Can 'the same earthing system' described in Regulation 411.3.1.1 be satisfied if both supplies originate from the same DNO PME cable in the street

I guess in most cases they will be - so the chances of harm are pretty low. Justifying that could be a lot more interesting - not only in a few cases will they not be on the same cable originally, there's no guarantee that things will remain the same into the future. (Shades of having to treat TN-S as PME in case the DNO change things later). I've seen situations where a DNO has cut an existing cable and fed some properties from the opposite end (from a different substation) to adjust things when demand increased due to a new development elsewhere (I got to know after we weren't told that planned work would cut our power - seemingly their records didn't have the location of the cut & re-route in quite the right place, so they thought we were on the other side and shouldn't have been affected).

I guess that open-PEN devices should mitigate the risks somewhat - as if either PE drifts too far away from true Earth at least one of the car should be disconnected from PE & left to float (notwithstanding that the devices won't detect all hazardous conditions and disconnection times in the region of 5s aren't entirely aligned with the usual ADS requirements, so won't help where one installation experiences a L-PE fault - but arguably we have that problem with unbonded extraneous-conductive-parts outdoors anyway).

Does 722.413.1.2 solve the problem?

Maybe. If you could guarantee that the vehicle side was genuinely earth free. I don't see much of an appetite for 7kVA/22kVA transformers though and there are still a lot of unanswered questions about the A722 approach altogether (e.g. actually meeting section 413 requirements). Has anyone ever seen that implemented?

   - Andy.

Hi Andy,

The 'new developments' is where we are going to have the issues. A number of these do not have off street parking but rather have dedicated parking space's elsewhere on the development which could be a distance away from their house/apartment and are mostly directly side by side with others parking spaces. 

We have been asked to look at one at the moment which requires 70 x 7kW chargers through out the development with none of them attached directly to the houses. We will therefor require a number of metered supplies located in feeder pillars through out and supply a number of chargers from each. 

I assume therefor to comply with 411.3.1.1, and where we are using PEN fault protection on a PME DNO supply, we would require any metal lamp post within touching distance to be from the same feeder pillar and DNO cut-out as the car on charge? As the lamp post are going to be located next to the parking bays, adopting a TT earthing system for the chargers is not really an option.

Regards

Mark

As the lamp post are going to be located next to the parking bays, adopting a TT earthing system for the chargers is not really an option.

Probably worth talking to the particular DNO in question - it's been reported that some aren't entirely convinced by open-PEN devices and will often refuse to supply a PME earthing terminal for on-street type charging installations. Their preferred solution would then to TT the charge points - together with anything else that's within reach (e.g. lamp posts). (I guess you could have a common TT earthing system even if the metering was split).

   - Andy.

Mark Fleming said:

The 'new developments' is where we are going to have the issues.

There should be no issues with open pen on a new development. If there are, sue the pants of the DNO! 
If they can’t guarantee the integrity of their network on a new development then they should dump the PME. 
However, no issues on new estates in Ireland which are always TNCS. It seems it’s only in the U.K. where folk get their knickers in a twist at the mere mention of EV and PME!

Hi Andy,

Sorry back to this one,

So where we might have an on street installation and the DNO have confirmed we could have a PME supply to the feeder pillar, along side an O-Pen device complying with 722.411.4.1 (iv) and the supply is from the same DNO cable in the street as the lamp post, do we still need to treat them as a sperate earthing systems and keep them apart as with a TT and PME?

Mark Fleming said:

So where we might have an on street installation and the DNO have confirmed we could have a PME supply to the feeder pillar, along side an O-Pen device complying with 722.411.4.1 (iv) and the supply is from the same DNO cable in the street as the lamp post, do we still need to treat them as a sperate earthing systems and keep them apart as with a TT and PME?

No that example is NOT a separate earthing system.

Mark Fleming said:

and keep them apart as with a TT and PME

The requirements of Regulation 411.3.1.1 regards simultaneously-accessible exposed-conductive-parts applies whatever the earthing arrangements wherever ADS is used. Therefore, would also apply if you had an existing building supply, powering lights in the car park, and you bring in a new supply for the EV charging points - the charging points and lights would have to be on the same earthing system.

How you might achieve that depends on a lot of different factors.

So where we might have an on street installation and the DNO have confirmed we could have a PME supply to the feeder pillar, along side an O-Pen device complying with 722.411.4.1 (iv) and the supply is from the same DNO cable in the street as the lamp post, do we still need to treat them as a sperate earthing systems and keep them apart as with a TT and PME?

Where everything is from the same DNO cable and using the DNO's earthing facility, then physically it's one common earthing system - I think that's clear enough.

The problem from a BS 7671 designer's perspective is that a) you typically don't know that for sure, and b) even if you do, no-one is going to guarantee that it'll remain so for the lifetime of your installation. (DNO's often re-configure things in the street and will certainly want to reserve the right to do so in the future). In effect BS 7671 is demanding reliance on things that are outside of the control of the designer (or indeed owner) of the installation. I think that's something BS 7671 needs to address.

It's not entirely unreasonable that BS 7671 should de-scope such requirements - there are already many holes in the equipotential zone theory already - it considers exposed-conductive-parts, but not extraneous-conductive-parts (at least not ones outside of a building), and not the general mass of the earth (very accessible outdoors) at all.  In these days of ADS/O-pen it's reasonably unlikely that exposed-conductive-parts of other systems will be at a hazardous potential for very long - so the hazard isn't that dissimilar from what already exists and is accepted.

   - Andy.

AJJewsbury said:

Where everything is from the same DNO cable and using the DNO's earthing facility, then physically it's one common earthing system - I think that's clear enough.

The problem from a BS 7671 designer's perspective is that a) you typically don't know that for sure

Forgive me for being simplistic, but can that not be ascertained by measuring the resistance between the two METS? If both supplies come from the same transformer, the the most it could be would be about the same as the Ze. (This assumes that R1 ≈ R2 and that the cables branch close to the transformer.)

By contrast, if the supplies came from different transformers, the resistance would be tens of Ohms or more because the circuit would be via the various (multiple) earths and the general mass of the earth.

Perhaps. In 'busy' places like large towns and cities, the DNOs mesh things so there are paths that connect the earths of different transformers, at HV and also at LV, but really they are supplying independent feeders, and at voltages that differ by the neutral voltage drop. Also links can be manipulated meaning that transformer secondaries may be in parallel, either permanently or just at times of peak load to give  volt drops and PSSC being lower and higher respectively than otherwise might be true.

The text book sketch of one transformer and separated HV and LV earths is not always the case in practice.

M.

By contrast, if the supplies came from different transformers, the resistance would be tens of Ohms or more because the circuit would be via the various (multiple) earths and the general mass of the earth.

Until consumers on two different supplies have bonds to a common water or gas main?

   - Andy.