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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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    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)?

Reply
  • 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)?

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