24Hour said:

When 2 vechiles are simultaneously accessible, ie in a street charging next to each other , will the open pen device ( assuming each charger has one ) operate if for example they are fed via 2 differing supply transformer earths ? .

It will if there's an open-PEN fault, but open-PEN is not the big issue here. In fact, it's an issue for any installation where ADS is used ... including TN-S and TT installations that are not affected by PME, as well as TN-C-S.

A more serious issue is that this arrangement is not conformant to BS 7671. BS 7671 is not guaranteed to protect against shock from simultaneously-accessible exposed-conductive-parts connected to different earthing systems, and is a fundamental requirement for Automatic Disconnection of Supply - see Regulation 411.3.1.1 (highlighted 2nd para):

411.3.1.1 Protective earthing
Exposed-conductive-parts shall be connected to a protective conductor under the specific conditions for each type of system earthing as specified in Regulations 411.4 to 411.6.

Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively.

Conductors for protective earthing shall comply with Chapter 54.

A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductive-parts and suspended from such a point.

The need for a Simultaneous Contact Assessment is discussed in Section 3.5 of the IET Code of Practice for EV Charging Equipment Installation.

24Hour said:

When 2 vechiles are simultaneously accessible, ie in a street charging next to each other , will the open pen device ( assuming each charger has one ) operate if for example they are fed via 2 differing supply transformer earths ? .

It will if there's an open-PEN fault, but open-PEN is not the big issue here. In fact, it's an issue for any installation where ADS is used ... including TN-S and TT installations that are not affected by PME, as well as TN-C-S.

A more serious issue is that this arrangement is not conformant to BS 7671. BS 7671 is not guaranteed to protect against shock from simultaneously-accessible exposed-conductive-parts connected to different earthing systems, and is a fundamental requirement for Automatic Disconnection of Supply - see Regulation 411.3.1.1 (highlighted 2nd para):

411.3.1.1 Protective earthing
Exposed-conductive-parts shall be connected to a protective conductor under the specific conditions for each type of system earthing as specified in Regulations 411.4 to 411.6.

Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively.

Conductors for protective earthing shall comply with Chapter 54.

A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductive-parts and suspended from such a point.

The need for a Simultaneous Contact Assessment is discussed in Section 3.5 of the IET Code of Practice for EV Charging Equipment Installation.

Are Granny chargers generally not equipped with integrated open PEN protection devices? I thought these chargers are commonly utilized as interim solutions pending the installation of a permanent charger.

AMK said:

Are Granny chargers generally not equipped with integrated open PEN protection devices?

No, there are two flavours of "granny charger:

"Mode 1" which is, sadly, just an extension lead. These won't work with new vehicles, and it's no longer permitted to sell EVs that can use them.

"Mode 2" has an in-cable control box (ICCB) that incorporates some residual current protection, and N-E monitoring via the "pilot" function between the ICCB and vehicle.

The ICCB, being international standards, doesn't include an open-PEN device (OPDD) - so if you want to dedicate a socket-outlet to EV charging using Mode 2, you'd have to provide something in the electrical installation to conform to Reg 722.411.4.1

Sorry for bringing this post up but I had very similar thoughts to the OP after watching the eFIXX video.

If we are not worried about the broken PEN scenario (as we would be using open PEN home chargers), then the remaining risk as I understand it is a car becoming live due to an internal fault inducing voltage via an adjacent car being charged or a street furniture.

But wouldn't the internal fault in the car simply be picked up by the Type B RCD despite the different earthing systems?

 then the remaining risk as I understand it is a car becoming live due to an internal fault inducing voltage via an adjacent car being charged or a street furniture.

When you have two different earthing systems in reach of each other, there's a risk of someone getting a shock when touching both at the same time. The problem is that earthing systems are rarely at exactly true earth potential - in TT systems normal leakage currents can raise the voltage on an earthing system (e.g. 50mA leakage * 500Ω rod yields 25V) and the one next door might similar magnitude but out of phase giving an even greater p.d. between them). TN-C-S systems will have their PE raised by the voltage drop of N currents on the supplier's network. Permitted voltage drop can be 16% of 230V - so around 37V - in worst case conditions half of which could be along the PEN, so raising the consumer's earth by that amount. Even TN-S systems aren't entirely immune as there can be current flowing in the PE conductors even in normal conditions (long term N-PE faults are common on TN systems and if there's no RCD they'll remain undetected). Then on top of that any kind of L-PE fault will drag the voltage on the earthing system up very significantly - and the fault doesn't have to be in the EV or even in the same installation - it could be almost anywhere on the system. L-PE faults within consumer's installations (where BS 7671 applies) could take up 5s to clear (and some older installations perhaps a little longer) but faults on the DNO's networks can take longer (maybe several tens of seconds or even minutes) - and there's no main bonding outdoors to mitigate the effects of faults imported into the installation from elsewhere. Then you can add broken PEN events on top of that. Hence the general concept of not having separate earthing system within reach of each other.

On the other hand, in practice things are very rarely that bad. Individually TT'd charge points will have limited leakage current and won't be import nasty voltages from elsewhere. In 3-phase TN-C-S distribution systems, N currents tend to cancel out, so much of the time the voltage drop long the PEN will be quite small, and broken PEN events are still pretty rare, so much of the time the earthing system for EVSE (and other things in the vicinity) will be pretty close to zero volts.

Then look at the regs again - while an earthing system has to be kept out of reach of another earthing system, there's no requirement to keep it out of reach of anything else that happens to be earthy - e.g. extraneous-conductive-parts outdoors or indeed the general mass of the Earth itself. So a certain level of risk is deemed acceptable it seems.

So, as an observation, as long as the "other" earthing system is pretty close to 0V, the risks even if your earthing system is at an elevated voltage, perhaps isn't any worse that a compliant situation where its accessible simultaneously with the general mass of the earth (or something metallic stuck into it - e.g. a sign post). And the changes of two separate faults affecting two separate earthing systems simultaneously quickly get vanishingly small - e.g. if the change of one fault was as low as 1 in1000, then two simultaneously would be one in a million.

   - Andy.

Thank you very much for this very comprehensive answer, it certainly gave me some further food for thought. It made me think about a few further points which I hope I haven't got wrong so I thought I would share below.

AJJewsbury said:

TT'd charge points will have limited leakage current and won't be import nasty voltages from elsewhere

But with the proliferation of open-PEN devices, we may unfortunately loose this benefit?

AJJewsbury said:

and broken PEN events are still pretty rare

In the past few years, the EVSE industry has primarily focused on mitigating the risk of broken PEN events leading to open-PEN devices having now become common. However, there are other issues linked to the DNO's earth being off true earth potential as highlighted in your post, which although less severe than a broken PEN (i.e. lower potential difference than 230V), remain hazards that could still have serious consequences. Paradoxically, TT'ing chargepoints helps mitigate both the broken PEN events AND the DNO's earthing being off the true earth potential.

AJJewsbury said:

In 3-phase TN-C-S distribution systems, N currents tend to cancel out

Does this beneficial effect occur only where all 3 phases are brought up to the property, or would it also happen to a property being fed by one of the 3-phases of the DNO?

AJJewsbury said:

Then look at the regs again - while an earthing system has to be kept out of reach of another earthing system, there's no requirement to keep it out of reach of anything else that happens to be earthy - e.g. extraneous-conductive-parts outdoors or indeed the general mass of the Earth itself. So a certain level of risk is deemed acceptable it seems.

Many properties now have shared driveways where their owners have to squeeze past between adjacent cars. These properties will likely be 1-phase and on a TN-C-S system (same earthing system) and owing to their phases being shifted, would be subject to earth potential difference values of similar magnitude as the scenarios mentioned above with different earthing systems.

These are valid points - I am not sure if you are aware of the HSL report

for  the IET looking at the dangers of electric car charging -it all relates to this and is  linked  above for anyone else who is not already familiar. The probabilities of various fault events are a little uncertain and seem to be patchy across DNOs with varying types of installation to manage.

Paul Meenan  and  other colleagues have been looking at this for a while, and the more general problem of diverted neutral currents, and their more recent info describes the problems well, and can give  a feel for the sort of diverted currents seen in practice. here

It does seem it can be  be a bit of a blind spot.

Mike.

But with the proliferation of open-PEN devices, we may unfortunately loose this benefit?

Indeed - but on the other hand if everything is connected to the same earthing system you reduce the risk. In many situations there are many items with exposed-conductive-parts within reach - the cars, outside lights, street lighting at the end of the drive, heat pumps outside the house and often fed from different installations. Even underground cables can mean the surface of the ground is closer to DNO earth potential than the general mass of the Earth. Keeping the on the same earthing system if it's not the DNOs has many practical problems. A detached garage many metres from anything else would be ideal TT territory, unfortunately most of suburbia isn't like that.

Does this beneficial effect occur only where all 3 phases are brought up to the property, or would it also happen to a property being fed by one of the 3-phases of the DNO?

It's all down to how well loads are balanced downstream of the break (which could be anywhere on the DNO's network of course). Even when you have 3-phases to a domestic property, it's unlikely the loads actually switched on at any moment will be particularly well balanced, so a broken pen affecting just one property is likely to be problematic whatever.  Breaks nearer the substation where many houses can contribute to the balance are better in that respect.

  - Andy.