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Separate CPC with SWA

jbrameld

I had a comment on a training course yesterday that someone had been told (by an electrician) that it is no longer acceptable to run a separate single core CPC alongside an SWA cable, and that it had to be a core within the cable.

I couldn't see anything in the Big Brown Book that prohibits use of a separate protective conductor. They state that it may be a single core cable, and provided it is run in the same wiring system as the circuit conductors or in close proximity to it then it should be OK.

Does anyone support the theory that use of a separate CPC is no longer allowed, and if so then which regulation might this contravene?  I can see situations where it may be regarded as not adequately mechanically protected if outside the SWA, but I can also see situations where that would not be a problem.

Thanks,

Jason.

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    This might come from 521.5.1  - the armour of a steel wire armoured cable being considered a ferrous enclosure - and that reg's requirement for all conductors, including the protective conductor, to be within the ferrous enclosure.  However some years ago a specific exemption was added to allow an additional protective conductor in parallel with SWA (but not steel conduit etc). So the comment might have been a correct interpretation a few years ago, but is clearly allowed now.

    There's also been some debate on how to size the additional conductor - whether it needs only to 'make up' what's missing from the SWA or whether it needs to be the full required c.s.a. by itself (on the basis that currents don't necessarily divide nicely between ferrous and non-ferrous conductors). Bit that's a different debate (I think the conclusion was that it's somewhere between the two).

       - Andy.

  • 0 in reply to AJJewsbury
    AJJewsbury said:
    There's also been some debate on how to size the additional conductor - whether it needs only to 'make up' what's missing from the SWA or whether it needs to be the full required c.s.a. by itself (on the basis that currents don't necessarily divide nicely between ferrous and non-ferrous conductors). Bit that's a different debate (I think the conclusion was that it's somewhere between the two).

    Everything you need to know to determine the proportion of the fault current in the armour and separate cpc is included in Annex NA.4.5 of PD CLC/TR 50480 (see Appendix D, section D5-5, of the 2022 IET Electrical Installation Design Guide which contains the formula you need to use.

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  • 0 in reply to AJJewsbury
    AJJewsbury said:
    There's also been some debate on how to size the additional conductor - whether it needs only to 'make up' what's missing from the SWA or whether it needs to be the full required c.s.a. by itself (on the basis that currents don't necessarily divide nicely between ferrous and non-ferrous conductors). Bit that's a different debate (I think the conclusion was that it's somewhere between the two).

    Everything you need to know to determine the proportion of the fault current in the armour and separate cpc is included in Annex NA.4.5 of PD CLC/TR 50480 (see Appendix D, section D5-5, of the 2022 IET Electrical Installation Design Guide which contains the formula you need to use.

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