The IET is carrying out some important updates between 17-30 April and all of our websites will be view only. For more information, read this Announcement

Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 25 replies
  • Subscribers 31 subscribers
  • Views 6209 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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

    Mike M:

    @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 ?

     




    Well, what I was getting at, is that if the nominal voltage is above 230 V (i.e. maximum voltage within occasional flits above this limit in accordance with BS EN 50160 being 253 V), then the equipment may be experiencing overvoltage. If you state your nominal voltage as 240 V, then these days it has a maximum voltage of 264 V (with occasional flits above that) ... so if the manufacturer has rated for 230 V, for most appliances for the past large number of years, then they won't be happy if you declare 240 V.

  • 0
    There seems to be some confusion about what "nominal" voltage is.


    It is just the number used for calculations.


    230 x 95% = 218.5  If you want to use 240V then Cmin will be 91.04% (ignoring the discrepancy between 218.5 and 216.2V)
  • 0

    geoffsd:

    There seems to be some confusion about what "nominal" voltage is.


    It is just the number used for calculations.




    Or for the rating of appliances and other electrical equipment, e.g. selection of appropriate semiconductor devices, tests for insulation, etc.

     






    230 x 95% = 218.5  If you want to use 240V then Cmin will be 91.04% (ignoring the discrepancy between 218.5 and 216.2V)




    In addition to a voltage factor Cmin, there's a voltage factor Cmax which is 1.1 for supplies in accordance with the ESQCR.


    Cmax appears in A722.3 of A1:2020 to BS 7671:2018.


    There's also a difference, for appliances, between the supply voltage and the utilization voltage, and Cmin relates to the latter - i.e. it takes into account voltage drop.

  • 0
    The upper and lower voltage limits in the UK are specified in the ESQCR regulations, a Statutory Instrument of British Law, the current version being made law in 2002. It should also be noted that if you are in the UK, the use of TNC-S in a private network is expressly forbidden in the ESQCR regulations, and could result in your HV supply being disconnected by the DNO.


    I suspect your installation isn’t really TNC-S, but if it is, the law says it must not be. In terms of the voltage limits, in a private network the nominal voltage and limits are up to the owner. It should however be remembered that equipment manufacturers will design to the ESQCR limits, as this represents the majority of their sales in the UK. 


    Regards,


    Alan.
  • 0

     



    In addition to a voltage factor Cmin, there's a voltage factor Cmax which is 1.1 for supplies in accordance with the ESQCR.

     


    Yes, but only for the "nominal" voltage of 230V.  Using 240V it would be 1.0542.


    Why not just use the relevant voltage in the first place?
  • 0

    geoffsd:




     



    In addition to a voltage factor Cmin, there's a voltage factor Cmax which is 1.1 for supplies in accordance with the ESQCR.

     


    Yes, but only for the "nominal" voltage of 230V.  Using 240V it would be 1.0542.

    BS EN 60909-0 tells us that 1.05 can be used where the tolerance is + 6 %, which used to be the case for 240 V supplies, but we don't have that any longer. You can't really mix the old money with the new ... and certainly not in this case?


    What is the agreed tolerance on the HV input to this transformer? Without that, you can only assume Cmax = 1.1, according to BS EN 60909--0 (Table 1 says that for supplies above 1 kV, Cmax = 1.1 and Cmin = 1.0)


    So if this transformer has fixed taps for 240 V, without further information, Cmax = 1.1 and the maximum voltage is 264 V, not the "old money" 254.4 V
  • 0
    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.



  • 0

    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?  



    And before that (for as long as I can remember), it was 240V +/- 6% - i.e. 225.6V to 254.4V - i.e. an even higher upper limit - and we still used 0.4s disconnection times in those days.


    True the specification was originally intended for public supplies - but the underlying physics (and physiology) of shock protection would be the same whoever owned the supply so I see no logical reason not to adopt the same approach. The definition of voltage, nominal in part 2 of BS 7671 (especially the NOTE) would also seem to suggest that the same fundamental approach can be taken regardless of supply ownership.


       - Andy.
  • 0

    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.


     




    That's not the case. As a simple example, Tables 41.2 to 41.5 in BS 7671 are based on 230 V supplies with Cmin in accordance with ESQCR. They can't be used for a 240 V system unless the Cmin of that system just happens to be 0.91. With fixed tap changer, the real value of is likely to exceed 0.95, meaning you would need to as the OP says, those Tables lead to an under-estimate of the possible earth fault loop impedance - not necessarily a safety issue though.


    HOWEVER, it's not always the case that dropping to 230 V puts you o the "safe side" - for example, the calculations in A722.3 - use of 230 V in that case, as my previous post indicates, would mean you calculate:


    (a) for a 230 V single-phase supply, with a MD of 100 A: RA ev ≤ 70 × 230 × 1.1 ÷ (100 × (230 × 1.1 - 70 )) = 0.97 Ω


    (b) for a 240 V single-phase supply, with a MD of 100 A: RA ev ≤ 70 × 240 × 1.1 ÷ (100 × (240 × 1.1 - 70 )) = 0.95 Ω


    So in that particular case, you would be calculating 2 % above the safety margin using the wrong nominal voltage ... Granted in this particular case it's a moot point because the value is too low to be practicable, but if you were doing the calculation for three-phase it would be a different story. It's quite important for that calculation, as things change regarding the impact of a shock only a few V above 70 V, so there's not a lot of margin for error or indeed assumption.

  • 0

    AJJewsbury:


    And before that (for as long as I can remember), it was 240V +/- 6% - i.e. 225.6V to 254.4V - i.e. an even higher upper limit - and we still used 0.4s disconnection times in those days.


    True the specification was originally intended for public supplies - but the underlying physics (and physiology) of shock protection would be the same whoever owned the supply so I see no logical reason not to adopt the same approach. The definition of voltage, nominal in part 2 of BS 7671 (especially the NOTE) would also seem to suggest that the same fundamental approach can be taken regardless of supply ownership.


       - Andy.

     




    It doesn't matter for disconnection times - as per my previous post, you're on the right side of safety.


    However, where the actual upper voltage is 264 V, not 254.4 V (the latter would make three shades of no difference over 253 V upper voltage of course),  things start to err on the wrong side of caution when calculating fault currents (by as much as 4 %), earth electrode resistances for diverted neutral currents (by as much as 2 %), etc.

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines