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Another "EV Charger mounted on a metallic building with PME supply" question

Former Community Member
Former Community Member
Hi All,


I'm new to the forum, but not new to being a sparky, but through it was time i got online and got involved in the community. I know this previously came up on this thread in the previous incarnation of the forum, but the OP finally resolved to move the EV points further away from the metallic building, so it didn't really get answered.

Firstly, i am an experienced sparky and an OLEV approved EV chargepoint installer with many installs under my belt, but i've been working up from installs in homes and small businesses knowing this question would come up one day - now that time is here.

I have been asked to install a double socket single phase charge point on this commercial unit which is metal clad (this is really the only option for a place to put the pint without massive excavation through the car park which is many sim of tarmac. It's a large industrial unit with a TN-S 3-phase supply, approx 500kVA demand after diversity. The point is coming from a sub board near the location of the charge point (about 15m). The only issue is the fact i need to assume the TN-S could be PME as the DNO will not guarantee otherwise, but the metal clad building is obviously bonded to the PME terminal. The customer does not want any additional structures or enclosures putting up, the loading can not be guaranteed to be balanced as there is various equipment starting a stopping all day on different phases which discounts 722.411.4.1(i), and there is literally no open ground for at least 150m so i've discounted 722.411.4.1(ii), so this only leaves Electrical separation as a protective measure against an open neutral fault as far as i can see. I have 2 questions:


1. Am i correct that this is the only option? Is there anything else i should consider. I have a quote for the transformer and it isn't too expensive, and although it will need a bit of manhandling in to place at 80kg, it's a viable option. But i know a lot of people have concerns over this method in practise.


2. In the CoP for EV Charging Equipment Installations 3rd Ed., Figure 5.3. it shows the CP for the EV charge point originating from the N between the transformer and the RCD for the chargepoint. Doesn't this contradict 722.312.2.1 in BS7671, as its effectively a PEN conductor at that point? Or does 722.413 override this?


Cheers


Lee


P.s. question 3. in addition, surely the risk here isn't any greater than with the EV chargepoint installed that without, and someone standing on the same tarmac and leaning on the buildings steelwork or cladding? I obviously want to meet the regs, but isn't there something missing for this sort of situation which will likely become more common in the next few years.
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    2. In the CoP for EV Charging Equipment Installations 3rd Ed., Figure 5.3. it shows the CP for the EV charge point originating from the N between the transformer and the RCD for the chargepoint. Doesn't this contradict 722.312.2.1 in BS7671, as its effectively a PEN conductor at that point? Or does 722.413 override this?



    I wouldn't have said it was a PEN conductor (partly because it's not reliably earthed, partly it's doing the same job as the N-PE link in a true TN-S system - which by definition can't have any PEN conductors) - I do however agree that the diagram (if it's the same one as in the DPC for AMD1 of BS 7671:2018) looks nothing like a separated system and contradicts many of the fundamental requirements spelled out in section 413 - with the result that it won't passively remain safe under some first fault conditions like a separated system should but instead relies on protective devices - i.e. it's actually ADS and should be treated as such. If you add in that the charge point (if class I construction) and/or vehicle under change may have fortuitous connections to true earth, it doesn't obviously look to have any reliable advantage over a local TN-S system, or even just TT'ing the charge point.


       - Andy,
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    2. In the CoP for EV Charging Equipment Installations 3rd Ed., Figure 5.3. it shows the CP for the EV charge point originating from the N between the transformer and the RCD for the chargepoint. Doesn't this contradict 722.312.2.1 in BS7671, as its effectively a PEN conductor at that point? Or does 722.413 override this?



    I wouldn't have said it was a PEN conductor (partly because it's not reliably earthed, partly it's doing the same job as the N-PE link in a true TN-S system - which by definition can't have any PEN conductors) - I do however agree that the diagram (if it's the same one as in the DPC for AMD1 of BS 7671:2018) looks nothing like a separated system and contradicts many of the fundamental requirements spelled out in section 413 - with the result that it won't passively remain safe under some first fault conditions like a separated system should but instead relies on protective devices - i.e. it's actually ADS and should be treated as such. If you add in that the charge point (if class I construction) and/or vehicle under change may have fortuitous connections to true earth, it doesn't obviously look to have any reliable advantage over a local TN-S system, or even just TT'ing the charge point.


       - Andy,
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