Temperature correction of Zs, Ze and R1 + R2

[sxdave]

I am looking at a scenario of temperature compensation in a text book (Practical Guide to Inspection, Testing and Certification of Electrical Installations, 5th Ed. by Christopher Kitcher). In the example (Scenario 2, pp 128-129) the measured value of R1 + R2 is adjusted for temperature effects to get the R1 + R2 value at 70 degC. This (70 degC) value is now added to a measured Ze to obtain Zs which "can be compared directly to the maximum Zs values provided in chapter 41 of BS7671" according to this book.
This appears to not apply any temperature compensation to the Ze part of Zs whereas BS7671, App. 3, Page 363 adjusts the whole of Zs for temperature effects.
Am I missing something or making a mistake?

 

[newfutile]

I am looking at a scenario of temperature compensation in a text book (Practical Guide to Inspection, Testing and Certification of Electrical Installations, 5th Ed. by Christopher Kitcher). In the example (Scenario 2, pp 128-129) the measured value of R1 + R2 is adjusted for temperature effects to get the R1 + R2 value at 70 degC. This (70 degC) value is now added to a measured Ze to obtain Zs which "can be compared directly to the maximum Zs values provided in chapter 41 of BS7671" according to this book.
This appears to not apply any temperature compensation to the Ze part of Zs whereas BS7671, App. 3, Page 363 adjusts the whole of Zs for temperature effects.
Am I missing something or making a mistake?

If you mean example 4 he has applied 80% to B7671 values to allow for the assumed ambient temperature of 20 C , these corrected values are also in onsite guide and GN3

 

[sxdave]

If you mean example 4 he has applied 80% to B7671 values to allow for the assumed ambient temperature of 20 C , these corrected values are also in onsite guide and GN3

I am looking at 'Scenario 2' which begins approx. half way down page 128 and continues to the top half of page 129 (in my copy of the book). This is before Scenario 4 I believe.

 

[Wilko]

Hi - I think the question becomes - do we try to account for an increased Ze with load?

I think folks commonly assume the DNO supply equipment temperature will not go up much and so the Ze will be fairly stable.

But I read it as you do and add measured Ze to (R1+R2) at 20C and then apply 0.8 factor. Along with using 0.95x230=218.5V this will give ADS in the worst worst case .

 

[sxdave]

Hi - I think the question becomes - do we try to account for an increased Ze with load?

I think folks commonly assume the DNO supply equipment temperature will not go up much and so the Ze will be fairly stable.

But I read it as you do and add measured Ze to (R1+R2) at 20C and then apply 0.8 factor. Along with using 0.95x230=218.5V this will give ADS in the worst worst case .

Thanks for your reply. I think you are probably correct about the DNO eqpt. I would be interested to know if the DNO cable does heat up much if anyone has any info on that.

 

[telectrix]

Thanks for your reply. I think you are probably correct about the DNO eqpt. I would be interested to know if the DNO cable does heat up much if anyone has any info on that.

depends on how big the cannabis farm in the attic is.

 

[pc1966]

Thanks for your reply. I think you are probably correct about the DNO eqpt. I would be interested to know if the DNO cable does heat up much if anyone has any info on that.

Obviously the DNO cable does heat up, and is also subject to ambient temperature changes due to the weather, etc.

However, the first point is DNO cable (or other up-stream feeds) generally has less variation due to diversity - when you measure it most of the network is probably at its typical temperature anyway as it is already under load by multiple properties.

The second point is that in most installations the Ze value is much smaller than the Zs limit, so variations in Ze are going to have a much smaller impact on the pass/fail for OCPD disconnection times based on end of final circuit Zs. If that is not the case (e.g. single very high current load in an industrial set up etc) you might need to look more closely at the uncertainty in Ze

Finally the wiring regs in the UK are very conservative: we take the worst-case OCPD trip point, the lowest supply voltage and allow for the heating of final circuit cables at max CCC of the cables. All of that means there is some wriggle-room for variations in Ze due to temperature as well as the modest accuracy of MFT measurements for impedances much less than 1 ohm.