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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
    He's right and wrong!

    I'm sure he is aware that despite the 230v figure, we in the UK regularly have voltages significantly higher than this on average. I blame the eu harmonisation thing.

    He is right that BS7671 2018 states that the figure of 230v must be used,

    But he is wrong if he says that Zs won't be achievable by using the old 240V figure too.

    Look at the +- tolerances for UK low voltage supply networks.

    Cmin is something of a nonsense distraction which was recently introduced. In reality it has little bearing upon Zs outcomes and I have never understood why it was included as a correction factor other than to serve to get us to install larger cables.

    Put it this way, ask yourself how many installations were completed satisfactorily prior to the Cmin correction factor being introduced, and how many have burst into flames or killed someone as a consequence?

    The regs are not retrospective in any case.


Reply
  • 0
    He's right and wrong!

    I'm sure he is aware that despite the 230v figure, we in the UK regularly have voltages significantly higher than this on average. I blame the eu harmonisation thing.

    He is right that BS7671 2018 states that the figure of 230v must be used,

    But he is wrong if he says that Zs won't be achievable by using the old 240V figure too.

    Look at the +- tolerances for UK low voltage supply networks.

    Cmin is something of a nonsense distraction which was recently introduced. In reality it has little bearing upon Zs outcomes and I have never understood why it was included as a correction factor other than to serve to get us to install larger cables.

    Put it this way, ask yourself how many installations were completed satisfactorily prior to the Cmin correction factor being introduced, and how many have burst into flames or killed someone as a consequence?

    The regs are not retrospective in any case.


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