Why 70V rms and not 50V rms - O-PEN faults

Question

Hi, always follow this forum as such a great source, so my first time with a question, hoping someone can advise. 
 Why `voltage between the cpc and earth exceeding 70V rms` for Reg. 722411.4.1 (iii) and not 50V as in Reg. 411 touch voltage threshold. Is 70V deemed acceptable ?

mapj1 · Accepted Answer

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.

lyledunn · Answer

AJJewsbury said: But one difference is that while an earth fault on a TN distribution circuit might be higher than 70V it's not going to persist for more than 5s - whereas 69.9999V could persist indefinitely as far as the open-PEN device is concerned. 
 Which is why an additional control measure would have merit. The Zappi by MyEnergi, for example, has an on board device that disconnects the supply to the EV, including the cpc when a current of circa 12mA is detected in the cpc. Obviously, someone has to touch the vehicle before the device operates but that is no different to additional protection by RCD to mitigate against direct contact.

AJJewsbury · Answer

including internal earth leakage able to detect a low as 6mA DC 
 
 That function isn't directly to prevent electric shocks - but to prevent "ordinary" kinds of RCD being locked out by a d.c. residual current (a feature that used to be considered useful and was made use of by certain "D-loc" loop testers, to prevent RCDs tripping during high current loop tests but these days is rather a drawback - especially when you have a tub full of batteries, already with deliberate connections to the c.p.c.). A-type RCDs are tolerant of d.c. residual currents up to 6mA - hence if you have one of these 6mA RDC-DDs in the charge point, you can use an ordinary A-type RCD upstream ... if not you'll be looking at a B-type which can directly monitor d.c. residual currents - but sit down before asking for the price of one of those! A 30mA B-type RCD will allow a d.c. residual current of up to 60mA before tripping. 
 - Andy.

Why 70V rms and not 50V rms - O-PEN faults

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