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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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  • 0

    geoffsd:

    There seems to be some confusion about what "nominal" voltage is.


    It is just the number used for calculations.




    Or for the rating of appliances and other electrical equipment, e.g. selection of appropriate semiconductor devices, tests for insulation, etc.

     






    230 x 95% = 218.5  If you want to use 240V then Cmin will be 91.04% (ignoring the discrepancy between 218.5 and 216.2V)




    In addition to a voltage factor Cmin, there's a voltage factor Cmax which is 1.1 for supplies in accordance with the ESQCR.


    Cmax appears in A722.3 of A1:2020 to BS 7671:2018.


    There's also a difference, for appliances, between the supply voltage and the utilization voltage, and Cmin relates to the latter - i.e. it takes into account voltage drop.

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  • 0

    geoffsd:

    There seems to be some confusion about what "nominal" voltage is.


    It is just the number used for calculations.




    Or for the rating of appliances and other electrical equipment, e.g. selection of appropriate semiconductor devices, tests for insulation, etc.

     






    230 x 95% = 218.5  If you want to use 240V then Cmin will be 91.04% (ignoring the discrepancy between 218.5 and 216.2V)




    In addition to a voltage factor Cmin, there's a voltage factor Cmax which is 1.1 for supplies in accordance with the ESQCR.


    Cmax appears in A722.3 of A1:2020 to BS 7671:2018.


    There's also a difference, for appliances, between the supply voltage and the utilization voltage, and Cmin relates to the latter - i.e. it takes into account voltage drop.

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