Can someone let me know the science behind this please. I’ve been told that if there is a fault on a circuit the ZS values in the regs are there to give ADS in a time that is not going to cause considerable harm. My question is if disconnecting times don’t comply what is the dangers that arise I know it’s physics but have seen marshalling bars carrying current through earthing conductors all day and we don’t get a shock of them. Thank you for you help in advance guys just want to clear up some unanswered questions.
"My question is if disconnecting times don’t comply what is the dangers that arise"
MrJack96
I think this thread could be helpful. It shows curves of touch-voltage versus time, that are supposed to be pretty safe (my sloppy choice of words) in 'normal' dry conditions or 'particular' conditions with more wetness of the skin. It's my plot (so not a copyright problem), but the curves are defined in an IEC document (1200-413) that describes some of the background of the requirements in the IEC standards that form the wiring regulations. In turn, the input data to these curves are from IEC 60479-1: the current-versus-time plots (often shown in brochures about RCDs etc) that show effects such as 5% or 95% probability of the heart stopping working effectively, and the body impedance plots for various contact types.
So, you could say that the dangers from going beyond the specificed times is that the risk of death from body currents changes from very unlikely to increasingly likely ... not a sudden step, of course, and it's based on quite a few assumptions.
The disconnection times are a simplification: it's relatively easy to say "it's a 230 V to earth system", "my loop tester tells me 0.2 ohm", (perhaps do a step or two more here with voltage factors etc), "the resulting fault current makes this fuse operate in less than the required 0.4 s for a 230 V Uo)", "therefore: it's OK". It would be more demanding to require that the touch voltages and durations don't go above the appropriate green curve" (in the above link): then you'd have to think more about the touch voltage as well as the time .. what are the sizes of PE conductor and L conductor, where is the bonding.
The 0.4 time is based on an assumption that the touch voltage is ~40% of the system voltage to earth. It could be a good deal less (e.g. within a bonded area in a TT installation with low fault current). Or it could be a good deal higher, such as in a TN* system where most of the loop impedance is the internal wiring and it's done in classic UK flat-twin-and-earth cable with a PE considerably smaller than the live conductors.
"My question is if disconnecting times don’t comply what is the dangers that arise"
MrJack96
I think this thread could be helpful. It shows curves of touch-voltage versus time, that are supposed to be pretty safe (my sloppy choice of words) in 'normal' dry conditions or 'particular' conditions with more wetness of the skin. It's my plot (so not a copyright problem), but the curves are defined in an IEC document (1200-413) that describes some of the background of the requirements in the IEC standards that form the wiring regulations. In turn, the input data to these curves are from IEC 60479-1: the current-versus-time plots (often shown in brochures about RCDs etc) that show effects such as 5% or 95% probability of the heart stopping working effectively, and the body impedance plots for various contact types.
So, you could say that the dangers from going beyond the specificed times is that the risk of death from body currents changes from very unlikely to increasingly likely ... not a sudden step, of course, and it's based on quite a few assumptions.
The disconnection times are a simplification: it's relatively easy to say "it's a 230 V to earth system", "my loop tester tells me 0.2 ohm", (perhaps do a step or two more here with voltage factors etc), "the resulting fault current makes this fuse operate in less than the required 0.4 s for a 230 V Uo)", "therefore: it's OK". It would be more demanding to require that the touch voltages and durations don't go above the appropriate green curve" (in the above link): then you'd have to think more about the touch voltage as well as the time .. what are the sizes of PE conductor and L conductor, where is the bonding.
The 0.4 time is based on an assumption that the touch voltage is ~40% of the system voltage to earth. It could be a good deal less (e.g. within a bonded area in a TT installation with low fault current). Or it could be a good deal higher, such as in a TN* system where most of the loop impedance is the internal wiring and it's done in classic UK flat-twin-and-earth cable with a PE considerably smaller than the live conductors.
