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Sizing of flylead to SWA

Graham Warner
Sorry about idiot-level question. 

3 no Distribution circuits, 3 phase, one each in 4 x 16, 4 x 25 and 4 x 35mm XLPE/SWA and each with separate 16mm cpc alongside.  The armour will of course be in parallel with the external cpc but ignored for sake of R1+R2 etc. however doing belt-and-braces by using Earthing Nuts at both ends.  Other than treating the armour as extraneous part to be supplementary bonded, in which case presumably the 4mm (544.2.1) rule applies, is there any proper guidance on the size of the fly lead to these Earthing Nuts?  Cable sizes quoted above are required primarily from voltage drop considerations.

These circuits are taken from a larger distribution circuit, overall on TN-C-S with additional earth rod at the end of the primary Distrib cct, which is in the middle of a field.  There are no other parts or services that require Supplementary Bonding.

Thanks for your advice.
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    There really is no point in going for a larger size than the live conductors, there is no credible fault that could require this, except an imperceptible change in R2. Whether 16 mm is adequate is presumably your actual question, and it will be, as under fault conditions we can allow a suitably high conductor temperature, using the adiabatic equation. Why have you got a 16mm CPC as well as the SWA? I assume that each cabin has an RCD-protected CU, 30 mA would be wise with equipment probably being extended outside. The only time where the current could get higher than a line conductor is if there are metal service pipes between multiple bonded points, PME, and a broken neutral. I assume this is not your case?
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    There really is no point in going for a larger size than the live conductors, there is no credible fault that could require this, except an imperceptible change in R2. Whether 16 mm is adequate is presumably your actual question, and it will be, as under fault conditions we can allow a suitably high conductor temperature, using the adiabatic equation. Why have you got a 16mm CPC as well as the SWA? I assume that each cabin has an RCD-protected CU, 30 mA would be wise with equipment probably being extended outside. The only time where the current could get higher than a line conductor is if there are metal service pipes between multiple bonded points, PME, and a broken neutral. I assume this is not your case?
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