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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

    I'm not sure whether they could have got any greater accuracy as the processing power of the time was something akin to 4 16-bit registers with 11-bit memory capability...



    You're not limited to the machine's word-size - you can calculate and store across several adjacent memory locations using the equivalent of 'long' calculations (e.g. do one byte/word at a time and carry/borrow from one to the next). The old 6502 based systems I used to work with (many years ago) supported calculations (IIRC) between 1E38 and 1E-38 with 6 significant decimal digits even though the hardware registers and memory were all just 8-bit (-128 to 127 integers only).


      - Andy.
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  • 0

    I'm not sure whether they could have got any greater accuracy as the processing power of the time was something akin to 4 16-bit registers with 11-bit memory capability...



    You're not limited to the machine's word-size - you can calculate and store across several adjacent memory locations using the equivalent of 'long' calculations (e.g. do one byte/word at a time and carry/borrow from one to the next). The old 6502 based systems I used to work with (many years ago) supported calculations (IIRC) between 1E38 and 1E-38 with 6 significant decimal digits even though the hardware registers and memory were all just 8-bit (-128 to 127 integers only).


      - Andy.
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