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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
    @gkenyon


    Thank you for your response. It can work both ways depending on the fault protective device:


    * higher permissible loop impedance using 240Vac in our calculations as per BS7671:2018 clause 411.4.4 (Zs × Ia ≤ U0 × Cmin).


    * lower permissible loop impedance due to the fault protective device having to operate more quickly when (Uo) is 240Vac as per BS7671:2018 table  41.1.


    Many ABB fuses require more tripping current (Ia) to operate in 0.2 seconds compared to a 0.4 second trip.


    What I am gathering is not to take table 41.1 at face value and instead say that a 0.4 second trip time is sufficient for a 240Vac (Uo) circuit ?

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  • Former Community Member
    0 Former Community Member
    @gkenyon


    Thank you for your response. It can work both ways depending on the fault protective device:


    * higher permissible loop impedance using 240Vac in our calculations as per BS7671:2018 clause 411.4.4 (Zs × Ia ≤ U0 × Cmin).


    * lower permissible loop impedance due to the fault protective device having to operate more quickly when (Uo) is 240Vac as per BS7671:2018 table  41.1.


    Many ABB fuses require more tripping current (Ia) to operate in 0.2 seconds compared to a 0.4 second trip.


    What I am gathering is not to take table 41.1 at face value and instead say that a 0.4 second trip time is sufficient for a 240Vac (Uo) circuit ?

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