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Voltage (Uo) for calculating permissible (Zs) earth loop impedance and disconnect times

Former Community Member
Former Community Member
Guys,


Ignoring BS7671:2018 appendix 3 for the purposes of simplifying this discussion.


Private installation, TNCS earthing system where the main transformers taps are set to give phase/phase voltage (U) 416Vac RMS on the secondary side. 


As ye are aware 416V/(√3) = 240Vac RMS line to earth voltage Uo.


I am also being told from an inspector that the permissible disconnection time in table BS7671:2018 41.1 is 0.4 seconds.


However when I look at table 41.1 it is stating that if Uo is 240Vac the permissible disconnection time is only 0.2 seconds.


I am being told by an inspector that we have to use 230Vac when performing permissible earth loop impedance calculations as per BS7671:2018 clause 411.4.4 Zs × Ia ≤ U0 × Cmin.


However if we perform the calculation using 230Vac will will get a reduced permissible Zs. This would seem to suggest to me that we could be failing Zs values that allow enough current to flow in the event of a fault to trip the fault protective device.


Is the inspector wrong ?


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  • Former Community Member
    0 Former Community Member

    lyledunn:

    I would agree with Mr Johnson. I would also suggest that if your U0 is not subject to the +10% tolerance then disconnection times as per Table 41.1 can be lifted from the 230v column.

    Just out of interest, new Irish Standard 10101 doesn’t bother with the cmin lark. 




    Yes the UK is the only country I have seen thus far that de-rates the voltage using Cmin for the purposes of calculating permissible Zs. Most European countries do not do use c-min as their national standards are all based the IEC 60364 series and that International IEC standard calculates permissible loop impedance as follows Zs = Uo/Ia. However it should be noted that when measuring Zs and accounting for the rise of temperature under IEC 60364 the permissible earth loop impedance is de-rated by 1/3 where as in the UK under BS7671 it is only de-rated by 20%.

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  • Former Community Member
    0 Former Community Member

    lyledunn:

    I would agree with Mr Johnson. I would also suggest that if your U0 is not subject to the +10% tolerance then disconnection times as per Table 41.1 can be lifted from the 230v column.

    Just out of interest, new Irish Standard 10101 doesn’t bother with the cmin lark. 




    Yes the UK is the only country I have seen thus far that de-rates the voltage using Cmin for the purposes of calculating permissible Zs. Most European countries do not do use c-min as their national standards are all based the IEC 60364 series and that International IEC standard calculates permissible loop impedance as follows Zs = Uo/Ia. However it should be noted that when measuring Zs and accounting for the rise of temperature under IEC 60364 the permissible earth loop impedance is de-rated by 1/3 where as in the UK under BS7671 it is only de-rated by 20%.

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