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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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  • Former Community Member
    0 Former Community Member

    Simon Barker:


    Given the long history of connecting the battery negative to the chassis, and then using the chassis as the negative "wire", I can't imagine it's that easy to create a double insulated EV.  And you couldn't assume any existing vehicles were double insulated when designing charging stations. 


    The traction battery is pretty much universally insulated from the chassis, usually with insulation monitoring (because double/reinforced insulation in the motor involves significant engineering compromise). The chargers are generally not isolated from the mains input so during charging the battery is subject to mains potentials and may well be bouncing around at 50 Hz and at the switching frequency. None of this means that the vehicle chassis has to be earthed, but I can see that it's a convenient (lazy?) way for the designers to avoid problems like tingles from capacitive coupling between battery and chassis. So long as the insulation monitoring system disables charging when it detects a leakage path between traction pack and chassis then earthing the chassis isn't needed for safety.


    The other issue may be that they're designed with other countries distribution systems in mind and the assumption (perhaps reasonable in many parts of the world, and maybe even here in practice) that PE is always a safe thing to connect to the chassis.


    Edit: typos.

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  • Former Community Member
    0 Former Community Member

    Simon Barker:


    Given the long history of connecting the battery negative to the chassis, and then using the chassis as the negative "wire", I can't imagine it's that easy to create a double insulated EV.  And you couldn't assume any existing vehicles were double insulated when designing charging stations. 


    The traction battery is pretty much universally insulated from the chassis, usually with insulation monitoring (because double/reinforced insulation in the motor involves significant engineering compromise). The chargers are generally not isolated from the mains input so during charging the battery is subject to mains potentials and may well be bouncing around at 50 Hz and at the switching frequency. None of this means that the vehicle chassis has to be earthed, but I can see that it's a convenient (lazy?) way for the designers to avoid problems like tingles from capacitive coupling between battery and chassis. So long as the insulation monitoring system disables charging when it detects a leakage path between traction pack and chassis then earthing the chassis isn't needed for safety.


    The other issue may be that they're designed with other countries distribution systems in mind and the assumption (perhaps reasonable in many parts of the world, and maybe even here in practice) that PE is always a safe thing to connect to the chassis.


    Edit: typos.

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