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

Former Community Member
Former Community Member
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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  • 0
    Put more simply, my tester measures to +/- 0.5% +/- 1d; or +/- 1% +/- 1d depending upon the range selected and gives a reading to two decimal places.


    So if it gives me a value of 1.46 Ω that could be anywhere from 1.45 Ω - 0.5% = 1.44275 Ω to 1.47 Ω + 0.5% = 1.47735 Ω, which easily includes your calculated value of 1.456 Ω.


    Note also that the values in Table 41.3 have been rounded to 2dp. The last column gives the calculation, so for example, the calculation for a 25 A Type D MCB gives 0.437 Ω, which has been rounded up to 0.44 Ω.
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  • 0
    Put more simply, my tester measures to +/- 0.5% +/- 1d; or +/- 1% +/- 1d depending upon the range selected and gives a reading to two decimal places.


    So if it gives me a value of 1.46 Ω that could be anywhere from 1.45 Ω - 0.5% = 1.44275 Ω to 1.47 Ω + 0.5% = 1.47735 Ω, which easily includes your calculated value of 1.456 Ω.


    Note also that the values in Table 41.3 have been rounded to 2dp. The last column gives the calculation, so for example, the calculation for a 25 A Type D MCB gives 0.437 Ω, which has been rounded up to 0.44 Ω.
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