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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

    As far as I am aware the voltage tolerances that you are referencing above stem from 1988 when the European electrical standards body CENELEC agreed on harmonization of low voltage

    electricity supplies within Europe as further detailed in BS7671:2018 appendix 2. My understanding is that this applies to public electricity supply systems. Does it also apply to private installations such as we are discussing here?  



    And before that (for as long as I can remember), it was 240V +/- 6% - i.e. 225.6V to 254.4V - i.e. an even higher upper limit - and we still used 0.4s disconnection times in those days.


    True the specification was originally intended for public supplies - but the underlying physics (and physiology) of shock protection would be the same whoever owned the supply so I see no logical reason not to adopt the same approach. The definition of voltage, nominal in part 2 of BS 7671 (especially the NOTE) would also seem to suggest that the same fundamental approach can be taken regardless of supply ownership.


       - Andy.
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  • 0

    As far as I am aware the voltage tolerances that you are referencing above stem from 1988 when the European electrical standards body CENELEC agreed on harmonization of low voltage

    electricity supplies within Europe as further detailed in BS7671:2018 appendix 2. My understanding is that this applies to public electricity supply systems. Does it also apply to private installations such as we are discussing here?  



    And before that (for as long as I can remember), it was 240V +/- 6% - i.e. 225.6V to 254.4V - i.e. an even higher upper limit - and we still used 0.4s disconnection times in those days.


    True the specification was originally intended for public supplies - but the underlying physics (and physiology) of shock protection would be the same whoever owned the supply so I see no logical reason not to adopt the same approach. The definition of voltage, nominal in part 2 of BS 7671 (especially the NOTE) would also seem to suggest that the same fundamental approach can be taken regardless of supply ownership.


       - Andy.
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