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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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    Mike M:

    @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 ?

     




    Well, what I was getting at, is that if the nominal voltage is above 230 V (i.e. maximum voltage within occasional flits above this limit in accordance with BS EN 50160 being 253 V), then the equipment may be experiencing overvoltage. If you state your nominal voltage as 240 V, then these days it has a maximum voltage of 264 V (with occasional flits above that) ... so if the manufacturer has rated for 230 V, for most appliances for the past large number of years, then they won't be happy if you declare 240 V.

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  • 0

    Mike M:

    @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 ?

     




    Well, what I was getting at, is that if the nominal voltage is above 230 V (i.e. maximum voltage within occasional flits above this limit in accordance with BS EN 50160 being 253 V), then the equipment may be experiencing overvoltage. If you state your nominal voltage as 240 V, then these days it has a maximum voltage of 264 V (with occasional flits above that) ... so if the manufacturer has rated for 230 V, for most appliances for the past large number of years, then they won't be happy if you declare 240 V.

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