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# 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

Parents
• 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.

• 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.

• Interesting,if we were installing a cable from a sub-board,me might just work on Zdb for

the cable size and not be too concerned about a nominal ze as high as 0.8ohms?

Regards,

Hz