BS7671 Max Zs and Maximum Measure Zs
I’ve asked 3 of my lectures, but have yet to get an answer to this question. I have my own theory, but would like to ask if I’m on the right track or not.
The question is why is the maximum permitted measured Zs = BS7671 Max Zs x 0.8 (appendix 3 page 363). I know it is to do with colder conductors, its the 0.8 factor used, as the table in the On Site Guide do not seem to match is they are at 10⁰C
Guidance note 3 gives the 0.8 as the temperature correction. With the values of Zs in the On Site Guide and Guidance note 3 given for Zs at 10⁰C
The equation in the On Site Guide for temperature correction is given as
(1+0.004(New Temp-20)) as 0.004 is the coefficient correction when starting at 20⁰. As we are not converting from 20⁰C to 10⁰C, we do not need the lower part of the equation.
This would give us a (1+0.004(70-20))=1.2 therefore this would equate to 80% or the 0.8. The quoted 0.8 multiplier.
As the table in the On Site Guide are given at 10⁰C, the equation would look like this (1+0.004(70-20))/(1+0.004(10-20))= 1.25. therefore this would equate to a 75% or 0.75. This correction factor is given as 1.25 in the Note on page 126 of the On Site Guide.
However the figures in the On Site Guide are given as Zs(BS7671) x 0.8 = Zs(On Site Guide).
The true calculation from appendix 3 of BS7671 includes a correction for U0 taking into account Cmin. With Cmin at 0.95, perhaps this would account for the missing 5%?
My theory is that the On Site Guide figures have been corrected for 20⁰C and not 10⁰C, which would bring them in line with Table I1 of the On Site Guide.
Although this is academic, as in design if our calculated Zs figures were equal to or approaching the Max Zs figures, we are likely if the circuit will allow, used additional protection with and RCD. It just my OCD nagging at me.
Thank you for your help.
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