have changed - and the only thing 'bonding' that armour to the MET would be the CPC of the cable supplying it, possibly as small as 1.5mm².
The bonding size being dependant on supply type etc. The minimum size for bonding is 4mm which may be adequate for TNS depending on the size of the earthing conductor. 4mm is also very likely ok for TT
All agreed - but a T&E cable would only have a CSA of ≥4mm² if it were a 10mm² or 16mm² T&E, and feeds to SWA in domestic situations are often/usually smaller than that. I have a lighting circuit in my garden consisting of 2.5mm² SWA fed by 1.5mm² T&E - so the only connection from SWA armour to the MET is via a 1.0mm² conductor.

As I've said, I doubt that there is any significant hazard in practice - but only because one will usually 'get away with' bonding connections from the armour to MET which are of much smaller CSA than would be required by the regs as a main bonding conductor.

Kind Regards, John
 
By host builiding I meant the building from where the supply is coming from.
I realised that - I can't see what else it could mean!
The SWA would be connected to the MET of that building and not the MET of the outbuilding.
Quite - as I said.
I have seen places where a SWA cable was earthed at the destination end. This was a communication site which had a very low impedance grounding system. I assume this was to avoid the SWA bringing in an "earth" that could be at a different potential to the site's grounding arrangements.
Fair enough, but that's an extremely unusual situation - as I said, any normal TT earthing system (certainly any likley to be found in a domestic environment) would not have low enough impedance to provide adequate ADS.

Anyway, none of this has really got anything much to do with the issue I'm discussing.

Kind Regards, John
 

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