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

Former Community Member
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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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    Anyway, this allows the use of an "Open PEN detector" and refers to an yet to be included section 722.411.4.1 (iv) in the regs, and then describes a voltage operated device for installations with single-phase supplies, that operates when the line to neutral voltage at the charging point is outside a specified range.



    I've got some worries about how reliable these can be when there's 3-phase distribution upstream of your single phase installation. If my O-level maths and back of an envelope phasor diagrams are anything like right, I think it's possible (with the wrong combination of loads on the various phases) to have the severed PEN hovering at more than 100V above Earth, but still have a nominal 230V L-N at the charge point.


    If you have access to all three phases it's a lot easier to reliably work out (without using an electrode for reference) if the the PEN is drifting away from earth potential - as you can generate an artificial N point to compare with (provided all three lines are still connected).


      - Andy.
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    Anyway, this allows the use of an "Open PEN detector" and refers to an yet to be included section 722.411.4.1 (iv) in the regs, and then describes a voltage operated device for installations with single-phase supplies, that operates when the line to neutral voltage at the charging point is outside a specified range.



    I've got some worries about how reliable these can be when there's 3-phase distribution upstream of your single phase installation. If my O-level maths and back of an envelope phasor diagrams are anything like right, I think it's possible (with the wrong combination of loads on the various phases) to have the severed PEN hovering at more than 100V above Earth, but still have a nominal 230V L-N at the charge point.


    If you have access to all three phases it's a lot easier to reliably work out (without using an electrode for reference) if the the PEN is drifting away from earth potential - as you can generate an artificial N point to compare with (provided all three lines are still connected).


      - Andy.
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