AJJewsbury said:

whereas 69.9999V could persist indefinitely as far as the open-PEN device is concerned

It could ... perhaps even higher if you think about measurement tolerances in the lab etc ... and in a real PEN fault, you might get touch-voltages in excess of C_maxU₀ for up to 5 s which could also be problematic.

But to fully understand what's going on here, we need to move away from the curves from IEC 60479-1 and use the values presented in IEC/TS 60479-5, which presents touch-voltage and duration values for different circumstances.

NOTE: IEC 60479 series is not really intended to be a de-facto standard in its own right ... it's a 'horizontal standard' for consideration by committees developing electrical safety standards. Hence, what you see in those 'horizontal standards' won't always be reflected in a product or installation standard.

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.

Thank you all, this query has generated quite a debate, and one that, apart from the science has many varied answers, for many various scenarios and eventualities, I guess every installation is different and would need careful consideration on each individual design. It is true now that a lot of EV car charger manufacturers have adopted a few extra integrated safety devices, including internal earth leakage able to detect a low as 6mA DC, and in-built PEN protection. I have been asked more often of late, the questions regarding touch voltage values and why the differences, this is what has led me to initiate this discussion. Real PEN fault conditions & values are difficult to gage normally (they can be calculated, but that's not the same) and are not easily understood by many contractors, but as long as the required measures are taken into account, to help reduce magnitudes & duration of touch voltages to a minimum, as I've always conveyed to customers, safety is paramount.

https://pes-spdc.org/sites/default/files/shocking-electricity.pdf  has some fun extracts from the standards.

Of course AC and wet feet will be rather different.

The IEC figures tend to skate over this - once upon a time when it was the  IEE , UK regs  recommended 25V in place of 50V touch voltages for places that were likely to be wet, but this changing of limits based on the environment has not been the case for some years.

You may find it amusing to note than the 1940s paper "Electric Shock as it Pertains to the Electric Fence," is still occasionally referenced, and that shows approximately a 4- 5 fold reduction in body resistance at low voltages when hands or feet are wet.

The paper itself is still under copyright, but this public domain review of it https://incompliancemag.com/article/body-resistance-a-review/

shows some key conclusions and re-creates the more interesting graphs.

Mike.

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.

mapj1 said:

UK regs  recommended 25V in place of 50V touch voltages for places that were likely to be wet, but this changing of limits based on the environment has not been the case for some years.

Well, that's not quite the case either - SELV voltage limits (including those under fault conditions in the source) are still lower in 'wet' areas, such as Sections 701 and 702.

BS 7671 has only really limited the magnitude of 'touch voltage' by supplementary local equipotential bonding - or, as above, in the permitted maximum voltage for SELV and PELV in some locations.

Well, true, but this thread is about car chargers and mitigation of network PEN lift,  and a car may be charged in the rain, and may  well be subjected to a greater rate of water flow than it would if it was installed under the shower in a lot of cheaper hotels, yet an EV charger  is scarcely operating at extra low voltage in any form !
Equally, it is under those very conditions that one, if not wearing the ideal footwear, may well have wet socks electrically connecting to the puddle underfoot, and also perhaps wet hands.

Personally I'm not sure that the '70V is OK for uninterrupted exposure for cars, but 50V for everything else' assumption is especially solid in such corner cases.

Note that a few unlucky folk have been killed by 28V DC in military vehicles, but as far as I know, only when already wounded so badly the skin resistance was reduced or removed from the loop. A  study by Peng and Shikui (in 1995) presented autopsy results of 7 cases of electrocution by AC or DC voltages ranging from 25-85 Volts. Common factors wer the contact site was on or near the chest, the contact time was '“'long” , and skin burns and damage to internal organs were observed. Victims were otherwise healthy 20-41 year old males.

In addition the authors note that the victims were working in high humidity and high temperature environments that

1) increase susceptibility to electric shock through decreased skin resistance;

2) decrease reaction time and ability to disengage from the voltage source;

3) increase the chance of heatstroke/ unconsciousness as a result of exertion/fatigue.

In terms of the 25V figure and UK regs,  I'm going back a long way - postwar I think, but long before regs were numbered in the current way, I'm pretty sure it was aimed at reducing accidents in shipyards etc.  I'll try and find out how far back tonight when I have more time to look at my archives of such ;-)
Mike.

EDIT

Later than I thought - 1966 14th edition,

Once again, thank you all for the very valid input, I`m certainly more enlightened about this subject now, it was not something a really considered too much, but of late we (my colleagues and I) have been dealing with a few EV car charging interested clients, so its great to have the extra knowledge.

Kindest regards

Colin

Later than I thought - 1966 14th edition,

25V lasted a lot longer than that - blue cover 16th Ed (2001) section 605 (Agricultural) modifies the usual R_A I_a ≤ 50V to 25V and likewise for supplementary bonding has R  ≤ 25/I_a

Ditto for constructon sites (604).

- Andy.

AJJewsbury said:

25V lasted a lot longer than that - blue cover 16th Ed (2001) section 605 (Agricultural) modifies the usual R_A I_a ≤ 50V to 25V and likewise for supplementary bonding has R  ≤ 25/I_a

Ditto for constructon sites (604).

And even longer than that. BS 7671:2018+A2:2022 in fact.

See Regulation 710.411.3.2.5, applies to ADS for all methods of protection against electric shock that use ADS, and Regulation 710.415.2.2 (esp NOTE).


It's also used in hazardous areas of filling station installations, which, whilst the hazardous area installation is outside the scope of BS 7671, is covered by the APEA/EI 'Blue Book'.