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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
    I don't think that there is a specific definition for 'private supplies' so just choose whichever route you wish to pursue.

    As for appliance manufacturers, with a purposely designed product life-cycle of 5 years max for most appliances, and given that so-called warranties are no longer worth the paper they are written on, I wouldn't worry too much about a bit of 'overvoltage' - I'm sure most manufacturers would not be prepared to stand the cost of coming out to your installation the measure the actual voltage present prior to deciding upon any potential pay outs if you are fortunate enough to actually be offered one in the 1st place).

    Generally, with warranties, you are referred into a circular game of ping pong between the retailer and the maker until you tire of the fight and give in and purchase a replacement.

    Caveat Emptor when it comes to warranties - especially those which are offered for a lifetime.
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  • 0
    I don't think that there is a specific definition for 'private supplies' so just choose whichever route you wish to pursue.

    As for appliance manufacturers, with a purposely designed product life-cycle of 5 years max for most appliances, and given that so-called warranties are no longer worth the paper they are written on, I wouldn't worry too much about a bit of 'overvoltage' - I'm sure most manufacturers would not be prepared to stand the cost of coming out to your installation the measure the actual voltage present prior to deciding upon any potential pay outs if you are fortunate enough to actually be offered one in the 1st place).

    Generally, with warranties, you are referred into a circular game of ping pong between the retailer and the maker until you tire of the fight and give in and purchase a replacement.

    Caveat Emptor when it comes to warranties - especially those which are offered for a lifetime.
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