You can no longer post new replies to this discussion. If you have a question you can start a new discussion
Table 41.1 Assumed Touch Voltage
Former Community Member
I am a bit confused by this. Why do the disconnection times in Table 41.1 appear to be based on a touch voltage of 100 volts rather than a touch voltage of 125 volts?
For example, 110% of 230= 253 volts. Assuming L and PE are of the same size and material, indirect contact touch voltage is 126.5 volts. Would 0.33 seconds not appear more realistic?
I suspect it's a mix of history and standardization with a bit of finger in the air approximation thrown in. I believe that 0.4s was agreed on for continental 220V supplies (110V touch voltage if equal sized line/c.p.c.) and we in (then) 240V land adopted the same - arguing that our per-installation bonding would likely reduce the touch voltage inside the building to well below 110V levels. There's a similar argument for permitting reduced c.s.a. c.p.c.s. which otherwise can similarly result in higher touch voltages. Portable equipment outdoors should be covered by a 30mA RCD so have a faster disconnection time anyway.
There are so many unknowns - body resistance, resistance of contact with the general mass of the earth, actual supply voltage, droop in supply voltage due to the short circuit that occurs during a L-PE fault on a TN system, exact effect of main bonding, not to mention variation between individuals - that it's far from being an exact science. There's likely some approximation gone into that table above - nice round whole numbers seem rather unlikely for real life.
I suspect it's a mix of history and standardization with a bit of finger in the air approximation thrown in. I believe that 0.4s was agreed on for continental 220V supplies (110V touch voltage if equal sized line/c.p.c.) and we in (then) 240V land adopted the same - arguing that our per-installation bonding would likely reduce the touch voltage inside the building to well below 110V levels. There's a similar argument for permitting reduced c.s.a. c.p.c.s. which otherwise can similarly result in higher touch voltages. Portable equipment outdoors should be covered by a 30mA RCD so have a faster disconnection time anyway.
There are so many unknowns - body resistance, resistance of contact with the general mass of the earth, actual supply voltage, droop in supply voltage due to the short circuit that occurs during a L-PE fault on a TN system, exact effect of main bonding, not to mention variation between individuals - that it's far from being an exact science. There's likely some approximation gone into that table above - nice round whole numbers seem rather unlikely for real life.
