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EVs, Street furniture, PME and TT configurations

Former Community Member
Former Community Member
Good afternoon all,


I'm part of one of the teams installing the EV charging points around London and we keep running into the same situations and problems when going through the site selection process - proximity of other electrified street furniture to the units we are installing (as well as potentially plugged in cars which is measured to the edge of the parking bay.)

Regs say that any EV installation cannot be connected to a PME system and must be converted to a TT in case of a damaged/faulty PEN conductor. Naturally if you're converting something to a TT system and not using the DNO TN-C-S earthing arrangement, there must be a reasonable distance between the TT and any other TN-C or TN-C-S systems (2m or so is reasonable).

If there were other services in the vicinity but can be proven that these have also been converted to TT and are 100% confirmed to not be using the DNO earth, would it be reasonable to say that the requirement for the 2m distance can be reduced or ignored completely? Another thought I've had is to bond the cabinets together - being on the same type of system, it makes logical sense that this would in turn reduce the Ze and improve disconnection times, both units have their methods of ADS and incorporate an RCD/RCBO of a 61008 or 61009 standard respectively.


Any other thoughts or ideas would be much appreciated as I try and figure a workaround for this issue. I understand this could work for smaller cabinets and for individual supplies, and not necessarily for street lighting which might not be adequately equipped for being converted to TT (bit of a bigger job to start installing RCDs and then giving a minor works cert etc.).
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    Andy, you're quite right, and this is what's described in section 5.3.4 of the IET Code of Practice for EV Charging Equipment Installation (3rd Edition, but it's been in there since the 1st Edition with no-one worrying about it). Provided that ALL of the following are met, it should be OK for Electrical Separation:


    • the protective conductor on the secondary is not connected to any conductor (including protective conductor) of the primary

    • Isolating transformer meets requirements for electrical separation

    • There is an RCD (30 mA for EV supply equipment) immediately after the secondary.

    • Only one vehicle is supplied by each transformer (one transformer per charge point).



    (Side note: for those that are still a little queasy about the situation, unearthed systems with protective conductors are discussed in BS 7430 ... as Andy says it's not really covered in sufficient detail in BS 7671 at the moment ... but I really don't think this particular situation needs to go down that rabbit hole as it's one separated system feeding one vehicle as per Section 722.)



    Just so I'm clear - what's proposed is a separated system but with c.p.c.s (and any exposed-conductive-parts) bonded to one of the live conductors - is that right? So the closest BS 7671 equivalent would be a separated system supplying more than one item of current-using equipment (418.3) but with a first fault already present. (Even though it's only supplying one item, the presence of c.p.c.s and their connection to exposed-conductive-parts makes it unlike 413 arrangements.)


    Has such an arrangement been tried & tested? I've got some worries about it. I'm familiar with SELV-like circuits from class II equipment suffering from capacitive coupling from the main supply (typically several interconnected items of domestic AV equipment) where disconnecting/reconnecting ELV interconnecting leads with the equipment live (contrary to the manufacturer's instructions) can sometimes produce a very obvious crackle and tingle and I'm sure someone here (Alan?) has described similar problems with DNO transformers where the LV system can float at around half the HV voltage if the LV earth is lost. So my worry would be with an isolating transformer supplying EVSE that there would be a risk of capacitive coupling with the earthed primary circuit coupling the secondary circuit to say something like half mains voltage above true earth potential (so each pole of the separated circuit might be at that +/- 115V say) - and so exposed-conductive-parts could be held at a significant voltage above true earth. Of course without galvanic connection to earth or the primary circuit any shock current would be limited by the capacitance between the transformer windings - almost certainly well below levels that would be fatal - but possibly high enough to be felt. But even a perceived shock could still be a significant problem - not just from the point of view of being disconcerting and causing members of the public to question the safety of the installation, but from in a highway environment any involuntary 'jump back' could put someone into the path of an approaching vehicle.


    Maybe I'm worrying unnecessarily about this - do isolation transformers necessarily have an earthed screen between windings to eliminate this effect? Or would things necessarily be wired in such a way (e.g. c.p.c. connected end of the secondary winding matches the N end of the primary winding) so things are cancelled out to an acceptable level?


       - Andy.
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    The floating secondary problem is very real, and even on small transformers used for charging portable kit the effect can be, as you say "tingly".

    Before the  mandated efficiency targets drove everything towards switch mode, (where the frequencies are higher, but the transformer is smaller, and now the leakage is not at 50Hz), the solution used to be the split bobbin mains transformer.
    https://www.belfuse.com/product-detail/signal-transformer-high-isolation-split-bobbin-low-power-transformers

    The problem is that these are fine for a few hundred watts, but the core, and therefore losses, end up quite a bit larger than if you allow the windings to interleave, and a toroid would be smaller still. However (of course) if you do that you bring the 'plates' of the coupling capacitance closer and so the interwinding capacitance gets larger, and is anyway more noticeable on on bigger transformers.

    It is possible to interrupt the capacitance and have an interwinding screen and either earth it, or in some odd special EMC related cases, connect it to another voltage that is not actually earth but does not have such a high impressed alternating voltage as the primary winding. After earthing the most common thing is to neutral it, followed by connecting it to a mains derived DC bus. In such a case the screen must then be insulated from the output windings to a similar degree as if it were another mains winding.

    Transformers like this are not hard to make, but are not the standard item, and this is reflected in the price at least for quantities where it is not worth tooling up a production line, and they are individually made.


    In the car situation, you are not too worried about this, as you would earth one side of the secondary anyway, but the bigger problem is that it is very little help, as while you side step the local PME you are still mixing earthing types if there is other class 1 kit using electricity nearby. It is no better than running an SNE cable back to the substation and saying 'this supply for the charger is TNS' Which is great, if it is in the middle of an otherwise deserted  car park, but no help at all if there are PME supplied items within reach of the car  that will be plugged in. And that may be another issue - as the car plus cable may be a few metres long and could in some cases will be parked on one side or the other, the exclusion area for simultaneously accessible ought if anything to be bigger, to include the area of the vehicle .


    The other question is if the risk is credible. There is a price to safety, and when it comes to vehicles  they are inherently a lot higher risk than electricity. It may well be that all this is disproportionate, and if we wanted to save more lives we'd ban the vehicles all together.

    Normally to see if an activity is worthwhile you assign a cost to a life lost which may be a few million pounds, then divide the cost of the extra precaution times the risk.

    For simple sums if my life cost is 1 million, and there is a new piece of safety kit that costs £10, then it  is only worth installing them everywhere if the risk it mitigates is more than 1 in 100,000. Now the hard part is always getting the probabilities and full costs right and for that it stands or falls by the accuracy of available data. It is hard enough to be sure if RCDs are a sensible idea, given the low numbers of electrical accidents both before and after they became commonplace, let alone with something  we have so few of at present. Once we get a decent number of accidents we will be in a better place to understand the risk quantitatively, rather than just emotionally.



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    Yep, adapt one of these to accommodate a trip delay timer and Bob's yer uncle and Fanny's yer aunt.

    https://www.flameport.com/electric_museum/old_elcb/crabtree_elcb.cs4


    722.411.4.1 (iii).


    Z.


  • Former Community Member
    0 Former Community Member

     



    I essentially am part of the install only as the client provides their own charge points which are all pre-built and shipped over, but I'm sure if we can come to an agreement or a redesign then this could change. We install mainly 7kW and 22kW charge points.

    500kW of EVC's Is one hell of a lot. I know Tesla have unveiled a 250kW charge point which requires a 1MW transformer to power 4 of them but we simply don't have the infrastructure to cope with that at the moment. I was asked to look at an install elsewhere as someone wanted to know how many 50kW rapid chargers would fit in the car park - long story short, they didn't go with it as it required a new HV supply to a brand new substation for a few charge points which would have been too powerful for the nature of the car park. (you wouldn't really have a 50kW in a "Park & Ride" as chances are you're going to be a good few hours!), if you need any help with this then I'll be happy to lend some assistance where I can.


      



     




    The site has a private HV network, there's space in the car park to add a dedicated Tx with a close coupled LV feeder pillar. 

    ​​​​​​

    ​​​​​​I haven't seen the EVC specification,  but have been advised they're likely to be 7kW. 


    Thanks for the link Graham.


    Regards

    Parsley


     

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    411.3.1.1 Protective earthing

    Exposed-conductive-parts shall be connected to a protective conductor under the specific conditions for each type of system earthing as specified in Regulations 411.4 to 411.6.
    Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively.



    Curiously, the same demand doesn't seem to be made of extraneous-conductive-parts (even though they are likely connected to the same MET as an exposed-conductive-part).


    Steel lighting columns, I would suggest, are generally extraneous-conductive-parts rather than exposed-conductive-parts (the innards generally being sheathed cables and enclosed terminals/devices). Even contact with the class I lighting head on the top and/or a deliberate bond to the lamppost's MET doesn't make it an exposed-conductive-part (even though it would obviously attain the same potential).


    So it might be that the only actual exposed-conductive-parts with reach are the charge point (if metallic) and the car itself (when present) - which would make satisfying 411.3.1.1 easy.


    What if we had a TT system for the EVSE and if we felt the need bonded the TT MET to any extraneous-conductive-parts in the vicinity (e.g. lighting columns). Very much like a TT'd house sharing a metallic water main with the PME'd house next door. Clearly nonsense from the point of view of avoiding nasty touch voltages under broken CNE conditions, but doesn't actually seem to be against either the general rules of BS 7671 or section 722 in particular (only the 'means of earthing' is prohibited from using PME - not parallel paths via extraneous-conductive-part).


        - Andy.
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    AJJewsbury:




    411.3.1.1 Protective earthing

    Exposed-conductive-parts shall be connected to a protective conductor under the specific conditions for each type of system earthing as specified in Regulations 411.4 to 411.6.
    Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively.



    Curiously, the same demand doesn't seem to be made of extraneous-conductive-parts (even though they are likely connected to the same MET as an exposed-conductive-part).


    Steel lighting columns, I would suggest, are generally extraneous-conductive-parts rather than exposed-conductive-parts (the innards generally being sheathed cables and enclosed terminals/devices). Even contact with the class I lighting head on the top and/or a deliberate bond to the lamppost's MET doesn't make it an exposed-conductive-parts (even though it would obviously attain the same potential).


    So it might be that the only actual exposed-conductive-parts with reach are the charge point (if metallic) and the car itself (when present) - which would make satisfying 411.3.1.1 easy.


    What if we had a TT system for the EVSE and if we felt the need bonded the TT MET to any extraneous-conductive-parts in the vicinity (e.g. lighting columns). Very much like a TT'd house sharing a metallic water main with the PME'd house next door. Clearly nonsense from the point of view of avoiding nasty touch voltages under broken CNE conditions, but doesn't actually seem to be against either the general rules of BS 7671 or section 722 in particular (only the 'means of earthing' is prohibited from using PME - not parallel paths via extraneous-conductive-part).


        - Andy.

     




    I'm happy that this situation would come under 542.1.3.3. If the extraneous-conductive-part is bonded to one installation already, and you then bond it again to another installation, the second protective conductor [main protective bonding conductor] surely becomes common to both installations.


    The DNO would also not be happy, as the arrangement would fail to meet their separation requirements for separation of TT earthing systems.


    I do agree that situations like this are left more to experience than being fully explicit in BS 7671 ... but to be fair it is covered in detail in Guidance (e.g. IET Code of Practice for EV Charging Equipment Installation).

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    If the extraneous-conductive-part is bonded to one installation already, and you then bond it again to another installation, the second protective conductor [main protective bonding conductor] surely becomes common to both installations.




    But the idea that they are not bonded is something of a mistake, reliable isolation cannot be acheived, adjacent buildings share gas and water pipes accross multiple methods of earthing (TT and PME is commonly mixed on farms, where the house is PME, and the farm proper is very much TT. Both have water. Mixed TNC-s and TNs on the same substation is common in built up areas with mixed ages of property), not to mention water and gas pipes also interlinking the CPCs of multiple substations.


    perhaps we need to permit fuses and breakers in the CPC after all ?


  • 0
    I can envisage in the future E.V.s being covered by a big heavy duty plastic or rubber insulating cover when on charge so that exposed conductive parts can not be touched. 

    https://www.classicadditions.com/en-GB/outdoor-car-covers/ultimate-outdoor-car-cover/prod_10077?gclid=EAIaIQobChMI_fbT5MXD4QIVLpPtCh3JJQWmEAYYASABEgJlY_D_BwE


    Z.
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    mapj1:




    If the extraneous-conductive-part is bonded to one installation already, and you then bond it again to another installation, the second protective conductor [main protective bonding conductor] surely becomes common to both installations.




    But the idea that they are not bonded is something of a mistake, reliable isolation cannot be acheived, adjacent buildings share gas and water pipes accross multiple methods of earthing (TT and PME is commonly mixed on farms, where the house is PME, and the farm proper is very much TT. Both have water. Mixed TNC-s and TNs on the same substation is common in built up areas with mixed ages of property), not to mention water and gas pipes also interlinking the CPCs of multiple substations.


    perhaps we need to permit fuses and breakers in the CPC after all ?



     




    So, point accepted with respect of water pipes, but if everything is connected on PME, it will be OK.  TT for a building is a different story to TT for caravans, and EV charge points - they are another thing entirely.


    In terms of "Metal pipes serve TT and TN systems equally" - Well, that's how it WAS.


    If you TT a NEW installation, or charge point, DNOs have physical separation requirements from metal parts connected to their global earthing system, including a requirement to separate the earth electrode below ground ... but this is no problem, as we can put insulating sections on our incoming pipework now.


    This has just as much to do with the changes brought in when we moved from the old BS approach for combining HV and LV earthing - and the definitions of what's acceptable in terms of overvoltage impulses due to EPR from a fault in the HV network. We are now working to BS EN 50522 / BS EN 61936-1 values instead, and the current definition of what's "HOT" and not, and I guess the DNOs are being mindful of that. BS EN 50522 and BS EN 61936-1 replaced the BS's back in 2010.


    It's not plain sailing out there.

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    I have just checked the latest version of the UKPN doc

    EDS 06-0017 CUSTOMER LV INSTALLATION EARTHING DESIGN;and indeed it has changed (as of 10/09/2018). Actually it is now much clearer than it was about such things, and very sensibly now includes labels at the cut-out for 'this is TNCS'  ;'this is TNS' 'this is TT' etc.

    This is good as over the next 50 years or so as kit gets updated the confusion about what earthing is any given site is supposed to be will slowly get cleared up.  I assume but have not checked that the other DNOs have similar updated advice, or soon will.


    However, I suspect that there won't be many 2m long insulating joints available for upgrading gas and water mains to current standards to permit TT and PME to co-exist, and  I think that the idea that the earthing systems can always be separated in this way remains very optimistic.

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