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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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    Other boxes such as cellphone equipment, bus stops, electrically illuminated adverts, and lamp-posts probably are on PME.

    The only easy way to tell after the fact will be to perform a Zs type test from L-E, if it is too low, assume CNE /PME


    There is no problem bonding multiple items on TT supplies, so the earth electrodes are in parallel (whether real electrodes or just connection via foundation steels or whatever) The problem is when they are not.
  • 0
    Outdoor bonding/equipotential zone always has been a bit of an oddity. Take regulation 714.411.3.1.2 in (the rather mis-named section on Outdoor Lighting Installation - it actually applies to lots of outdoor things, including highways power supplies and street furniture, not just lighting). That says a metallic structure such as a fence (or non-electrified sign post) need not be bonded even though it might be within easy reach of metalwork connected to the PE of the outdoor circuit (which will very likely be PME). Given that the metallic fences etc, rooted in the ground, will likely be at true earth potential - just like a a typical TT earthing system. So if a PME'd lamppost is allowed to be within reach of a effectively TT'd fence (or cycle stand or bench), what can't it be within reach of a TT'd car? People might be more likely to be touching a car than a fence, but are they any more likely to be touching the lamppost?

       - Andy.
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    part of the problem is that the regs make no distinction between the case of (extraneous) conductive parts that are lightly earthed like the fence post, or even an earth electrode, where yes, it is grounded, but via significant electrode resistance, such that current is limited, and a person holding it is probably no more connected to earth and at risk than if the metal was not there, but they were barefoot on the ground.


    and the other case of parts that are solidly connected to the neutral or earth elsewhere, and can carry a large current,  potentially enough to start a fire, and really are part of the earthing system, and need either solidly bonding or keepoing out of reach.


    A  palms of hand contact with say a galvanised handrail connected to a PME earth at a bus stop, is quite different shock proposition to a similar looking rail 'planted'  on concrete pads on the earth, even if the footings are buried, and mud comes up round the bottoms of the bare metal for a bit.

    I am reminded of the railway fence dilemma to bond or not to bond that came up last year. Sadly I do not think we know the conclusion the OP reached on that one.
  • 0

    SScho:

    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).




    2.5 m is 'arm's reach' and this is what the latest IET CoP for EV Charging Equipment Installation says.


    BUT, there's a different separation difference that's required by DNO's - that between earth electrodes and buried conductive parts connected to the TT system, and the same connected to the PME system. In fact, one particular DNO requires a lot more than 2 m - they are asking for 3.5 m ... but let's be clear, this is below ground not  arm's reach as you are describing.



    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?



    Definitely NOT. BS 7671 has a clear requirement in 411.3.1.1, a basic requirement for fault protection (my highlight):

    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.

    ...


     


    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.




    That initially sounds like a good suggestion, and if both TT, might have addressed the 411.3.1.1 requirement. BUT, there are a couple of other considerations. For example, if they are not connected to the same supply transformer, or any of the TT earth electrodes are too close to PME buried earth (or worse, HV!) then you might have an issue with impulses and fault transfer potential.


    Which is why there is another fundamental requirement of BS 7671 - 542.1.3.3, which says you need to be very, very careful bonding earths of different installations together, and you take full responsibility for that.


    Neither of the above points are new - and to that end, they are clearly signposted in section 5.1.2 of the IET Code of Practice for EV Charging Equipment Installation (3rd Edition).

     



    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.).



    Well, isolating transformer is one option (although there are constraints this brings of course - cost, size, weight, one per charge point required, perhaps inrush currents and selection of primary circuit OPD) but then, what is the price of safety?


    Perhaps encourage suppliers to develop the device stated in 411.3.3 (iii)?

  • 0

    Well, isolating transformer is one option



    How would that work? You can't use it to create a separated system as most EVs check for an earth connection before commencing charge. If you attempt to create your own local TN (or TT) system then you need a source earth electrode - so back to square 1.


    The only other option I can think of is a floating system with PE connected to N (but not earth) - but as far as I know BS 7671 doesn't recognise such arrangements - which makes it quite a limb to go out on.


      - Andy.
  • 0

    AJJewsbury:




    Well, isolating transformer is one option



    How would that work? You can't use it to create a separated system as most EVs check for an earth connection before commencing charge. If you attempt to create your own local TN (or TT) system then you need a source earth electrode - so back to square 1.


    The only other option I can think of is a floating system with PE connected to N (but not earth) - but as far as I know BS 7671 doesn't recognise such arrangements - which makes it quite a limb to go out on.


      - Andy.

     




    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.)
  • 0

    SScho:

    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.)



    I appreciate that increasing numbers of EV points, or even non-electrified street furniture, mean that the available spaces are disappearing, but what has been done to date? How have you coped so far?
  • Former Community Member
    0 Former Community Member

    SScho:

    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.).




    Are you responsible for the design or the install only?

    I can see the earthing of EVC points being a major headache in a cramped city centre with existing street lighting, CCTV, ANPR, comms cabinets and so on. 


    I've never been involved in EVC.  I was talking to an estates manager this morning that has been instructed to arrange for 500kW of EVC to be installed at their site.

    Time for some CPD.


    Regards


    Parsley

  • 0

    Parsley:


    Time for some CPD.



    Regards


    Parsley


     





  • Former Community Member
    0 Former Community Member
    Good morning all, quite a lot of responses - more than I expected - which is good! - I'll try to reply to everyone with questions in this,


    Graham, I mentioned about the two systems both being converted to a TT system and not using the DNO PME, forgot to mention that in an ideal world I would have a link between both of the grounding spikes/mats. The main cause for concern is that if there were a fault with the PME and someone could reach out and touch a lamp column whilst charging their car, this could create an alternative path to earth and pose the risk of electric shock.

    That initially sounds like a good suggestion, and if both TT, might have addressed the 411.3.1.1 requirement. BUT, there are a couple of other considerations. For example, if they are not connected to the same supply transformer, or any of the TT earth electrodes are too close to PME buried earth (or worse, HV!) then you might have an issue with impulses and fault transfer potential.


    Which is why there is another fundamental requirement of BS 7671 - 542.1.3.3, which says you need to be very, very careful bonding earths of different installations together, and you take full responsibility for that.



     If we have the same path to earth (coming from the same DNO cable - be it a 300wv, 180wc, 0.3:4c Al, etc. confirmed by uncovering the joint in the ground and comparing to drawings) then a risk of having multiple paths would be reduced or nullified. Each supply in the TT system would also be covered by a 300mA RCD (as standard) and charge points protected by a 30mA 61009-1 RCBO (as standard). Of course, installing around London has its difficulties - especially when I bring up an electrical map which has 30 HV cables, dates back to the 60's and looks like a child has scribbled over it! So we do need to take many precautions so as not to interfere with any of the existing infrastructure.

    I had a look through 542.1.3.3 and as far as I can see, as long as the requirements are met that it's capable of carrying the maximum fault current of either installation then this is a satisfied prerequisite.

    My idea is to essentially turn the surrounding furniture/installations into a grouped, single installation - the same as if you were to have multiple distribution boards from the same LV switchgear - Everything is essentially the same in terms of voltage, phasing and earthing arrangements - just different locations. I hope this makes sense (it's still early!).


    I feel that I might need to do a video or at least some diagrams to explain what I mean coherently!


    I did also consider the use of isolation transformers, but we are very limited on space and of course, everything is related to cost. You can't put a price on safety, unless it costs too much. ? 



     


    Chris -

    I appreciate that increasing numbers of EV points, or even non-electrified street furniture, mean that the available spaces are disappearing, but what has been done to date? How have you coped so far?




    It's incredibly difficult and we are trying to find an electrical solution which reduces or negates the need to be a certain distance from other installations or street furniture,  we also have to comply with many other factors as well, such as noise pollution to the adjacent properties, light intrusion if there are windows next to where we want to put them, if there's any underground structures we can't dig through... the list goes on! There's actually a pretty good document which outlines the requirements of EV charging points - http://ukevse.org.uk/resources/procurement-guidance/

     






    Parsley -

    Are you responsible for the design or the install only?

    I can see the earthing of EVC points being a major headache in a cramped city centre with existing street lighting, CCTV, ANPR, comms cabinets and so on. 


    I've never been involved in EVC.  I was talking to an estates manager this morning that has been instructed to arrange for 500kW of EVC to be installed at their site.

    Time for some CPD.


    Regards


    Parsley




    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.


      


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