Hi Guys,
Hopefully a simple one to answer.
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, for example where 2 neighbours are both charging their EV's on the drives which are side by side or a drive is next to a metal lamp post already installed on the street?
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.
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?
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.
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.
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.
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.
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