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

I find it very regrettable that British Standards are not affordable. At one time, I had access through Hampshire Libraries, but that went years ago. Fortunately, the Open University gives me access whilst I remain an undergraduate.

The earliest edition of BS 6004 which I can see is 1984. Even then, the minimum CSA to correspond with 2.5 mm² live conductors was 1.5 mm².

What sort of micrometer were you using please?

The standard to look at for the conductors themselves is BS EN 60228.

When new (before drawing into the cable), conductors are circular. They may, occasionally, become non-circular by slight deformation in cable manufacture, but because copper is soft deformation can occur over time.

The actual measure of "conformity" to BS EN 60228 is not by diameter (or cross-section) alone, but by resistance (because of the deformations and variances possible in manufacture).

Ignoring degradation, the maximum resistance of a solid conductor when new is as given in Table I1 of OSG (Table B1 of GN3).

Sadly, saying that the resistance of the cpc (max 12.10 mΩ/m, but could be less) is exactly half that of either of the live conductors (max 7.41 mΩ/m) is not a definitive "check".

The difficulty you have, is that, unless you know the actual length of the cable, there is no way of determining whether the end-to-end R1, Rn or R2 meets the requirements.

For example, the cable would "pass" if the resistance of the cpc worked out at its maximum of 12.10 mΩ/m, and the live conductors were only 6.00 mΩ/m ... regardless of their actual diameter or cross-sectional area.

Alan B said:

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.

I think this is the measure ... whether the disconnection requirements for ADS are met ... faults to Earth in terms of thermal conditions there may be more leeway if (when bonding and fortuitous earthing is in place) there are relatively low prospective fault currents (certainly below the current-carrying capacity of the circuit protective conductor).

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.

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.

Alan B said:

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 answering your query, but within the last 5 years or so, I suspect many cables are not made to the relevant standards. This was clear to me a few years ago when crimping 16mm cable. Previously the crimps were quite a tight fit on the cable when put on to be crimped, but i had one reel that was very loose, in so far as I thought it was 10mm. The 10mm crimp wouldnt go on, so it was larger that 10mm, but I suspect it was more like 12 or 13mm CSA rather than 16mm.

The cable originated from Turkey, suppled by CEF. We're currently using 2.5 and 4 mm singles supplied by CEF, made in India, and I have to read the writing on the cable to determine the CSA, as the difference is so small, whereas in the past, I could easily tell the difference between the two by looking at the conductors.

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.