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

Question

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.

gkenyon · Answer

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 m&Omega;/m, but could be less) is exactly half that of either of the live conductors (max 7.41 m&Omega;/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 m&Omega;/m, and the live conductors were only 6.00 m&Omega;/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).

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

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