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 32 subscribers
  • Views 6260 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 ?


Parents
  • 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
Reply
  • 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
Children
No Data

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