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Split concentric ccc?

Farmboy

Hi all



Basically, what ccc table, if any, in apdx 4 would be used for split concentric? I've had a look on the old forum but couldn't see a definitive answer, so sorry if I've just missed it.



The scenario is an existing domestic installation has an approx 7 metre run of what appears to be 25mm2 cable running from the external meter box, under the floor, to the internal CU, on a 100A main fuse, with no switch fuse.



The installation method is awkward to determine because the cable comes up internally through the floor screed, making it seem to be buried under the concrete floor. However, there also appears to be an approx rough/faint 100mm circular pattern around the cable which makes me think there may be a duct (probably brown plastic drainage pipe) just below the surface, but can't be sure, could just be the way the screed was smoothed around the cable.



So, if there's a duct, will that satisfy reg 522.8.10 (probably no cable making though), so the concentric can remain? But if there's no duct, should it be disused/replaced because the cables protective earth only surrounds 1/3 of the cable?



So if the cable can remain, what ccc and size of switch fuse?



Thoughts please - apart from run the other way ?


F


  • 0
    Assuming it is this stuff  see the table of current ratings  on page 2.  No more than an odd % or two different to the SWA figures. for the same copper cross section.

    As you say it is not one of the cable types already listed in the regs, so some professional judgement is needed, but if in doubt, makers figures are always the trump card.

    If it is indeed 25mm sq, then a 100A fuse will be safe.

    Because the earth is not a full all-round armour like SWA, then the regs do not consider it to be any safer than twin and earth or any other non-armoured cable in terms of recommended routes, so needs to be 50mm or deeper, or obvious , or in conduit/trunking etc 

    Or of course a note that compliance to current regs cannot be verified..
  • 0
    Whilst this case is somewhat unusual, and not compliant with several regs. it is not dangerous in the usual meaning of that and should probably only get a comment as Mike says. Remember that a nail or screw (the usual danger cited) would need to travel through the N conductor strands to contact the potentially dangerous live conductor. This would probably blow the DNO cutout fuse without any difficulty.  You should be aware that 522.6 applies to walls and partitions, and this floor is not a partition, so slightly sticky anyway. The requirement for Fusing tails of more than 3 metres is a DNO one not BS7671, so just an observation. Split concentric is not a specified cable type either, but again not a cause of danger, just difficult to terminate.


    The outcome of this is that you should note the deviation from BS7671 but not code it. In my view as described it is not a danger, or potential danger. You do need to watch the letter of the regs. too, the actual words may not be those one expects!


  • 0
    You need to be careful with split concentric. As others have said, it should be regarded as offering the same protection as twin & earth from BS7671’s point of view. The earthing copper “armours” cover about 2/3 of the diameter of the cable, with the neutral and a couple of separators covering the remaining diameter. DNO’s regulations allow the neutral to be treated as the same voltage as earth, BS7671 calls it a live conductor. It does not qualify as meter tails, as the conductors are not double insulated and should be protected by a customer-owned fuse as you would with an SWA submain. In fact, in some cables the thickness of the neutral insulation means it is “covered” for identification, not “insulated”. 


    Regards,


    Alan.
  • 0
    I agree Alan, but that does not make an existing installation "dangerous", but simply non-compliant. Sheathed cables are not "double insulated" (class 2) but you know that! I don't like this split con tails install, but I also think that ripping up a concrete floor is not very useful, on the information provided. Does SWA make this any safer, I rather doubt it. Also I presume the DNO connected these tails to the meter, are you saying that they did not? There is an answer, and that is to note the deficiencies in the EICR notes, then it is up to the customer to decide to change it if they wish. BS7671 does not say anything about the alleged problem, and so the EICR should reflect this.
  • 0

    davezawadi:

    I agree Alan, but that does not make an existing installation "dangerous", but simply non-compliant. . . 




    I agree. I was pointing out the shortcomings of split concentric, as not everyone will realise that the neutral isn’t insulated to the same extent as the phase. 

     



    . . . I presume the DNO connected these tails to the meter . . . 



    We have to. In connecting them, the meter supplier / DNO is deemed to have accepted them. 


    As a correction to my post above which the new forum software prevents me from editing, the neutral of course covers two thirds of the diameter, the earth the remaining third. 


    Regards,


    Alan. 

  • 0

    davezawadi:

    I agree Alan, but that does not make an existing installation "dangerous", but simply non-compliant. Sheathed cables are not "double insulated" (class 2) but you know that! I don't like this split con tails install, but I also think that ripping up a concrete floor is not very useful, on the information provided. Does SWA make this any safer, I rather doubt it. Also I presume the DNO connected these tails to the meter, are you saying that they did not? There is an answer, and that is to note the deficiencies in the EICR notes, then it is up to the customer to decide to change it if they wish. BS7671 does not say anything about the alleged problem, and so the EICR should reflect this.




    Sorry, it's not an eicr. I was attending the property to look at something else and noted this cable. There's a cct to put in to the CU and thinking about the existing setup must be adequate, so thinking about installing a switch fuse, which led on to the cables installation method.


    I'd already seen the pdf mapj1 posted, and saw that the installation method wasn't listed, which led to the question of ccc for the cable under/in concrete, and applying 0.9 rating factor for buried cables, hence the question about ccc and size of switch fuse.


    Regards


    F

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