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Using the drain wire in FP200 plus as a signal conductor... can I?

Michael Brennan
Hi Everyone,


I am looking to find out if it is against BS7671:2018 to use the drain wire in a cable (specifically FP200 plus) as a signal conductor (switched 24VDC signal), or whether this must be used for the earth/shield?  Is there anything in the regs that states what the drain wire must be used as?


Many thanks in advance,


Michael
  • 0
    what voltages are on the other cores and if it is not a CPC how are things at the far end earthed ? What you are proposing is unusual.

    If you are using FP200 in a PELV system, then there should be a CPC. The outer jacket is not a guaranteed insulator, rather it is a mechanical cover for a bare wire, so you can only do things you would be happy to do with a bare wire, which in some cases (a 12V battery powered system for example ) may be just fine.

    The short answer is that the regs may not expressly prohibit it, but they do assume you will use cable in the way the maker's expect, and more info on the proposal

    is needed to advise sensibly.


  • 0
    Also begs the question of why FP 200 type cable was selected in the first place?  If it was for its fire resistance then what happens to your signal if the foil shorts out to earth or the screen of other cables when the outer plastic sheath melts in a fire? (as I understand it, the bulk of the fire resistance is due to the core insulation and mica wrap rather than the outer sheath) If it was for its EMI screening properties then substituting a changing signal for earth on the foil isn't going to be beneficial for the other cores and your signal wire wouldn't have the benefit of any screening.


    If on the other hand you're only using FP becuase it was 'free' in some sense - say you had a bit left over in the van, or re-using some that had previously been installed for something else, and otherwise a bit of bell wire would have done, then you might be fine.


       - Andy.
  • 0
    The other option if you really needed more cores, would have been to start out with the correct number of cores. FP200 is readily available as 2 core plus earth, three core plus earth and four core plus earth. Some of the suppliers even sell the “unusual” sizes per metre. 


    Regards,


    Alan.
  • 0

    Alan Capon:


    FP200 is readily available as 2 core plus earth, three core plus earth and four core plus earth. Some of the suppliers even sell the “unusual” sizes per metre. 


    Regards,


    Alan. 






    FP200 is even available as a multicore cable with seven, twelve or nineteen cores in with cross-sectional areas from 1 mm² to 2.5 mm² - although probably more of a special order item rather than something to be found in your local electrical wholesaler!


    I wouldn't be overly keen on the use of the drain wire other than for its intended purpose - it just doesn't feel right in the same way as the mis-use of the CPC in 6242Y to provide a third conductor for multiway switching.


    Depending on what the actual application is, there may also be EMC (putting a signal on what should be a screen) and fire resistance (the drain wire isn't protected from fire in the same way as the actual cores) considerations to take into account.


     

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