IEC 60364 Table 48A

gkenyon:

ProMbrooke:

That is unless you change the values for a TN-S supply. But ultimately I don't like the idea of those values because 0.35 ohms may be impossible to achieve with a small pole pig despite its massive drop during a fault where on the other hand doable with a 1000kva unit but limited drop on a 100 amp fuse's max EFLI. 


I will admit I've never run the voltage drop numbers in significant depth for a typical UK supply transformer however. 


I think this is where the IET needs to do more work.


Ideally transformer size vs the conductor size would have a hand in the play.    

What do you mean by "change the values"? It's a function of the distributor's cpc?


Not sure why you're making reference to "pole pig" ... values are different in rural areas anyway, although isn't that why they use either TT or PNB with pole-mounted transformers?


What work do you think the IET should do, specifically?


There's also a trade-off between conductor size and cost. Do you have something specific to share, as all this seems a little shrouded in mystery, and dancing around willow-the-whisps?

That if testing and/or calculations can prove high touch voltage in relation to IEC 60479-1, that disconnection times are reduced for circuits over 63 amps.


I bring up pole pigs in that a 25kva fed 63-200amp customer vs a 1000kva fed 63-200amp customer will have a very different touch voltage to remote earth. A fault on sub feed will cause significant drop on the output terminals of a 25kva transformer vs 1000kva transformer. As such a 5 second disconnection would be acceptable for a the prior, but perhaps not the latter.


The IET should investigate actual touch voltages during a fault for various scenerios.   

 

gkenyon:

ProMbrooke:

That is unless you change the values for a TN-S supply. But ultimately I don't like the idea of those values because 0.35 ohms may be impossible to achieve with a small pole pig despite its massive drop during a fault where on the other hand doable with a 1000kva unit but limited drop on a 100 amp fuse's max EFLI. 


I will admit I've never run the voltage drop numbers in significant depth for a typical UK supply transformer however. 


I think this is where the IET needs to do more work.


Ideally transformer size vs the conductor size would have a hand in the play.    

What do you mean by "change the values"? It's a function of the distributor's cpc?


Not sure why you're making reference to "pole pig" ... values are different in rural areas anyway, although isn't that why they use either TT or PNB with pole-mounted transformers?


What work do you think the IET should do, specifically?


There's also a trade-off between conductor size and cost. Do you have something specific to share, as all this seems a little shrouded in mystery, and dancing around willow-the-whisps?

That if testing and/or calculations can prove high touch voltage in relation to IEC 60479-1, that disconnection times are reduced for circuits over 63 amps.


I bring up pole pigs in that a 25kva fed 63-200amp customer vs a 1000kva fed 63-200amp customer will have a very different touch voltage to remote earth. A fault on sub feed will cause significant drop on the output terminals of a 25kva transformer vs 1000kva transformer. As such a 5 second disconnection would be acceptable for a the prior, but perhaps not the latter.


The IET should investigate actual touch voltages during a fault for various scenerios.