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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 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.
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  • 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.
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