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GN8 Chapter 6 Extraneous-Conductive-Parts and their connections Equation 6.1

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In Guidence Note 8 where the equation 6.1 is used to illustrate the range of values of resistance RCP for the currents IB of 0.5mA, 10mA & 30mA in equations 6.2, 6.3 & 6.4, what is the second value of 1000 thats deducted from the value of RCP for? Looking at equation 6.1 the first value of 1000 thats deducted is for the impedance of the human body ZT taken to be 1000Ω from BS IEC 60479-1:2018 for the purpose of this calculation, but I'm struggling for this moment to understand what the subtraction of the second value of 1000 is for?

It looks to me like the equations have been incorrectly printed and the sum of each should be 459,000Ω, 22,000Ω & 6670Ω respectively and the second value of 1000 should be dividing the sums by 1000 to convert the values to KΩ?

If this is the case then these equations have been miss printed and miss represented in GN8 for the last 20+ years! Not to mention the publications and documents that they've been copied across to over these years...

Any thoughts on this would be much appreciated...

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  • 0

    should be dividing the sums by 1000 to convert the values to KΩ?

    Not really. 459,000Ω is exactly the same as 459kΩ - mathematically there's no need to divide by 1000. The k prefix itself means x1000.

    Strictly speaking, In the metric/SI system the there's only one "unit" for a dimension (in this case Ω for resistance)  Unlike the imperial system where you might divide by 12 to convert between feet and inches, in metric there is only the metre - so really the prefixes are just to 'scale' the number to something more convenient as your write them down (or display them).

       - Andy.

  • 0 in reply to AJJewsbury

    There are always exceptions to the rule.

    Whilst it is true that, according to SI units, 1 km2 = (1,000m)2 = 1,000,000 m2, this would lead us to believe that 100,000 A2s ≠ 100 kA2s ... but when reading standards and data sheets of let-through energy of protective devices, you would be a factor of 1000 out if you assumed SI rules are applied ... it seems the "great and the good" of our industry are happy to consider [A2s] as a 'derived unit' and hence use  1 kA2s = 1 k[A2s]

    Confused yet?

  • 0 in reply to gkenyon

    There are always exceptions to the rule.

    Indeed - probably the biggest blunder is that the SI system uses kg rather than g as a base unit for mass (or perhaps that the metric system adopted a unit that was 1000x too small for consistency). If we knew then what we know now things would certainly have been done differently. But for resistance at least it should be fairly straight forward.

       - Andy.

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  • 0 in reply to gkenyon

    There are always exceptions to the rule.

    Indeed - probably the biggest blunder is that the SI system uses kg rather than g as a base unit for mass (or perhaps that the metric system adopted a unit that was 1000x too small for consistency). If we knew then what we know now things would certainly have been done differently. But for resistance at least it should be fairly straight forward.

       - Andy.

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