r1 + r2 for 3 core 10mm XLPE - Wiring and the Regulations BS 7671 - IET EngX - IET EngX
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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.

• 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.

Children
• 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

• It does raise some questions though. On a 100A TN- S system, must you design everything as if Ze is 0.8Ω, even if it measures much less? Even if it means bearing the cost of up-sized cables? When doing an EICR, if the cabling complies with the measured Ze but would fail to trip in time if Ze hypothetically increased to 0.8Ω, is that a pass, or a C2, or what?  If Ze measures 0.9Ω, should you be asking the DNO nicely to reduce it, or should the installation be designed to 0.9Ω?

• I think an (apparent) Ze of 0.9 would certainly raise a question to the DNO and in my experience (smaller installations, I know) we would work on the premise that we have 0.8 even if we measure much smaller. Around here just about all TNS Ze`s are about 0.34 max on TNS (or apparent TNS) and TNC-S about 0.22 but often 0.18 ish max as measured. But we always imagine "what if 0.8 TNS became the norm for some reason?"

• When doing an EICR, if the cabling complies with the measured Ze but would fail to trip in time if Ze hypothetically increased to 0.8Ω,

One of the criteria for initial design is a consideration for a reasonable life of the installation.

EICR is effectively MOT - safe on the day.

Agree you don't know what will happen when you leave ... but you could have a "finger in the air" whether the original designer was sailing close to the wind or not from resistance measurements (or estimated cable lengths ) - which would tell you whether something might be going wrong in the street?

• If Ze measures 0.9Ω, should you be asking the DNO nicely to reduce it

My personal experience of that was that they were round in under a couple of hours and the supply had been converted to PME within another couple of hours so SSE at the very least takes these things seriously.

In a built up environment, it is difficult to see how (or why) a substation might be moved, although it is conceivable that two small ones could be replaced by one larger one leading to longer cables under the road.