This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

# r1 + r2 for 3 core 10mm XLPE

I,m trying to design a  submain for an outbuilding.The circuit comprises 50m of 3 core 10mm xlpe

on a bs88 40A fuse.Have worked out R1+R2 for the cores as 50x3.66x1.2/1000 =0.0216 ohms

Can the armour be included in the calcs to lower ZS and how would it be done?The supply is TNS 0.8 ohms (understand we have to work on that value)

Thanks for any help,

Regards,Hz

• 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.

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

• 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.

• 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'

• 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.

• 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.

• 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

• and normal meters out in the field nay not have the accuracy that resolution suggests . So our reading might be to plus or minus 1 ohm of the real value but to a resolution therein of one thousandth of an ohm.

• I do think there is something in the response he quoted from the NICEC though. Sometimes you can overthink it and make it needlessly more complex than is actually required. As a scholarly exercise there is nothing wrong with chasing it down to the last decimal point, but with the practical limitations ever present on the job, it's a different ballgame out in the field.

For example, I recently did a job whereby I ran 19 metres of 16mm 3 core SWA for a submain, protected by a type s 100A DP RCD in the tails, then a switch fuse with a 63A BS88 fuse. The system is TT with a electrode resistance of 14 ohms, so I just paralleled the armour with the third core and exported the TT earth. I'm getting 14.8 ohms at the destination consumer unit end.

I initially thought about doing a 'island' but there wasn't anywhere pratical to sink another rod so I just exported the existing instead.

I was going for a 40A or 45A fuse but I could not get one lower than 63A with the correct form factor for the switchfuse.

• Mind you, I'm not afraid of the maths, and I'm inclined to agree with the sentiment.

What, using an AC formula for DC "dead tests" ?

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.

Again, for DC resistance measurements OK, but  where reactance comes into it, AC measurement (at the relevant frequency) would be necessary.

Not all the time, but I'm coming on to that.

I do think there is something in the response he quoted from the NICEC though. Sometimes you can overthink it and make it needlessly more complex than is actually required. As a scholarly exercise there is nothing wrong with chasing it down to the last decimal point, but with the practical limitations ever present on the job, it's a different ballgame out in the field.

Yes I'd agree with that also ... but where calculation with DC resistances shows 100 m run would be OK ... yet AC impedance affects that run length by 5 to 10 %, and you're talking about runs at or close to the maximum calculated by DC resistances, and the AC impedances say "hang on a moment" it certainly matters a lot.

So, really it's not a case of "does it matter for the job I'm looking at" but "does it matter if I'm at the limits".