Table 41.1 Assumed Touch Voltage

ProMbrooke:

AJJewsbury:

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.


   - Andy.

I'd say so by the looks of it. 

Agreed, too (that it's give or take a tens of percent of more, and much about convention).


A thread from about a year ago ( 5 s disconnection times ) has a plot showing the time/voltage curves that IEC TR 1200-413 1996 considers acceptable for normal conditions or for 'particular' (non-dry) conditions. IEC TR 1200-413 is a report that gives explanations of the thinking behind the shock-protection part of the IEC wiring regulations at that time. The voltage for 0.4 s disconnection is claimed to be based on 80% of 50% of the source voltage, with some rounding. The 80% factor comes from assumptions about bonding and the relative influence of internal and external cabling; as you mention, the protective conductors are assumed to be as big as the live conductors, which may be true for most of the countries involved in IEC committees but is not common in the UK.

So - as discussed in that thread - the IEC reasoning behind 0.4 s can easily be severely wrong in UK conditions. Yet people still work rigidly to 0.4 s. Fortunately the times will often be much quicker anyway, particularly when there are RCDs. And there's plenty of conservatism behind the chosen voltage/time curves, at least as long as dry conditions are true. I haven't heard of any death resulting from a standard-compliant 0.4 s disconnection of e.g. a 4 mm2 FTE cable (with its much thinner cpc). During much of the 1900s the wiring regulations appear to have been concerned more with getting a reliable disconnection (e.g. 5 s), rather than with aiming at a good chance of survival even for someone who's unlucky enough to be in contact with an object when it has a fault; so the shorter times based on best-guesses about bodies were an increase in safety anyway.

ProMbrooke:

AJJewsbury:

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.


   - Andy.

I'd say so by the looks of it. 

Agreed, too (that it's give or take a tens of percent of more, and much about convention).


A thread from about a year ago ( 5 s disconnection times ) has a plot showing the time/voltage curves that IEC TR 1200-413 1996 considers acceptable for normal conditions or for 'particular' (non-dry) conditions. IEC TR 1200-413 is a report that gives explanations of the thinking behind the shock-protection part of the IEC wiring regulations at that time. The voltage for 0.4 s disconnection is claimed to be based on 80% of 50% of the source voltage, with some rounding. The 80% factor comes from assumptions about bonding and the relative influence of internal and external cabling; as you mention, the protective conductors are assumed to be as big as the live conductors, which may be true for most of the countries involved in IEC committees but is not common in the UK.

So - as discussed in that thread - the IEC reasoning behind 0.4 s can easily be severely wrong in UK conditions. Yet people still work rigidly to 0.4 s. Fortunately the times will often be much quicker anyway, particularly when there are RCDs. And there's plenty of conservatism behind the chosen voltage/time curves, at least as long as dry conditions are true. I haven't heard of any death resulting from a standard-compliant 0.4 s disconnection of e.g. a 4 mm2 FTE cable (with its much thinner cpc). During much of the 1900s the wiring regulations appear to have been concerned more with getting a reliable disconnection (e.g. 5 s), rather than with aiming at a good chance of survival even for someone who's unlucky enough to be in contact with an object when it has a fault; so the shorter times based on best-guesses about bodies were an increase in safety anyway.