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

sae70

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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    Maybe I should have made it a Debate rather than a question as I can see what is for me, incorrect. And having put together a Tech Talk for a dozen electricians on the omission of supplementary bonding in a location containing a bath or a shower and the correct determination of potential extraneous-conductive-parts. I realised whilst putting this talk together that after more than 20+ years of staring at this equation that for me I now see it as an incorrectly conceived and lazily written working example of the equation (6.1).

    Looking at equation (6.2) as with the others, it's not so clear that everything to the right of the (=) in the middle of the equation is the same equation just with the bracketed part of the equation completed. This surely should have been the symbol for 'equivalent' or 'is equivalent to'? Like an equals symbol but with three lines?

    Why is this simplification or staged aproach required?

    If followed mathematically then the outcome would be incorrect...

    (230 / 0.0005) -1000 = 459 000 - 1000 = 458 000Ω (not 459KΩ)

    Surely written correctly it should be:

    (230 / 0.0005) - 1000 / 1000 = 459kΩ

    It's just messy and if assumes that the person reading it needs help with the bracketed part of the equation, but then goes on to jump from 459 000Ω to 459KΩ without any further guidance for those that required help with the bracketed part of the equation as to how we got from Ω to KΩ.

    It's just my opinion of course ;0)

  • 0 in reply to sae70
    sae70 said:

    If followed mathematically then the outcome would be incorrect...

    (230 / 0.0005) -1000 = 459 000 - 1000 = 458 000Ω (not 459KΩ)

    Surely written correctly it should be:

    (230 / 0.0005) - 1000 / 1000 = 459kΩ

    That is not correct at all.

    Why have you in the first example subtracted 1000 (Ω) twice, and divided by 1000 in the second?

    I would prefer to have each = sign starting a new line.

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  • 0 in reply to sae70
    sae70 said:

    If followed mathematically then the outcome would be incorrect...

    (230 / 0.0005) -1000 = 459 000 - 1000 = 458 000Ω (not 459KΩ)

    Surely written correctly it should be:

    (230 / 0.0005) - 1000 / 1000 = 459kΩ

    That is not correct at all.

    Why have you in the first example subtracted 1000 (Ω) twice, and divided by 1000 in the second?

    I would prefer to have each = sign starting a new line.

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