Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 44 replies
  • Subscribers 32 subscribers
  • Views 3076 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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
    It’s easy to find guidance telling electricians to earth the armour of SWA cables, such as this:

     

    Providing automatic disconnection of supply (ADS)



    Regardless of whether the armouring of the cable serves as a protective conductor, or not, it should always be effectively earthed to ensure the requirements of ADS are met. Should the armouring not be effectively earthed and a fault develops due, for example, to mechanical damage of a line conductor incorporated in the cable, the condition may leave the armouring attaining line voltage and pose a danger to persons in the vicinity of the installation or using the installation.

    https://www.voltimum.co.uk/articles/guide-installing-steel-wired-armoured


    Can anyone post a link to what seems to be an authoritative article saying that it’s not required? There’s plenty of “how to do it” videos on Youtube in which electricians say it’s not required, but I don’t count consider those authoritative. 


     


  • 0
    The law in England is permissive, that is to say that you can do whatever you will unless it is forbidden.


    If you can demonstrate that not earthing the armour of SWA as we have discussed is forbidden, so be it. Otherwise, leave well alone.
  • 0
    I will carry on earthing it, I’m not being convinced that earthing is not required.


    In reality earthing it is not much extra work, you have to trim the cable and armour utter regardless. The only extra work fitting a SWA gland instead of a stuffing gland is wiggling the conductors about to bell out the armour to take the cone of the gland and fitting an earth fly lead in the enclosure, which is hardly an excessive amount of work, it also ensures compliance with 526.1


    Andy B
  • 0


    Btw – domestic – through cavity wall to outside.

     


    Just out of interest, please give more details. Length of run. Height above ground. Is the cable liable to receive knocks or thumps by anything? What does it run to? Why was S.W.A. chosen in the first place for this run? Have you any pictures of the run.


    Thanks,


    Z.


  • 0
    It’s easy to find guidance telling electricians to earth the armour of SWA cables, such as this: Providing automatic disconnection of supply (ADS)

    But you're presuming shock protection is by ADS. For exposed plastic sheathed cables it would usually be by double/reinforced insulation via 412.2.4.1 (how could a T&E cable provide shock protection by ADS when there's no earthed part between the live conductors and any victim?). Is this setup really any worse than if T&E had been used?

       - Andy.
  • 0
    This discussion has gone on for a long time. In an EICR, if everything is fully insulated I would give a C3. It is not a standard method and it requires improvement. It is only potentially dangerous if the cable is damaged sufficiently to expose the armour, and connect it to a live conductor, which is very unlikely. The "other" electrician has condemned it, presumably on the basis of an exposed conductive part as my previous post, but if it is all mechanically protected, similar to a sheathed cable, it is not immediately dangerous, unless as the previous sentence. In my opinion, SWA with unconnected armour is simply bad workmanship, there is no sense in not Earthing it, and the EVSE install mentioned by Lyle is simply perverse. From the OP, write out an EICR form, only covering this cable on the limitations section, code it a C3 referring to 411 and give it to the client. His problem, he can get it fixed or not, you have provided a report in place of certification, everyone should be happy.
  • Former Community Member
    0 Former Community Member
    On Friday, I had a chat with the technical manager of a leading cable manufacturer. He stated that best practice dictates that the steel wire (not used as a cpc) should be earthed at least one end, but he did not have a problem with unearthed steel wire as long as the installation complied with BS7671.

    So, back to my original post… applying BS7671, and commonsense, the installation is compliant.

  • 0
    Just remember that in the smaller sizes of SWA the bedding is an extruded plastic that looks like and gives the impression of being supplementary insulation between the basic insulation on the conductors and the steel armour.


    But, on the larger sizes of SWA, generally 25 mm and above, the bedding is plastic string similar to the plastic garden twine I have just used to tie my tomato plants to support canes in my greenhouse, there is absolutely no way that this plastic string can be considered as supplementary insulation.


    So as far as I am concerned SWA has single insulated conductors within metal containment and that metal containment needs to be earthed, which isn’t difficult.


    On the odd occasion I only earth one end of the armour I use heat shrink or as tape to ensure the end of the armour cannot be touched from within the enclosure on the end that isn’t earthed to avoid there being two accessible earthing systems within the same enclosure.


    To sum up, I’ll stick with guidance from the authors of the IET GN, incidentally I did read the page in the GN that acknowledges some of the authors and contributors ?
  • Former Community Member
    0 Former Community Member
    Schip - Your contribution has been helpful. I would follow your example when installing myself and advise others to do the same, but the G in GN is for guidance. I need to quote Regulations and keep in mind the end of Chapter 12 when writing reports that could be used in a court of law.
  • 0
    So as far as I am concerned SWA has single insulated conductors within metal containment and that metal containment needs to be earthed

    But in this particular case, unlike most steel conduit, trunking etc., the "metal containment" isn't exposed - it's protected by a plastic sheath - rather like the sheath that covers basic insulation on T&E cables. If it's not an exposed-conductive-part how can you insist it must be earthed?


    Rather like the debate about insulated & sheathed cables run inside unearthed steel conduit (for purely mechanical protection) - (a common practice on the continent) - I think it's accepted that that's OK since we already have the equivalent of double/reinforced insulation between the conductor and the inside of the conduit. This case only differs in the order of the metallic and insulating sheath layers.


       - Andy.

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines