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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

    The problem in "engineering" terms or any other terms for that matter is 1.4566666667 Ω does not equal 1.46 Ω.

    :

    In any case I have not heard a valid reason as to why the IET would round up Zs values above the maximum allowed by BS7671 instead of rounding them down if they want to display them to 2 decimal places.



    OK, imagine (just theoretically) you had a circuit where max Zs precisely equalled 1.4560 Ohms - is that a pass or a fail situation?


    You then measured the loop impedance with a 100% accurate loop meter that could display to 2 d.p. What would you expect the meter to display? What would you conclude from that reading?


    (You might also like to research how the 80% figure comes about (clue: 0.004 per degree C) and what effect actual conditions during measurement might have on results.)


      - Andy.
Reply
  • 0

    The problem in "engineering" terms or any other terms for that matter is 1.4566666667 Ω does not equal 1.46 Ω.

    :

    In any case I have not heard a valid reason as to why the IET would round up Zs values above the maximum allowed by BS7671 instead of rounding them down if they want to display them to 2 decimal places.



    OK, imagine (just theoretically) you had a circuit where max Zs precisely equalled 1.4560 Ohms - is that a pass or a fail situation?


    You then measured the loop impedance with a 100% accurate loop meter that could display to 2 d.p. What would you expect the meter to display? What would you conclude from that reading?


    (You might also like to research how the 80% figure comes about (clue: 0.004 per degree C) and what effect actual conditions during measurement might have on results.)


      - Andy.
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