r1 + r2 for 3 core 10mm XLPE

hah no he does not like it or mince his words does he. Mind you, I'm not afraid of the maths, and I'm inclined to agree with the sentiment.

The problem is that unlike pure copper at ~ 16 milliohms per meter length per mm square, the word  'steel' covers a multitude of different  alloys and it's resistance is not predictable without knowing the exact composition. It is typically about ten times more resistive than the same wire diameter in copper but it may be more or less, and is far from exact. 
The cable makers are the only folk who actually know for sure what they have used, so  if they do not publish it, then a measurement beats 4 aces.
Mike

presumably irrelevant for TT, as that has far more to do with the user's electrodes than those at the transformer, normally, but not always and RCDs dominate the ADS sums, and an admission that  what presents in the meter box as a TNS is often really more a sort of
"TNS-C-S-C-s"  when you look under the road, and you may as well expect a Zs and PSSC that is more or less the same as  the house next door that has TNC-s..

I think it formalises what we sort of knew - the 0.8 ohm and 0.35 ohm figures were "aspirational" rather than guranteed, but usually met, but no real way to know that does not involve using a meter.
Mike.

Article here explains the current situation: electrical.theiet.org/.../

Thanks Graham - that's interesting. So no 0.35Ω TN-C-S or 0.8Ω TN-S any more - just might be less than 0.34Ω (90% of premises), or less than 0.64Ω (98% of premises) or anything above that (remaining 2% of premises) - with no official relationship with earthing type - so we can't tell in advance which group any given premises will fall into?

   - Andy.

GN 6 is not as helpful as initially anticipated -

See comments from 43:46 onwards for a prime example of it's use 'out in the field'

www.youtube.com/watch

Or as above when it gets more complicated even if you cannot measure the inductance, if you have a meter that does capacitance you can deduce the surge impedance and therefore the inductance of a line pair from first principles for your own specific layout, rather than rely on a tabulated value that relates to an idealized hairpin inductor geometry where core to core spacing is twice the cable diameter, as that may well not be quite what you have. A measurement done well beats paying for an estimate based on the work of a distant committee.

Like the OSG, it is useful as a first cut cookbook, but not in situations where things are very near the limit, or indeed with non 50Hz or non- sinusoidal waveforms.

All TR 50480 does is scale for groups of conductors from these figures..

For that twin wire layout, as a more general form than the 2:1. for wire diameters 'd' spaced centre to centre by any distance  'D'

the L and C per metre of a line pair are given as  (ref)

Because the geometry factor is the same you can deduce the short circuit inductance from a measurement of open circuit capacitance (or vice versa) and knowledge of the material properties to get the e and u factors.

Mike.

Agreed AJJewsbury .  Article here explains the current situation: electrical.theiet.org/.../

whjohnson said:

No need to buy the BS book

You still need the reactance formulas from PD CLC/TR 50480, as even with small csa SWA, it makes quite some difference over using resistance alone (between 5 and 10 % for SWA used as armour alone, but a greater difference when considering a parallel cpc).

But still, no need for the standard and the cable construction BS ... the formulas from PD CLC/TR 50480 are available in Guidance Note 6 and the Electrical Installation Design Guide, and tables of armour resistance available in a few places as you say.

The measured ZE was 0.15 ohms,but I thought we had to work on the published value

of 0.8 in case there was a change in the network.

I don't think BS 7671 says we have to - it's certainly a good idea in the general case (although I think it unlikely that modern changes to the network would likely raise Ze much above 0.35 Ohms if it's below that already). In some (rare) situation that actual Ze can exceed the published figures - so you can't completely ignore the actual Ze. In others (e.g. sites with own transformers) the chances of significant increases in Ze are extremely unlikely.

   - Andy,

Batt Cables publishes a useful table on SWA armour resistance - No need to buy the BS book.

www.batt.co.uk/.../Armour-Resistance

Hi Mike,

The measured ZE was 0.15 ohms,but I thought we had to work on the published value

of 0.8 in case there was a change in the network.

                                                                  Regards,

                                                                       Hz