EVC Points & Earthing

Eddie


Sorry to be a party pooper but your plan needs a major re-think.


Firstly if this building has it's own transformer it will highly likely be a TN-S supply. The other alternative it might be TN-C-S (PNB) but not likely to be the other flavour of TN-C-S (PME) unless the transformer supplies other buildings? The special provisions only apply to TN-C-S (PME) and as your supply probably lacks the "M" it is not going to be PME. A more detailed survey is required to determine the actual means of earthing.


TTing the charging points with your earth electrodes with circa 2m of the legs of the metal canopy legs is likely to overlap the earth resistance fields of the 2 effectively connected your 2 earthing systems together. You cannot have simultaneous access between 2 earthing systems, see the Part 2 definition and Regulation 411.3.1.1.


I would say that a 11kW 3 phase load is a bit tight for a 16A MCB. This Type A RCBO you speak of is it 4 pole? If I was you, and I have carried out a design recently for 2 No pillar unit each with 2 No. 11kW 3 phase units, I would use units with a 3 phase Type A inside with DC sensing so all you need to do is run out the required 3 phase supplies from an MCB for each socket. I use a company starting with "R" and ending in "C", other makes are available.


Also 20 11kW units is potentially a very large load on the installation. Will the installation take the additional load and have you sort permission from the DNO before you think about starting the installation work? You may need to fit load control to you charging points given the no diversity rule.


You cannot start the installation without verify the earthing system, any bonding required and ensuring the installation will take the additional load, see Regulation 132.16.  Also the installation needs to be designed which is a process that most contractors seem un-aware of but are happy to sign the declaration on the EIC to the effect the installation has been designed to BS 7671. 


You might want to wait until this Monday when the IET will make available the  18th Edition AMD 1, free to view on line £5 to download and have a good read before starting your design.

Each  are 11kw 3 phase so 16A TP apiece via 16A Type A RCBOs. Each EVC will be independanly spiked using either a traditional earth spike or a Condudisc to make them TT. All SWAs will terminate at an adaptable box in LV Switchroom mounted below D/Board thus keeping all EVC earths seperate to that of the TNCS. The plan is to install a metal construction solar panel canopy over the 20 parking bays. Presumably this will need earthing but to which system? The legs of the canopy are metal and may fall within 2m of a number of EVCs. I'm guessing bond them to the local TT Charging point?

Good point from JP - is the supply isn't PME then you can keep the TN earthing system and everything is much simpler.


If you do have to create a TT island, I can't help thinking it would be a lot simpler just to have one TT earthing arrangement for all the EVSEs (and PV as well) - rather than one each and running into problems where they're within reach of each other.


I might even be tempted to keep the SWA on the TN system - gapping it at the charge points - then as JP suggests you (probably) don't need an extra RCD on the supply.

 

You might want to wait until this Monday when the IET will make available the  18th Edition AMD 1, free to view on line £5 to download and have a good read before starting your design.

Very good point - I know there were some fairly radical suggestions made to overcome some of the 'within reach' problems during the DPC process - it'll be very interesting to see if any made they way into the final version.


   - Andy.

Andy


Even if you TT the whole installation or just TT the EV charging socket the problem is getting the TT electrode away far enough from any under ground metallic pipes or metal work that is connected to the PME earthing system or connected to your neighbors PME earthing system.


I feel sure that the AMD 1 722 published on Monday may provide some useful guidance but we will have to wait until then. 2 big sleeps before we find out!


Of course if you came to the Elex show this week you will be all clued up on 722. If not come along to Manchester Elex to see the the 2 No excellent presentations from the IET.

Even if you TT the whole installation or just TT the EV charging socket the problem is getting the TT electrode away far enough from any under ground metallic pipes or metal work that is connected to the PME earthing system or connected to your neighbors PME earthing system.

Hence one of the comments directed at the DPC - what we should be seeking is equipotentiallity with the ground under/around the EV, rather than equipotentiallity with some theoretical zero volts (the potential of the centre of the planet?). Hence if the ground in the vicinity of the EVSE/EV is subject to potentials from a PME system it makes sense for the TT electrode to match the very same potentials if we're to minimise shocks between the EV and the surface of the ground (or random metalwork - posts for signs etc - stuck in the same ground). Make the electrdode(s) local to the EVSE (a buried grid under the parking space if you're really worried about voltage gradients across such a small area) and it all just comes out in the wash.

   - Andy.

"a buried grid under the parking space"

Never mind, by all accounts the 18th guide book on Monday will reveal all. Meanwhile, in the real world around 4000 units are being sold each month, connections being dealt with by the DNOs, as they have been for years. This is the problem with the guide book, its always years behind. Typical example is Marinas, swimming pools and bathrooms. You would think they hadn't existed before the 16th ed. And to flog the book the called them "special locations" 


Regards, UKPN???

connections being dealt with by the DNOs

Yeah, by the penny pinching DNO's unilaterally changing over to a PME "earthing" system that's not safe to use for EVSE (or much else outdoors), so leaving a complete mess for the rest of us to sort out!


   - Andy.

Some great feedback there, thanks guys.

After requesting further clarity from DNO they said they cannot confirm what earthing system the building is because the transformer was installed by an independant DNO. I contacted the Company who last worked on the Transformer with the independant DNO who said its a TNCS. The Test report for the building however suggests it's a TNS. The building has an independant transformer which feeds no other buildings except this one so its a little wooly me making assumptions as to whether TNS or TNCS. Assuming it is TNS and not TNCS this will make it much simpler and I would be tempted to feed the EVCs from a dedicated D/Board located in the Main LV Switchroom adjacent the TX room. I would feed all the EVCs via individual TP MCBs and ensure the Chargers contain the 16A Type A RCBO within the individual Charging Units. Forget the TT Island and this way the building and the 20 EVCs and the Solar Canopy are all at the same earth potential leaving no room for error. I hadnt realise AMD 1 is released this week so will take a good read when available.

We also applied for a 300KVA uplift on the TX to cover the additional demand on the building created by the 20 x 11kw EVCs