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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.

  • Former Community Member
    0 Former Community Member
    wallywombat:

    The main difference between T&E and SWA is that damage may make the armour live along its whole length, compared to T&E where any "liveness" is just at the nail or exposed bit of conductor or whatever at the one point. Thus making any other minor damage to the sheathing or termination potentially far more dangerous


    I would agree - if it were exposed. In this case, it is not. The cable is a relatively short run, clipped at a height where it is unlikely to encounter any mechanical damage.  


    I don't believe anyone on this forum doesn't disagree with the earthing of the steel wire, but back to my original post, I asked for regulation, not opinion, to support why steel wire 'Must be earthed' in this instance. To date, I don't believe anyone has come up with one. 


  • 0
    The main difference between T&E and SWA is that damage may make the armour live along its whole length, compared to T&E where any "liveness" is just at the nail or exposed bit of conductor or whatever at the one point. Thus making any other minor damage to the sheathing or termination potentially far more dangerous
  • 0
    You are making an assumption Andy, and that is that the sheath is undamaged.

    Not quite. My assumptions are that the SWA sheath is not lesser than the sheath found on an insulated & sheathed (e.g. T&E) cable, and in this particular case, SWA has been employed in a situation where T&E would have been acceptable (which seems to be the case from the OP's description).


    Yes, a single event could damage both the sheath and basic insulation at the same time - but that's exactly the same issue with T&E (if anything the mechanical properties of the armour in the SWA cables make simultaneous damage to the basic insulation somewhat less likely). In situations where we're relying on the armour for protection (underground, concealed in walls outside of zones, or in harsh environments) then yes clearly we can't rely on the sheath remaining intact. In this case however the situation seems no worse at all than if T&E had been used - as if that was acceptable, how can this one not be?


      - Andy.
  • 0
    You are making an assumption Andy, and that is that the sheath is undamaged. As it is only mechanical protection (the definition of a cable sheath) you cannot assume that. If it is damaged and there is a fault, two things which may well happen together as that is why you are using SWA, it is an exposed conductive part at a dangerous potential to external Earth. That is why I class it as containment and exposed. Real SWAs do get damaged, and do sometimes get Earth faults to the armour. If they didn't no one would use them!
  • 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.
  • 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
    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
    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
    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.
  • 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.

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