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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

    Alan Capon:

    If you are working to BS7671, then 400V. 


    If you are working to manufacturer’s data for your loop impedance values, you need to remember that this may differ between manufacturers. If a different make device is fitted, the calculations may not work. 


    Regards,


    Alan. 




    Yes absolutely the permissible Zs values differ depending on the protective device manufacturer that's is why it is important to use the manufacturer's data for the fault protective device otherwise you run the risk of failing earth loop impedance values that are actually compliant with BS7671. 


    In regards to your 400V statement, can you reference any particular clause in BS7671 whereby it states 400V or is that just your personal experience/opinion?

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  • Former Community Member
    0 Former Community Member

    Alan Capon:

    If you are working to BS7671, then 400V. 


    If you are working to manufacturer’s data for your loop impedance values, you need to remember that this may differ between manufacturers. If a different make device is fitted, the calculations may not work. 


    Regards,


    Alan. 




    Yes absolutely the permissible Zs values differ depending on the protective device manufacturer that's is why it is important to use the manufacturer's data for the fault protective device otherwise you run the risk of failing earth loop impedance values that are actually compliant with BS7671. 


    In regards to your 400V statement, can you reference any particular clause in BS7671 whereby it states 400V or is that just your personal experience/opinion?

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