# 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

• Again, for DC resistance measurements OK

It had occurred to me that theory and practice are not necessarily aligned.

In the case of the OP where a third core is used for the CPC, having determined the maximum load, and then having used the CCC tables and Table I1 of the OSG, I would have an estimate of R1 + R2. Then I install it and keep my fingers crossed when measuring R1 + R2 with my trusty DC low residence ohmmeter. Any lowering of R2 due to the armour is a bonus.

With larger installations, or when the figures are marginal gets above my pay grade. If in doubt, select a bigger cable.

• 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.