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surface-mounted SWA – earthing

Former Community Member
Former Community Member
Just trying to find a regulation that states a reason why the armoured metal of surface-mounted SWA needs earthing. Obviously, when buried underground, it does - 522.8.10.  I have read previous posts on this, but the question seems to have remained unanswered.

It would be considered best practice to at least earth the supply end, but best practice is not regulation. And, considering the statement at the end of Chapter 12, could it not be argued that short runs of surface-mounted armoured without earthing are ‘safe’? Where is the risk?

The armour does not meet the definition of an exposed conductive part when neatly terminated so it can not be touched – under what fault conditions could it become live?

SWA conductors are not double insulated, but is the risk any less than conductors in a plastic conduit?

Manufacturer instructions… it could be that they stipulate that the armour needs to be earthed, but where these are not available for review, how can a non-conformity be raised?

I’d appreciate any replies that point to a specific regulation or group of regulations.

Thanks in advance.

  • 0
    Thank you for the further clarification.


    If I came upon this installation as described, I would struggle to go worse than C3 and might not code at all. If the cable had been made off into normal conducting glands then C2 would be justified on the basis that a single fault (e.g. something pierces the cable) would present a danger not only at the point of damage, but also at the ends of the cable. 411.3.1.1


    If you are going to invoke manufacturer's instructions, then I think you need to identify them. If they exist for SWA, do they also exist for T&E or singles. I suspect not.


    As for compliance, presumably there is an EIC where the installer has certified it.


  • 0
    But do the cable manufacturer's instructions say anything about Earthing the armour? I don't recall ever seeing such a thing. I've seen things like 'Should not be installed at temperatures below 0OC or above +60°C' or 'should be installed according to national wiring rules'.


    I'd suspect the manufacturers would want to avoid such detailed instructions such as the need to Earth things - partly to avoid any liability (better to pass the buck onto JPEL/64) and partly not to want to unnecessarily rule their products out from the more unusual situations (e.g. separated systems, d.c. PV or SELV).


      - Andy.
  • Former Community Member
    0 Former Community Member
    I think the conversation has drifted a little from my OP. We are all in agreement that the steel wire should be earthed.

    I don’t know why the installer chose to use SWA, but the SWA installation did not have any exposed conductive parts. The SWA was terminated at both ends using non-metallic boxes and glands, the steel was not brought into the box, the steel was not used as a cpc – a scenario not covered in Guidance Note 8. So if your reason for earthing the Steel is ECP, then that wouldn’t stand up in this situation.

    The installation was neat and tidy, and on the whole, the electrician looked as if he knew what he was doing.  Now, if that electrician was Ajjewsbury or Chris Pearson, and I told them they had not complied with BS7671 they would come straight back at me with – “prove it!”…. So how do I prove it? My ‘opinion’ is irrelevant; Guidance is helpful but not enough. I would need to offer up a Regulation to demonstrate non-compliance or produce a regulation that reverses the situation where the installer has to demonstrate compliance.

    In my view, although technically compliant with BS7671 from a safety perspective, the installation method is not compliant with cable manufacturer instructions – Hence my choice of Regulation 134.1.1 – “…The installation of electrical equipment shall take account of manufacturers instructions. “. If I raise this as a non-conformity, it will put the onus of the installer to demonstrate how their installation is compliant.

  • 0
    Is the armour an unused conductor?

    https://d2z8ufzpcqvblm.cloudfront.net/sites/www.voltimum.co.uk/files/fields/attachment_file/gb/others/Z/2007061145227060883572007060863222007060882044162bk1snag22_0.pdf
  • 0
    In principle you could mis-use SWA and put it in plastic conduit or something and terminate the ends in plastic boxes- it might be perfectly safe but is a senseless waste of cable. GN8 is very sensibly not considering that case, and I do not think that is what is happening here either.


    To the OP, you could do worse than to print out that scan of guidance note 8 and wave it at the person trying to kid you it is OK...


    Mike.
  • 0
    Sparkingchip:
    Chris Pearson:

    If the cable is surface mounted, AND a core is being used as the CPC, AND the cable is neatly terminated into enclosures with insulating glands, then I agree that there are no ECPs, so the armour does not have to be earthed.

     


    So you completely disagree with the extract from the IET Guidance Note 8 which I posted?




    Yes, because exposed-conductive-part = conductive part of equipment which can be touched ... .


    I cannot think of any circumstances when you would specify SWA, but not use it as intended, but that wasn't the issue in the OP.


  • 0
    Presumably SWA has been chosen because the protection afforded by the armour is required, but then the armour only provides limited protection because of inadequate installation methods.


    It seems fair to say either install the SWA correctly and earth it or use something else.
  • 0
    So you completely disagree with the extract from the IET Guidance Note 8 which I posted?

    It all depends on the assumptions the authors of the GN had in mind. In perhaps 99% of situations SWA is selected because something with lesser performance (e.g. T&E) wouldn't have been suitable for the conditions - thus some demand is being placed on the armour and therefore it needs to be earthed. If you want to write one statement that covers 100% of situations from a design point of view, then always earth it seems a logical approach.


    In the perhaps 1% of situations where SWA has been installed where T&E would have been perfectly adquate, I agree it's still not clear cut. For me it boils down to whether the insulation, armour and sheath of an SWA cable meet the demands of 412.2.4.1(ii)(a) - or whether the presence of the armour detracts from the protection offered by the plastic sheath. It's not clear to me that it doesn't.


      - Andy.
  • 0
    I think they both say the same thing.  Unless this cable is installed very oddly, the armour is an ECP and should be earthed.


    If they really do not want to they would have to  totally insulate it ,but that would be silly.

    Show them that quote....

    Mike.
  • 0
    Chris Pearson:

    If the cable is surface mounted, AND a core is being used as the CPC, AND the cable is neatly terminated into enclosures with insulating glands, then I agree that there are no ECPs, so the armour does not have to be earthed.

     


    So you completely disagree with the extract from the IET Guidance Note 8 which I posted?


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