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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 ?


Parents
  • 0
    The figure is given as a nominal voltage figure to use in the same way the 230V figure is.

    Right, back to 1st principles -

    Idiot question 1. Can someone kindly explain to me in plain idiot-proof language (a tall order I know, especially when you don't know your audience) how and why the Cmin thing became a requirement?

    What drove it's inclusion into BS7671 and why?

    I confess that I find it confusing to have to factor in something over which one has no direct control.

    I gather that it has something to do with DNO transformer taps but know nothing more.

    Q.2 - So, transformer voltage taps changing? Why would the DNO do that and how regularly?

    Q.3 - Why was this factor never included before? i.e. what has made it become necessary all of a sudden?

    Q.4 - What are the negative effects of not using it in our calculations now, which did not exist previously?

    Q.5 - Who benefits the most from it's use and why?


    Comments welcome.
Reply
  • 0
    The figure is given as a nominal voltage figure to use in the same way the 230V figure is.

    Right, back to 1st principles -

    Idiot question 1. Can someone kindly explain to me in plain idiot-proof language (a tall order I know, especially when you don't know your audience) how and why the Cmin thing became a requirement?

    What drove it's inclusion into BS7671 and why?

    I confess that I find it confusing to have to factor in something over which one has no direct control.

    I gather that it has something to do with DNO transformer taps but know nothing more.

    Q.2 - So, transformer voltage taps changing? Why would the DNO do that and how regularly?

    Q.3 - Why was this factor never included before? i.e. what has made it become necessary all of a sudden?

    Q.4 - What are the negative effects of not using it in our calculations now, which did not exist previously?

    Q.5 - Who benefits the most from it's use and why?


    Comments welcome.
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