kenyon

Yes but ... that only works indoors. And even there, slightly over 70 V is too much for 5 s ... perhaps even 1 s.


What about when you supply equipment outdoors, and have no control over the voltage at a person's feet as you might have within a building (what we used to term "equipotential zone")?


The worst case is a TT system, because the full U0 is available outdoors, and 1 s is way too long.


Next up comes a TN-S system where the installation is some distance from the transformer supplying it.



The actual answer, as Mike alluded to, is that it's an engineering compromise between what's practicable, and the likely risk of shock in a fault condition (larger conductors perhaps less likely to break, often in more robust wiring systems etc.).


 

In all of this I am assuming remote earth and a minimum body impedance of 1000 ohms. Zero bonding or equal potential taken into account.


I'm well aware the IEC used to allow (and still does to a degree) equal potential bonding as a means to manage greater disconnection time however this is not what I have in mind.


In a TN system voltage from the fault point to remote earth is far lower than is being assumed here. The transformer is far from an infinite source, it is rather weak.


It has nothing to do with compromise, rather the IEC knows voltage and body resistance will typically never reach values that will violate the IEC's body graph.


I'm going to go out and say that with circuits protected over 200-400 amps could easily get away with a 10 second disconnection time and this should be researched/considered further.  

kenyon

Yes but ... that only works indoors. And even there, slightly over 70 V is too much for 5 s ... perhaps even 1 s.


What about when you supply equipment outdoors, and have no control over the voltage at a person's feet as you might have within a building (what we used to term "equipotential zone")?


The worst case is a TT system, because the full U0 is available outdoors, and 1 s is way too long.


Next up comes a TN-S system where the installation is some distance from the transformer supplying it.



The actual answer, as Mike alluded to, is that it's an engineering compromise between what's practicable, and the likely risk of shock in a fault condition (larger conductors perhaps less likely to break, often in more robust wiring systems etc.).


 

In all of this I am assuming remote earth and a minimum body impedance of 1000 ohms. Zero bonding or equal potential taken into account.


I'm well aware the IEC used to allow (and still does to a degree) equal potential bonding as a means to manage greater disconnection time however this is not what I have in mind.


In a TN system voltage from the fault point to remote earth is far lower than is being assumed here. The transformer is far from an infinite source, it is rather weak.


It has nothing to do with compromise, rather the IEC knows voltage and body resistance will typically never reach values that will violate the IEC's body graph.


I'm going to go out and say that with circuits protected over 200-400 amps could easily get away with a 10 second disconnection time and this should be researched/considered further.