IEC 60364 Table 48A

gkenyon:

ProMbrooke:

Higher rated circuits has a much lower R1+R2, producing voltage drop on the transformer and in turn a lower touch voltage whereby disconnection can be longer without worry of physiological harm. 

That is a huge assumption - particularly for public TN-S supplies - that most of the supply resistance is in the transformer and line conductor.


The "standard" figures for 100 A supplies doesn't support this view:


TN-C-S, Ze = 0.35 Ohm


TN-S Ze = 0.8 Ohm


Both supplies have the same line conductor and same transformer, so the difference, > 50 %, must be the distributor's protective conductor.


So, you really aren't going to be able to reduce the potential touch voltage in TN-S systems of 100 A or less below 115 V nominal.


Touch voltages can be better in PME systems.


Alternatively, if you actually do have exposed-conductive-parts, inside buildings with TT, these present the lowest touch voltages in an earth fault. Sadly, outdoors they produce the highest touch voltages. But then again, you're far more likely to have at least two RCDs in series (RCD main switch and an RCBO for the final circuit) to help you in a TT system ...


Perhaps we'd just better all go TT?

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.    

gkenyon:

ProMbrooke:

Higher rated circuits has a much lower R1+R2, producing voltage drop on the transformer and in turn a lower touch voltage whereby disconnection can be longer without worry of physiological harm. 

That is a huge assumption - particularly for public TN-S supplies - that most of the supply resistance is in the transformer and line conductor.


The "standard" figures for 100 A supplies doesn't support this view:


TN-C-S, Ze = 0.35 Ohm


TN-S Ze = 0.8 Ohm


Both supplies have the same line conductor and same transformer, so the difference, > 50 %, must be the distributor's protective conductor.


So, you really aren't going to be able to reduce the potential touch voltage in TN-S systems of 100 A or less below 115 V nominal.


Touch voltages can be better in PME systems.


Alternatively, if you actually do have exposed-conductive-parts, inside buildings with TT, these present the lowest touch voltages in an earth fault. Sadly, outdoors they produce the highest touch voltages. But then again, you're far more likely to have at least two RCDs in series (RCD main switch and an RCBO for the final circuit) to help you in a TT system ...


Perhaps we'd just better all go TT?

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.