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 ?
