@ mapj1 


People are well aware that guidance notes are for guidance. However the guidance notes should not be contradicting the regs, especially when they are both produced by the IET.


To follow your train of taught lets run the calculation from start to finish without limiting to 2 decimal places.


Table 41.3 from BS7671 17th Edition A3 requires:


0.4sec trip D6A = (230 x 0.95 / 20x6) = 1.8208333333 Ω


Adjusted to 80% as per appendix 14 = 1.4566666667 Ω


Page 121 from Guidance Note 3 Inspection and Testing 17th Edition A3 states:


0.4sec trip D6A = 1.46 Ω



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


If IET are stating values are the maximum then they should not be contradicting in the same breath by rounding up Zs figures above the maximum.


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.


Thanks for the feedback.

@ mapj1 


People are well aware that guidance notes are for guidance. However the guidance notes should not be contradicting the regs, especially when they are both produced by the IET.


To follow your train of taught lets run the calculation from start to finish without limiting to 2 decimal places.


Table 41.3 from BS7671 17th Edition A3 requires:


0.4sec trip D6A = (230 x 0.95 / 20x6) = 1.8208333333 Ω


Adjusted to 80% as per appendix 14 = 1.4566666667 Ω


Page 121 from Guidance Note 3 Inspection and Testing 17th Edition A3 states:


0.4sec trip D6A = 1.46 Ω



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


If IET are stating values are the maximum then they should not be contradicting in the same breath by rounding up Zs figures above the maximum.


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.


Thanks for the feedback.