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Cable thoughts?

Hi All, 


So I am looking at a job to install a cable inside a commercial building from an existing 100A 3ph service head to a 9 way TP Ryefield; circa 20M


The client has aggreed with the DNO that it will be a BNO installation, there will be no meters until after the Ryefield fuses.


Choices would seem to be,


External trench around the building or run internally (client prefered)


Stright Con

Split Con

SWA

Singles in trunking (nightmare)


Thoughts? 


I wont be deciding, it’s not going on my tickets! 


Thanks


Martyn


Jesus this new forum malarkey is crap on mobile, it seems to break all the text integration rules.

  • I love S.W.A with copper conductors. Straightforward and reliable.


    Z.

  • Martynduerden:

    . . . Choices would seem to be . . . 


    Straight Con

    Split Con

    SWA

    Singles in trunking (nightmare) 


    Thoughts?  . . . 




    Ok, your BNO network will have to comply with BS7671: so that is split con out, and singles are out as they will not be allowed by the DNO or Meter Operator. Straight con can be used, but you need four-core as you need the armouring as an earth. Summing up, I think SWA is probably your best bet. 


    Retards,


    Alan. 

  • Always been slightly confused on BS7671 for cables before a Meter? 


    If as you suggest it will need to meet BS7671, How would Concentric meet BS7671 with CNE after the head?


    Why do you contend Split Con does not meet BS7671, there are millions of meters of it installed in to flats from Ryefields? 


    To my mind, its absence of mention does not equal prohibition.


    Why would I need armouring? 


    Why would the Meter operator have any knowledge of a cable that does not anywhere connect to a meter? 


    5c SWA *seems* a crazy choice 


    Thanks


    Martyn

  • Check with your  DNO, but at least round here, they expect (and will only connect a supply if ) the BNO works also comply with BS7671, for anything and everything after the DNO cut-out, so the wring to secondary cutouts and the meters if they are some distance away do end up in SWA, as split concentric, is not really suitable for cables that are routed in the fabric of the building, as the earth is not an over-all armour.  Fine however anywhere that T and E could be routed.Certainly true in flats, not so sure of your situation, but probably. There are plenty of legacy cables of course, but they seem stricter on the new stuff. I'd suggest to call the DNO and clarify before installing something they don't like. There will also be some duplication of bonding to services, if this spits one supply to multiple 'origin'

  • Martynduerden:

    . . . If as you suggest it will need to meet BS7671, How would Concentric meet BS7671 with CNE after the head? . . . 


    . . . Why do you contend Split Con does not meet BS7671, there are millions of meters of it installed in to flats from Ryefields? . . . 


    . . . Why would I need armouring? . . . 




    Four core concentric has four cores with an overall screen/armour. With four cores you can use the screen/armour as the earth - it can be easier to get round corners than SWA sometimes. It must be SNE after the head. 


    Split con does not comply with BS7671 as the neutral surrounds 1/3 of its diameter, a conductor that BS7671 refers to as a “live conductor”. It could be used in places where T&E is acceptable. 


    You need armouring partly to protect it from damage, and it is also the easiest approved way to protect it against abstraction. 


    Regards,


    Alan. 


  • Always been slightly confused on BS7671 for cables before a Meter? 


    If as you suggest it will need to meet BS7671, How would Concentric meet BS7671 with CNE after the head?



    BS 767 doesn't prohibit CNE any more - it just stipulates a number of conductions (see section 543.4) - which seeing as we don't seem to have any requirement to obtain some Secretary of State's authorization any more, they don't seem too onerous for a supply situation. Alternatively you could use straight concentric with the outer as PE rather than CNE (so needing an extra core, or extra single cable, for N).


    Why do you contend Split Con does not meet BS7671, there are millions of meters of it installed in to flats from Ryefields? 


    To my mind, its absence of mention does not equal prohibition.



    There was a debate a few years ago on the old forum which apparently lead to it's removal from BS 7671 - from memory it was partly that the N part of the armour didn't satisfy the requirement for an earthed armour/sheath for buried/concealed cables - and I think partly that the thin insulation on the N strands wasn't rated for full line voltage - so falling foul of BS 7671 approach of treating N as a live conductor. There's also the issue of providing fault protection to the N conductor - being it's in effect many separately insulated small c.s.a. strands in parallel you'd be looking at an Appendix 10 approach - which can rapidly get very messy (potentially requiring a separate fault protective device at boths ends of each strand).


    The DNOs with their assumption of reliable earthing of N conductors (e.g. no switching in N) and more relaxed approach to fault protection, continue to use it of course.


      - Andy.
  • I used split con to move my consumer unit to the back of my hose. Much easier getting it through joists than 25mm x 3 SWA!

    25mm centre core (live) 25mm eq neutrals and a bare 16mm eq cpc all in one easily handled package. I couldn't ask for more!