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Why STEEL in SWA?

AJJewsbury

Admittedly I don't make off a lot of SWA, but do occasionally and can usually do a decent job in a reasonable time but the other day I had to terminate a couple of SWAs into an awkward position (back of  cupboard, restricted space, having to work left handed, and not quite enough space to get a spanner in and so on) and got to thinking there must be a better way…

Most of the difficulty was around glanding off the actual steel armour - trying to get everything aligned and tightened in a confined space seemed more like the less desirable aspects of being a plumber rather than an electrician. Split-con would have appealed - as then a simple stuffing gland could have been used and the copper outers just pig-tailed into the terminals, but as the cables go underground split-con isn't permitted any more. Which got me thinking - why is the armour in SWA steel? (apart from the name of course) - had it been copper it could be terminated like split-con. The physical robustness of steel seems rather wasted since if the cable is penetrated by something the steel strands are easily displaced (as in the garden fork experiment) - so really it is ADS that the armour gives us in way of safety rather than an impenetrable mechanical barrier - and copper if anything would be better than steel at enabling ADS. The DNOs use concentric cables with just copper “armour”, if as a PEN rather than just PE, but the principle is the same.

So I guess I'm coming around to copper concentric cables, but with an extra core for a separate N - it could still be glanded off using brass glands if you really wanted to, but you'd have the option of just pig-tailing the c.p.c. where that was more appropriate.

That might feel like it's going to be more expensive (copper instead of steel), but as most people use and extra core in SWA for c.p.c. in parallel with the armour, it's really just moving that copper to the armour instead - so really it's a saving of the steel with no extra copper required. So perhaps slightly cheaper and slightly smaller o.d. cables.

Any other takers for “CWA" cables?

     - Andy.

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    Zoomup: 
    Perhaps in certain situations a length of underground Hi-Tuff cable run in steel scaffold pole would comply. Especially if the whole lot was to be run under a concrete drive or patio, with suitable warning tape over it of course.

    I think that there is a distinct danger of the pudding being over-egged here.

    When we had the concrete, etc. relaid at the back of the house, provision was made for Mrs P's fountain, which is visible from the kitchen window (and my lathe room). 

    The ground worker was instructed to lay some black ducting, but in the event, he used a bit of blue water pipe. I couldn't help feeling that putting in some SWA to the pump was a waste of effort. Said pipe sits in the ballast an inch or so below the block paving. Frankly, I think that any act which could disrupt the cable would be either reckless stupidity or malicious.

    ETA: here it is. And yes, that is an effin' fox. ?

    39cb3bc1ecee894c3858f42af17a14e4-original-ffbc1e26-d2e2-436f-ac16-287736c19f5e.jpg
  • 0

    Over the years there’s been a few times we needed to get a pipe or cable under a private road, not having a mole we dug a trench then drove a scaffold tube with a spike off a tractor front end loader adapted to go into the front end of it under the road with a sledge hammer.

    When needs must and all that ?

  • 0

    Perhaps in certain situations a length of underground Hi-Tuff cable run in steel scaffold pole would comply. Especially if the whole lot was to be run under a concrete drive or patio, with suitable warning tape over it of course.

     

    Z.

  • 0

    It sounds like you are installing a wiring system with the anictipation that the mechanical protection is likely to be compromised?

    That's always a risk with hidden cables in situations where digging/nailing/drilling is to be anticipated - I don't know of any wiring or containment system that would mechanically resist that. Hence BS 7671's requirements for concentric c.p.c.s or (for internal situations) additional protection by 30mA RCD.

       - Andy.

  • Former Community Member
    0 Former Community Member

    AJJewsbury: 
     

    “Equivalent” to what exactly?

    To my mind, to provide the same safety in the same circumstances - if an armoured cable remain ‘safe’ when hit by a spake/fork/mattock/pick/mechanical excavator bucket (by virtue of ADS) - the same attack on the duct/conduit system should be no less safe (e.g. if not by ADS then by some other means such as preventing the damage getting through to the cable).

    To my mind a bit of plastic “twinwall” duct doesn't quite do that - at least in uncontrolled environments.

       

    It sounds like you are installing a wiring system with the anictipation that the mechanical protection is likely to be compromised? Have you complied with BS7671?

     

     

  • 0

    “Equivalent” to what exactly?

    To my mind, to provide the same safety in the same circumstances - if an armoured cable remain ‘safe’ when hit by a spake/fork/mattock/pick/mechanical excavator bucket (by virtue of ADS) - the same attack on the duct/conduit system should be no less safe (e.g. if not by ADS then by some other means such as preventing the damage getting through to the cable).

    To my mind a bit of plastic “twinwall” duct doesn't quite do that - at least in uncontrolled environments.

        - Andy.

  • 0

    I think you will find the rules are different. For DNOs, the Supply Regulations require all live conductors to be enclosed by a metallic screen, at or about earth potential. Either an earth or System Neutral will satisfy the requirement. 

    I quite agree the DNO's rules require an earthed(ish) screen - effectively BS 7671 does too  for buried or likely to be damaged cables (if anything more strictly as it must be PE not N) - my question was if the DNO's can achieve that with copper instead of steel these days, why can't we?

       - Andy.

  • 0

    Weirdbeard: 
     

    Zoomup: 
     

    Weirdbeard: 
     

    AJJewsbury: 
     

    Most of the difficulty was around glanding off the actual steel armour - trying to get everything aligned and tightened in a confined space seemed more like the less desirable aspects of being a plumber rather than an electrician. Split-con would have appealed - as then a simple stuffing gland could have been used and the copper outers just pig-tailed into the terminals, but as the cables go underground split-con isn't permitted any more. 

         

    Hi Andy, you can run unarmoured cables underground if they are in a duct that provides normal mechanical protection, ie the duct is at an appropriate depth to any expected stress.

     

    If the concern is offering additional protection to anyone who may mechanically stress a buried cable system then the additional protection should be via a 30mA RCD?

     

    That must be Reg. 522.8.10 then. What is “equivalent protection?”

     

    Z.

    Any duct or conduit that is installed underground to contain a wiring system in accordance with BS7671.

    “Equivalent” to what exactly?

     

    Z.

  • 0

    AJJewsbury: 
     

    . . . just wondering why we continue to use steel when others (e.g. DNOs) seem to have moved away from that and are having an easier life as a result.

    I think you will find the rules are different. For DNOs, the Supply Regulations require all live conductors to be enclosed by a metallic screen, at or about earth potential. Either an earth or System Neutral will satisfy the requirement. 

    Regards,

    Alan. 

  • Former Community Member
    0 Former Community Member

    Zoomup: 
     

    Weirdbeard: 
     

    AJJewsbury: 
     

    Most of the difficulty was around glanding off the actual steel armour - trying to get everything aligned and tightened in a confined space seemed more like the less desirable aspects of being a plumber rather than an electrician. Split-con would have appealed - as then a simple stuffing gland could have been used and the copper outers just pig-tailed into the terminals, but as the cables go underground split-con isn't permitted any more. 

         

    Hi Andy, you can run unarmoured cables underground if they are in a duct that provides normal mechanical protection, ie the duct is at an appropriate depth to any expected stress.

     

    If the concern is offering additional protection to anyone who may mechanically stress a buried cable system then the additional protection should be via a 30mA RCD?

     

    That must be Reg. 522.8.10 then. What is “equivalent protection?”

     

    Z.

    Any duct or conduit that is installed underground to contain a wiring system in accordance with BS7671.

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