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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 ?


  • 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.


  • 0
    I would agree with Mr Johnson. I would also suggest that if your U0 is not subject to the +10% tolerance then disconnection times as per Table 41.1 can be lifted from the 230v column.

    Just out of interest, new Irish Standard 10101 doesn’t bother with the cmin lark.
  • 0

    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.



    The table is based on nominal rather than actual voltage - 230V nominal is usually taken to be anything within the -6% to +10% range - i.e. between 216.2V and 253V. It's very common for DNOs to have far above 230V at their transformers to compensate for voltage drop in the distribution system - typically a 433/250V.  So as long as your supply doesn't go above 253V (during times of low load say) I'd say it's acceptable to use the 230V nominal column (e.g. 0.4s for TN small final circuits).

       - Andy.
  • 0
    If it was a curve of disconnection times versus voltage in the regs and not a table no one would care, as 230V and 240v would have times very similar.

    But it is not really a sudden step in risk between 230v and 231, more of a gentle rise in risk from quite safe at about 50V to really very bad for you by perhaps 1500V , from which 2 handy values have been converted into safe times for the table.


    The origin of the 0.2 secs or 0.4 secs  is all about shock currents, and the time it takes to destabilize a human heart. For shocks that last longer than about half a heart beat, a lower current is dangerous, compared to shorter duration events, where we can stand quite a lot more peak current. (we still swear a lot though...)

    The assumption on a TN x system is that on fault the appliance will be mid rail between L and E, and the voltages will drop equally along the wiring there and back.

    So a barefoot victim gets exposed to about 120V, and maybe we can have an argument about reduced CPC in twin and earth.

    For a TT system we assume most of the volt drop is in the earth path, not the live, so the voltage from faulty kit to earth is more like 200V plus, so the maximum safe exposure time is reduced.


    For both cases the safe time has to be reduced for 400V, and again by 690, and again by 1200.

    Hope this helps.

  • 0
    Andy has the correct answer.


    If you want to claim you've got a higher nominal voltage than 230 V, and hence can use higher loop impedance values, then some appliance manufacturers would be within their rights not to guarantee their equipment if it fails in your installation.
  • Former Community Member
    0 Former Community Member
    @gkenyon


    Thank you for your response. It can work both ways depending on the fault protective device:


    * higher permissible loop impedance using 240Vac in our calculations as per BS7671:2018 clause 411.4.4 (Zs × Ia ≤ U0 × Cmin).


    * lower permissible loop impedance due to the fault protective device having to operate more quickly when (Uo) is 240Vac as per BS7671:2018 table  41.1.


    Many ABB fuses require more tripping current (Ia) to operate in 0.2 seconds compared to a 0.4 second trip.


    What I am gathering is not to take table 41.1 at face value and instead say that a 0.4 second trip time is sufficient for a 240Vac (Uo) circuit ?

  • Former Community Member
    0 Former Community Member

    AJJewsbury:




    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.



    The table is based on nominal rather than actual voltage - 230V nominal is usually taken to be anything within the -6% to +10% range - i.e. between 216.2V and 253V. It's very common for DNOs to have far above 230V at their transformers to compensate for voltage drop in the distribution system - typically a 433/250V.  So as long as your supply doesn't go above 253V (during times of low load say) I'd say it's acceptable to use the 230V nominal column (e.g. 0.4s for TN small final circuits).

       - Andy.

     



    @AJJewsbury


    Thank you for your response. As far as I am aware the voltage tolerances that you are referencing above stem from 1988 when the European electrical standards body CENELEC agreed on harmonization of low voltage

    electricity supplies within Europe as further detailed in BS7671:2018 appendix 2. My understanding is that this applies to public electricity supply systems. Does it also apply to private installations such as we are discussing here?  



     

  • Former Community Member
    0 Former Community Member

    lyledunn:

    I would agree with Mr Johnson. I would also suggest that if your U0 is not subject to the +10% tolerance then disconnection times as per Table 41.1 can be lifted from the 230v column.

    Just out of interest, new Irish Standard 10101 doesn’t bother with the cmin lark. 




    Yes the UK is the only country I have seen thus far that de-rates the voltage using Cmin for the purposes of calculating permissible Zs. Most European countries do not do use c-min as their national standards are all based the IEC 60364 series and that International IEC standard calculates permissible loop impedance as follows Zs = Uo/Ia. However it should be noted that when measuring Zs and accounting for the rise of temperature under IEC 60364 the permissible earth loop impedance is de-rated by 1/3 where as in the UK under BS7671 it is only de-rated by 20%.

  • 0
    What is the problem here?

    Don't look at any tables. Work it out yourself.


    For maximum Zs we use the worst case scenario (almost)


    For example, for BS60898 MCBs Ia = In x 5  although it might be anything from 3 to 5.

    230V is used and then corrected by a Cmin of 95% so use 218.5V  (which should actually be 94% for the minimum allowed voltage and 216.2V).


    So, for a 32A BS60898 it is 


    218.5 / 160 = 1.365625  (should be 216.2 / 160 = 1.35125)

    Then a temperature correction from ambient to conductor temperature in expected use.


    Any variations of higher voltage or lower Ia or lower conductor temperature will make the situation better.


  • Former Community Member
    0 Former Community Member

    mapj1:

    If it was a curve of disconnection times versus voltage in the regs and not a table no one would care, as 230V and 240v would have times very similar.

    But it is not really a sudden step in risk between 230v and 231, more of a gentle rise in risk from quite safe at about 50V to really very bad for you by perhaps 1500V , from which 2 handy values have been converted into safe times for the table.


    The origin of the 0.2 secs or 0.4 secs  is all about shock currents, and the time it takes to destabilize a human heart. For shocks that last longer than about half a heart beat, a lower current is dangerous, compared to shorter duration events, where we can stand quite a lot more peak current. (we still swear a lot though...)

    The assumption on a TN x system is that on fault the appliance will be mid rail between L and E, and the voltages will drop equally along the wiring there and back.

    So a barefoot victim gets exposed to about 120V, and maybe we can have an argument about reduced CPC in twin and earth.

    For a TT system we assume most of the volt drop is in the earth path, not the live, so the voltage from faulty kit to earth is more like 200V plus, so the maximum safe exposure time is reduced.


    For both cases the safe time has to be reduced for 400V, and again by 690, and again by 1200.

    Hope this helps.

     




    In many European countries there is no table, they only have the formula in their national standards. Note that BS7671:2018 clause 411.4.202 does state that the permissible Zs can be calculated and in my opinion it is the best method to ensure that we are not failing Zs values that are still low enough to achieve the required disconnect time. I will give you an example:

    Type C 10A MCB - BS7671:2018 table 41.1 states that the max max permissible Zs for a 5 second trip is 2.19 Ohms.


    However if we calculate it we get a higher permissible Zs while still being in compliance with BS7671 411.4.202.


    ABB S202M-C10 Type C 10A MCB


    Using ABB curves software we can quickly obtain the required tripping current (Ia) for a 5 second trip = 77.2A


    Using formula as per BS7671:2018 clause 411.4.4 Zs × Ia ≤ U0 × Cmin.


    Zs = (230V*0.95)/77.2A = 2.83 Ohms


    That's quite a large difference and hence the reason why the national standards in other European countries do not include Zs tables, only the formula is provided.


    People using the Zs tables in BS7671 are failing BS7671 compliant circuits in some instances...


    Using BS7671:2018 clause 411.4.4 formula with manufacturers data is always the best way to go.

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