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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 ?


  • Former Community Member
    0 Former Community Member

    geoffsd:

    You are missing the point.


    Cmin and/or Cmax values are only required to convert pre-calculated tables and only applicable for the voltage used in those table calculations.


    What I am saying is to not use those tables but do your own calculations using the appropriate actual voltage needed for your calculations - whether it be a minimum or maximum voltage depending on what is being considered.



     




    I also agree with using calculations and ignore the tables. However when performing calculations use the manufacturers data as stay away from using BS-EN-60898 tripping current values such as 5xIn for B type MCB, 10XIn for C type MCB as this can result in incorrect conclusions regards the maximum permissible earth loop impedance.


    However back to my original question that started all this discussion. What value Uo should be used when calculating max disconnect times and Zs values for the 416Vac phase/phase circuits ?

  • 0
    If you are working to BS7671, then 400V. 


    If you are working to manufacturer’s data for your loop impedance values, you need to remember that this may differ between manufacturers. If a different make device is fitted, the calculations may not work. 


    Regards,


    Alan.
  • Former Community Member
    0 Former Community Member

    Alan Capon:

    If you are working to BS7671, then 400V. 


    If you are working to manufacturer’s data for your loop impedance values, you need to remember that this may differ between manufacturers. If a different make device is fitted, the calculations may not work. 


    Regards,


    Alan. 




    Yes absolutely the permissible Zs values differ depending on the protective device manufacturer that's is why it is important to use the manufacturer's data for the fault protective device otherwise you run the risk of failing earth loop impedance values that are actually compliant with BS7671. 


    In regards to your 400V statement, can you reference any particular clause in BS7671 whereby it states 400V or is that just your personal experience/opinion?

  • 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.
  • 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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