It is a bit of an odd one - we do not allow higher touch voltages on other outdoor things unlikely to be touched with two hands (if a car, why not a caravan or a bus-stop) and it is arguable the great (wet) British Outdoors would require a lower touch voltage than indoors, rather than a higher one. I suspect however  there is a desire to co-ordinate with other standards related to substations, and also to set the limit as high as reasonably possible without appearing too reckless. 

In practice factors we do not usually like to rely on come into play, like the ground surface (gravel is safer than a lawn, as it drains better, and is sharp, so bare feet are less likely paved surfaces are not such a good connection to terra firma as a bare earth surface), the fact that cars are painted, and door handles are probably not shiny bare metal, though they may be - all these unreliable factors do help to reduce the risk, but not always and are not guaranteed.

Note also that the effective body resistance is a voltage dependent thing - or at least the breakdown of the surface skin is - once under the skin we are all pretty wet and salty, and  the current that flows through a person in a particular configuration at 110V is not half the current in the same victim at 220V, but in fact rather less. Also voltages that force enough current  and are present for a long enough to heat the entry and exit areas are more dangerous, as that heating lowers the resistance, leading to a positive feedback, and in extremis creating a burnt area.

M.

It is a bit of an odd one - we do not allow higher touch voltages on other outdoor things unlikely to be touched with two hands (if a car, why not a caravan or a bus-stop) and it is arguable the great (wet) British Outdoors would require a lower touch voltage than indoors, rather than a higher one. I suspect however  there is a desire to co-ordinate with other standards related to substations, and also to set the limit as high as reasonably possible without appearing too reckless. 

In practice factors we do not usually like to rely on come into play, like the ground surface (gravel is safer than a lawn, as it drains better, and is sharp, so bare feet are less likely paved surfaces are not such a good connection to terra firma as a bare earth surface), the fact that cars are painted, and door handles are probably not shiny bare metal, though they may be - all these unreliable factors do help to reduce the risk, but not always and are not guaranteed.

Note also that the effective body resistance is a voltage dependent thing - or at least the breakdown of the surface skin is - once under the skin we are all pretty wet and salty, and  the current that flows through a person in a particular configuration at 110V is not half the current in the same victim at 220V, but in fact rather less. Also voltages that force enough current  and are present for a long enough to heat the entry and exit areas are more dangerous, as that heating lowers the resistance, leading to a positive feedback, and in extremis creating a burnt area.

M.

Great, thanks for all your help, it`s starting to make a lot more sense now, I agree, the science behind this can be unreliable in reality faced with all the many varied situations.

mapj1 said:

we do not allow higher touch voltages on other outdoor things

Well, that's not quite the case. With ADS and Class I equipment outdoors, the voltage could reach U₀ for the duration of the fault. The general 'assumption' is that for TN systems, the voltage is not likely to exceed U₀/2 (although there is no fixed statement about this), and for TT systems (shorter disconnection times) it may well approach U₀

Sorry, yes, I should really have said that we do not allow higher touch voltages to persist for long enough to have a significant

chance of fibrillation.

In a house with a TN supply, a distribution circuit to a detached garage has a disconnection time of 5 s. I feed a Class I item of equipment (or at least, one with earthed metal) such as an LED flood from the CU, which contains RCBO's, and mount it on the garage, within arm's reach of the ground, above the lawn.

It's entirely possible I could have an arrangement, with a TN system, fully compliant with BS 7671 (if I use SWA, for example, and it's a very long run fed from a fuse) where a touch-voltage of > 50 V, perhaps up to 100 V, would appear at the LED flood with respect to the lawn, for well over 1 s, right up to the max 5 s, if there is a fault at, or in the SWA cable near, the garage CU, on the supply side of the RCBO's. The cpc voltage at the point of fault is transferred to the LED flood for the duration of the fault.

In most installations these days, of course it's not likely to be a problem, because the garage will be supplied from the house via Type B or Type C mcb, so for Zs to achieve 5 s, you actually get a < 0.1 s disconnection time ... but it is possible to have the described arrangement, and fully compliant with BS 7671.

All I'm illustrating is that BS 7671 does not limit touch-voltage in all cases to < 50 V, or the time to < 0.4 s, so choosing 70 V for 722.411.4.1 is not all that 'alien' a concept.