# 80% Maximum ZS from OSG and GN3

we use the 80% to  compare against our measured ZS to compensate for the temperature of the wiring being less than 70 C .

we apply this to the whole maximum ZS  ,should it not be only applied to the R1+R2 ,as the ZE part may be at a different temperature and possibly wont be loaded up as much.

Or is it just safer and good engineering to apply the 80% to all?

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• we apply this to the whole maximum ZS  ,should it not be only applied to the R1+R2 ,as the ZE part may be at a different temperature and possibly wont be loaded up as much.

You can if you like. BS 7671 says, just below this method in Appendix 3 (Page 410 of BS BS 7671:2018+A2:2022):

NOTE: The above is one method of correcting for the effects of temperature difference; other methods are not precluded.

So, you could, for example, subtract a Ze reading from Zs, temperature correct the remainder using the correction factor for temperature coefficient of resistance from ambient to 70 deg C (roughly, multiply by 1.2), and add Ze again and compare the result with Tables in Chapter 41 (rather than the tables in OSG and GN3 which have the corrected values in them) ...

BUT Ze (and sometimes Zs) readings aren't all that accurate, so it might not make a lot of difference in the round.

• So, you could, for example, subtract a Ze reading from Zs, temperature correct the remainder using the correction factor for temperature coefficient of resistance from ambient to 70 deg C

Not so sure about that. I think R1+R2 would have to be measured separately. You may well find Zs-Ze is far from any reasonable notion of R1+R2, it could even work out as a negative number. Raising R1+R2 to conductor operating temperature and adding Ze is probably more applicable to a design exercise. However, in testing, if you are over the 0.8* Zs7671 but under the value of Zs7671 and Ze is relatively high, there may be merit in digging deeper.

• Not so sure about that. I think R1+R2 would have to be measured separately.

BS 7671 does not require that, nor advise that.

You may well find Zs-Ze is far from any reasonable notion of R1+R2, it could even work out as a negative number.

I agree, that is why I mentioned errors in measurement.

aising R1+R2 to conductor operating temperature and adding Ze is probably more applicable to a design exercise. However, in testing, if you are over the 0.8* Zs7671 but under the value of Zs7671 and Ze is relatively high, there may be merit in digging deeper.

I think there are circumstances that would lead to "does not compute" between any of the methods.

• Not so sure about that. I think R1+R2 would have to be measured separately.

BS 7671 does not require that, nor advise that.

You may well find Zs-Ze is far from any reasonable notion of R1+R2, it could even work out as a negative number.

I agree, that is why I mentioned errors in measurement.

aising R1+R2 to conductor operating temperature and adding Ze is probably more applicable to a design exercise. However, in testing, if you are over the 0.8* Zs7671 but under the value of Zs7671 and Ze is relatively high, there may be merit in digging deeper.

I think there are circumstances that would lead to "does not compute" between any of the methods.

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