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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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    Mike M:

    All digital Fluke and Megger loop impedance meters I have used are accurate enough to tell the difference between 1.45 Ω and 1.46 Ω .




    That is not correct for the reason that I have given above.


    Also if you measure the Zs of a given circuit, it will not give the same reading every time. For example, I use a socket in my workshop for checking my MFT. The mean reading is 0.85 Ω with a standard deviation of 0.04 Ω.


    Precision in measurement is quite a complicated matter.

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  • 0

    Mike M:

    All digital Fluke and Megger loop impedance meters I have used are accurate enough to tell the difference between 1.45 Ω and 1.46 Ω .




    That is not correct for the reason that I have given above.


    Also if you measure the Zs of a given circuit, it will not give the same reading every time. For example, I use a socket in my workshop for checking my MFT. The mean reading is 0.85 Ω with a standard deviation of 0.04 Ω.


    Precision in measurement is quite a complicated matter.

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