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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

    geoffsd:

    You are missing the point.


    Cmin and/or Cmax values are only required to convert pre-calculated tables and only applicable for the voltage used in those table calculations.


    What I am saying is to not use those tables but do your own calculations using the appropriate actual voltage needed for your calculations - whether it be a minimum or maximum voltage depending on what is being considered.



     




    I also agree with using calculations and ignore the tables. However when performing calculations use the manufacturers data as stay away from using BS-EN-60898 tripping current values such as 5xIn for B type MCB, 10XIn for C type MCB as this can result in incorrect conclusions regards the maximum permissible earth loop impedance.


    However back to my original question that started all this discussion. What value Uo should be used when calculating max disconnect times and Zs values for the 416Vac phase/phase circuits ?

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  • Former Community Member
    0 Former Community Member

    geoffsd:

    You are missing the point.


    Cmin and/or Cmax values are only required to convert pre-calculated tables and only applicable for the voltage used in those table calculations.


    What I am saying is to not use those tables but do your own calculations using the appropriate actual voltage needed for your calculations - whether it be a minimum or maximum voltage depending on what is being considered.



     




    I also agree with using calculations and ignore the tables. However when performing calculations use the manufacturers data as stay away from using BS-EN-60898 tripping current values such as 5xIn for B type MCB, 10XIn for C type MCB as this can result in incorrect conclusions regards the maximum permissible earth loop impedance.


    However back to my original question that started all this discussion. What value Uo should be used when calculating max disconnect times and Zs values for the 416Vac phase/phase circuits ?

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