- Reaction score
- 52
Hi, as an aspiring sparky I'm needing help with the temperature adjustment formula for Zs. I'm using GN3 (17th). I understand that the values given in Appendix A - Table A4 (for BS 60898) are assumed at a test ambient of 10C whereas the values in BS7671 are at the "normal operating temperature" of the conductor (70C) and adjusted by using a divisor of 1.24.
The example given under Table A6 in Appendix A is - 32A Type B 60898 at an ambient of 25C gives a Zs of 1.23Ω
I understand how they get this - BS7671 gives 32A Zs as 1.44 (at 70C), divided by 1.24 = 1.16Ω (at 10C), then using table A6 an adjustment factor of 1.06 accounts for the test temperature being 25C, gives 1.16 x 1.06 = 1.23 Ω.
The problem I have is that under Appendix A "A3 Methods of adjusting tabulated values of Zs" the example there is "twin and CPC 70C thermoplastic protected by 6A type B 60898 at 20C". They give the answer as 7.67 (the Zs from BS 7671) divided by 1.20, gives a Zs of 6.39Ω They get the 1.20 divisor from Table B3 in Appendix B which accounts for the conductor "incorporated in a cable or bunched".
Using the first formula the Zs comes out at 7.67 divided by 1.24 = 6.18Ω (at 10C), then using Table A6 to account for test temperature of 20C multiplying by 1.04 gives a Zs of 6.43Ω
I know that difference is very small between these two Zs values but I don't understand why they use two different calculations - what am I missing?
The example given under Table A6 in Appendix A is - 32A Type B 60898 at an ambient of 25C gives a Zs of 1.23Ω
I understand how they get this - BS7671 gives 32A Zs as 1.44 (at 70C), divided by 1.24 = 1.16Ω (at 10C), then using table A6 an adjustment factor of 1.06 accounts for the test temperature being 25C, gives 1.16 x 1.06 = 1.23 Ω.
The problem I have is that under Appendix A "A3 Methods of adjusting tabulated values of Zs" the example there is "twin and CPC 70C thermoplastic protected by 6A type B 60898 at 20C". They give the answer as 7.67 (the Zs from BS 7671) divided by 1.20, gives a Zs of 6.39Ω They get the 1.20 divisor from Table B3 in Appendix B which accounts for the conductor "incorporated in a cable or bunched".
Using the first formula the Zs comes out at 7.67 divided by 1.24 = 6.18Ω (at 10C), then using Table A6 to account for test temperature of 20C multiplying by 1.04 gives a Zs of 6.43Ω
I know that difference is very small between these two Zs values but I don't understand why they use two different calculations - what am I missing?
