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Max Zs BS7671 17th Edition / Guidance Note 3 Inspection and Testing

Former Community Member
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Hello,


There seems to be some slight differences with regards to acceptable Zs values when comparing the two documents BS7671 17th Edition A3 and Guidance Note 3 Inspection and Testing 17th Edition A3.


For example table 41.3 from BS7671 17th Edition A3 states:


0.4sec trip D6A = 1.82 Ω


Adjusted to 80% as per appendix 14 = 1.456 Ω



However page 121 from Guidance Note 3 Inspection and Testing 17th Edition A3 states:


0.4sec trip D6A = 1.46 Ω




These two documents are produced by IET. Guidance Note 3 Inspection and Testing 17th Edition A3 is rounding up Zs values above the maximum allowable values detailed in BS7671 17th Edition A3, why is that ?




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    Once again taking into account all that we know about tolerances and circuit variations, if something is defined as the maximum then surely it should it should never be acceptable to have a value higher than the maximum as far as the regs are concerned.



    Have you considered the possibility that the printed values in BS 7671 have been rounded down to 2 decimal places? Calculations based on the (slightly higher) original value and then also rounded down might give the kind of result you observe?


       - Andy.
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  • 0

    Once again taking into account all that we know about tolerances and circuit variations, if something is defined as the maximum then surely it should it should never be acceptable to have a value higher than the maximum as far as the regs are concerned.



    Have you considered the possibility that the printed values in BS 7671 have been rounded down to 2 decimal places? Calculations based on the (slightly higher) original value and then also rounded down might give the kind of result you observe?


       - Andy.
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