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

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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
    The appendices to the regs and the guidance notes are just that, guidance. 

    In the end  the design is something for which the designer is responsible for, and part of that is to decide if you cone in  or cone out when you look at the test instrument accuracy, you may prefer to set your own tighter limits, or you may know more about the installation and set wider.

    .For the reasons given in my first response, for engineering purposes, I see two indentical numbers, just with different no.s of redundant trailing digits.


Reply
  • 0
    The appendices to the regs and the guidance notes are just that, guidance. 

    In the end  the design is something for which the designer is responsible for, and part of that is to decide if you cone in  or cone out when you look at the test instrument accuracy, you may prefer to set your own tighter limits, or you may know more about the installation and set wider.

    .For the reasons given in my first response, for engineering purposes, I see two indentical numbers, just with different no.s of redundant trailing digits.


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