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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

    gkenyon:


    The issue we are discussing here is a little different - that of a TT supply separated from the network (specifically, the PME system). However, there are situations for EV charging installations where physical separation above and below ground cannot be achieved also.



    mapj1:

    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.




    I've seen one DNO that requires 3.5 m separation. Above ground at least, arm's reach (2.5 m) would be required by BS 7671.


    My understanding is that metal gas and water mains, at least in the street, are being (or have been) replaced by plastic. Once this is completed, the place any remaining pipework (the bit up the driveway of a house, for example) is connected to the PME earthing system is in consumer's premises.


    The situation may well be muddied by supply cables which have a conductive (or effectively conductive) earthed outer sheath.


    But would it mean I can convert a whole installation to TT? Not always simple if an adjoining property is on PME. There are a number of situations where you might get caught out, e.g. two properties have roof-mounted TV antennas - the antenna systems may [should, but I know they aren't always] be earthed at one point, so if they are in arm's reach (or 3.5 m in one case) ...





    Morning Graham,


    What I'm understanding from this so far is that it may be possible to convert the adjacent installations to a TT to satisfy the requirements of removing the potential of being within arms reach of a PME system, as long as a rather long list of prerequisites are met. As 5.3.3.1 in the CoP for EVSE states;



    "Providing a dedicated TT earthing system is only acceptable if there is no possibility of simultaneous contact between exposed- and/or extraneous-conductive-parts of the TT earthing system and exposed- and/or extraneous-conductive-parts of the TN-C-S (PME) earthing system of the main installation or any other earthing system of any other nearby installation.



    This would mean that if the nearby installations are also on a TT system or a TN-S system, they would still require to be minimum 2.5m away.

    If you had 2 TT systems (2 joints from the same DNO supply cable) assuming both systems were either 3ph or one system 3ph and one 1ph, or both on the same phase, you would be able to convert both installations into, essentially, a singular TT system with a link between the earthing rods/mats used with a large enough CSA to carry the fault current to satisfy disconnection times for either installations ADS protective devices and additional protection.

    As for the DNO restriction of 3.5m of underground services, this would have to be by enquiry and also by means of exploratory excavations where needed.


    I've also been trying to think of a way you could design something as mentioned in section 5.3.5 of the EVSE CoP yet as soon as I think of something, I think of how it wouldn't work... very frustrating! 


    My main concerns and reasoning for asking this initial question, is that we are going to get to a point when all criteria is going to be very tricky to meet and will hinder the uptake of EVs with the public scratching their heads saying "Why can't they just put a charger here?!". If we can agree a method of working around the TT - PME proximity issue, then this will allow for a much easier install with fewer restrictions.

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  • Former Community Member
    0 Former Community Member

    gkenyon:


    The issue we are discussing here is a little different - that of a TT supply separated from the network (specifically, the PME system). However, there are situations for EV charging installations where physical separation above and below ground cannot be achieved also.



    mapj1:

    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.




    I've seen one DNO that requires 3.5 m separation. Above ground at least, arm's reach (2.5 m) would be required by BS 7671.


    My understanding is that metal gas and water mains, at least in the street, are being (or have been) replaced by plastic. Once this is completed, the place any remaining pipework (the bit up the driveway of a house, for example) is connected to the PME earthing system is in consumer's premises.


    The situation may well be muddied by supply cables which have a conductive (or effectively conductive) earthed outer sheath.


    But would it mean I can convert a whole installation to TT? Not always simple if an adjoining property is on PME. There are a number of situations where you might get caught out, e.g. two properties have roof-mounted TV antennas - the antenna systems may [should, but I know they aren't always] be earthed at one point, so if they are in arm's reach (or 3.5 m in one case) ...





    Morning Graham,


    What I'm understanding from this so far is that it may be possible to convert the adjacent installations to a TT to satisfy the requirements of removing the potential of being within arms reach of a PME system, as long as a rather long list of prerequisites are met. As 5.3.3.1 in the CoP for EVSE states;



    "Providing a dedicated TT earthing system is only acceptable if there is no possibility of simultaneous contact between exposed- and/or extraneous-conductive-parts of the TT earthing system and exposed- and/or extraneous-conductive-parts of the TN-C-S (PME) earthing system of the main installation or any other earthing system of any other nearby installation.



    This would mean that if the nearby installations are also on a TT system or a TN-S system, they would still require to be minimum 2.5m away.

    If you had 2 TT systems (2 joints from the same DNO supply cable) assuming both systems were either 3ph or one system 3ph and one 1ph, or both on the same phase, you would be able to convert both installations into, essentially, a singular TT system with a link between the earthing rods/mats used with a large enough CSA to carry the fault current to satisfy disconnection times for either installations ADS protective devices and additional protection.

    As for the DNO restriction of 3.5m of underground services, this would have to be by enquiry and also by means of exploratory excavations where needed.


    I've also been trying to think of a way you could design something as mentioned in section 5.3.5 of the EVSE CoP yet as soon as I think of something, I think of how it wouldn't work... very frustrating! 


    My main concerns and reasoning for asking this initial question, is that we are going to get to a point when all criteria is going to be very tricky to meet and will hinder the uptake of EVs with the public scratching their heads saying "Why can't they just put a charger here?!". If we can agree a method of working around the TT - PME proximity issue, then this will allow for a much easier install with fewer restrictions.

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