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minimum CSA for a 1.5mm CPC in 2.5/1.5 twin & earth cable.

Alan B

Doing an EICR today and two ring finals dead tests indicated that the CPC was exactly half the CSA of the line and neutral. If it had been old cable I wouldn't be surprised. But one was wired with brown/blue and the other relatively new looking red/black, They should have been 2.5/1.5. When I measured the cable with a micrometer the cpc worked out at a csa of about 1.2mm and the main conductors 2.5mm.

Not an issue from a safety point of view as it was on a TT earth with low fault currents, but then I started to think what is the allowed minimum and maximum diameter/csa of the cores according to british standards and I haven't been able to find any data, hope others can point me in the right direction.

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  • 0
    Chris Pearson said:
    What sort of micrometer were you using please?

    I was using a proper micrometer rather than Vernier caliper if that's what you are asking, admittedly not been calibrated for 20 plus years.

    I am glad that I checked the diameter of the CPC as the alternative would have been chasing around two fairly big ring finals trying to find a poor joint that I am fairly sure doesn't exist. Lesson for me is to not automatically assume an unexpectedly high test result is due to a poor connection. But does leave a lot of uncertainty on when to spend time chasing down a high loop test result, especially when there's a mixture of different age cables in a circuit.

  • 0 in reply to Alan B

    It is possible that this is a bad batch of cable or that it has been pulled during coiling or something but unlikely.

    One might argue it is not desperately important unless it is miles out - a CPC even as thin as about 1mm2 will be safe for the user, even when only covered by a BS3036 hot wire fuse, although in that case it is possible that  repeated large faults may cook the cable.
    But we may decide that the failure of a section of cable that is out of spec like that after a heavy fault is acceptable - as the sort of fault that will do that, rather than blow a 13Afuse, probably involves spiking the cable in the wall, and needs it's replacement anyway; -)

    If the impedances suggest that all credible faults will be cleared, and the ring buzzes out and is actually a ring, until it is way out, it is reasonable to let sleeping dogs lie unless there are other observations as well that make you suspect a genuine problem.

    Another way to look at this, is with an extra half ohm in the earth path, will the RCD still trip ? Especially half an ohm that will weld itself into a lower impedance once a high current flows.

    I agree measurements on an existing set up that has been modified over the years are always going to be the hard case, and the meter cannot tell you everything -- in the same way that bare wires in dry air can pass an insulation test.

    it can only ever be 'as far as can reasonably be ascertained,,, safe for continued use'

    Mike.

  • 0 in reply to Alan B
    Alan B said:
    I was using a proper micrometer rather than Vernier caliper if that's what you are asking, admittedly not been calibrated for 20 plus years.

    Difficult to see how a screw thread and markings on the barrel could fall out of calibration, though of course you would always start with a zero check. ;-)

    It seems to me that a micrometer could easily over-estimate the diameter compared with the tips of a pair of callipers (Vernier, dial, or electronic) because the cable could be very slightly kinked.

    I would be very cautious measuring to within a tenth of a millimetre (4 thou) in absolute terms.

    I have had a peep at BS EN IEC 60228. It specifies maximum diameters of conductors for a certain CSA in an appendix. Measurements are given to the nearest 0.1 mm/mm². By contrast BS 6004 specifies the minimum CPC nominal CSA for given conductor sizes.

    It is fairly easy to stretch a copper conductor, in which case the diameter shrinks. I wondered whether your cables might have been stretched when being pulled in, but in that case the live conductors would also have stretched, so it does all seem a little odd.

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  • 0 in reply to Alan B
    Alan B said:
    I was using a proper micrometer rather than Vernier caliper if that's what you are asking, admittedly not been calibrated for 20 plus years.

    Difficult to see how a screw thread and markings on the barrel could fall out of calibration, though of course you would always start with a zero check. ;-)

    It seems to me that a micrometer could easily over-estimate the diameter compared with the tips of a pair of callipers (Vernier, dial, or electronic) because the cable could be very slightly kinked.

    I would be very cautious measuring to within a tenth of a millimetre (4 thou) in absolute terms.

    I have had a peep at BS EN IEC 60228. It specifies maximum diameters of conductors for a certain CSA in an appendix. Measurements are given to the nearest 0.1 mm/mm². By contrast BS 6004 specifies the minimum CPC nominal CSA for given conductor sizes.

    It is fairly easy to stretch a copper conductor, in which case the diameter shrinks. I wondered whether your cables might have been stretched when being pulled in, but in that case the live conductors would also have stretched, so it does all seem a little odd.

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