The open PEN fault risk is the same with a granny lead as it is with a non-open PEN fault protected charge point. All outlets intended for charging EVs (not just charge points) are supposed to comply with BS7671:2018 Section 722, but the reality is that many do not, and anyway, there is an argument that an outlet installed to run a Class 2 appliance (like a lawnmower) doesn't need open PEN fault protection (as there are no exposed conductive parts), so the outlet could be said to be compliant with the regs if it wasn't put in specifically for car charging.

The regs do not apply to the granny lead itself, as it's not installed electrical equipment, it's classed as an appliance. That doesn't change anything in terms of the possible risk, but does mean that there is no obligation on granny lead manufacturers to put in any protection measures that may be required by the wiring regs.

In practice, the view regarding appliances that connect to any outdoor exposed conductive part is that the outlet supplying them should have open PEN fault protection. This goes back way before EVs, and includes things like metal framed greenhouses, metal buildings, caravans and hot tubs, all of which really need open PEN fault protection and have done for many years (not that it's ever been formalised in the wiring regs, though, it was always just a "good practice" thing, or sometimes in the MIs for things like hot tubs and caravan hook up boxes).

In terms of risk, no one has yet really done much about the risk that granny leads present, because the assumption is that they are really a "get out of jail" device, that won't be used very often. The theory is reasonable, we get around 300 to 500 open PEN faults a year, and each only usually lasts a very short time. The probability of someone using a granny lead at the same time as their supply has an open PEN fault, and at the same time as someone is standing on the ground and touching the car, is assumed to be very low, much lower than for a fixed charge point that may be in use for several hours every day. Of course, if someone chooses to use a granny lead as their only, or primary, means of charging then the risk is actually greater than that from a fixed charge point, just because the charge rate is lower so the time plugged in is much longer.

 

The probability of someone using a granny lead at the same time as their supply has an open PEN fault, and at the same time as someone is standing on the ground and touching the car,

..and that the RCD wouldn't provide enough protection against serious injury...

 

Thank you Jeremy - that’s incredibly helpful.

 

Got it thanks.

One more risk related question then: how does the risk of going without open PEN fault protection compare with the risk of going without DC leakage protection in the RCD?

 

That's very dependent on a number of factors, very hard to quantify as a DC leak will only happen due to a fault on the car, and in some cases the car will detect it and disconnect the pack contactors. In most cases the HV system is isolated from the rest of the car's electrical system, and it monitors insulation resistance.
One issue is that a DC fault can de-sensitise an upstream RCD to other faults, but this is primarily a problem if that RCD is shared with other loads, which it usually won't be in most installations.

 

This is very interesting, how many people are using granny chargers with the view they just work but without understanding the risk they run?

 

Exactly - this was what got me worried.

 

What I wonder about is how many public charging stations haven't got proper PEN fault detection because they were installed before it was required and aren't properly maintained...

 

Almost all have it from what I've seen, in fact I've not seen one that hasn't had it. This isn't anything new, as it's been normal practice to consider the impact of a PEN fault ever since we first adopted TN-C-S/PME. Before it was added to Section 722 in the 18th Edition, it was just assumed that installers would consider the risk, as they already do for things like hot tubs, caravan hook up points, etc, and install them as TT, which is how every public charge point I've seen is installed. All the 18th Edition did was make it explicit in the regs for any power outlet installed to charge a vehicle. I'm near-certain that was done largely because quite a lot of installers of charge points were ignoring their obligation to consider the open PEN fault risk. The same issue applies to DC tolerant earth leakage protection, as there's been a need for this since the IET guidance was published around 2013 (which was adopted as a condition for grant funded charge points). That was also being ignored, so the IET added it to Section 722 in the regs to make it abundantly clear that it needed to be done.