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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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Parents
  • I believe Megger recommends as a general rule,  taking 3 readings when doing loop impedance measurements and then using the average value as some supplies can be quite volitile in their waveforms Therefore accuracy to more than 2 decimal places is often very hard to justify in the field as opposed to on paper.


    Legh
Reply
  • I believe Megger recommends as a general rule,  taking 3 readings when doing loop impedance measurements and then using the average value as some supplies can be quite volitile in their waveforms Therefore accuracy to more than 2 decimal places is often very hard to justify in the field as opposed to on paper.


    Legh
Children
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