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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.).
  • Former Community Member
    0 Former Community Member
    Unfortunately separating HV and LV earthing systems on many existing sites isn't practicable.


    Regards

    Parsley



    ​​​​​
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    Parsley:



    Unfortunately separating HV and LV earthing systems on many existing sites isn't practicable.​​​




    Parsley, agreed. The current standards do permit combined HV/LV, and recommend as far as practicable something they call a global earthing system.


    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) ...

     

  • 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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    SScho:





    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 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.


     




    Yes, definitely. It is recognised, and there is a lot of work ongoing in this area at the moment.


    I also agree that mixed bag of earthing systems doesn't make things easy and is not really a desirable solution - a device as discussed in §5.3.5 of the IET Code of practice for EV charging equipment installation 3rd Edition, and of course meeting BS 7671 reg 722.411.4.1 (iii), would be a big help.

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    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.



    I wouldn't bank on that. Around these parts at least the water authority's method of "replacing with plastic" seems to consist of breaking the old iron main at junctions and sliding the new plastic tube inside (presumably saves a lot of digging that way). So what we end up with is a continuous plastic main, but surrounded by long lengths of metal pipes with only comparatively short breaks of muddy soil between them. To get the equivalent of 3.5m of soil gap between metalwork you might need half a dozen or more such gaps in series - which might mean many tens if not hundreds of meters of upgraded "plastic" water main.


        - Andy.
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    Andy,


    Yes, you can't legislate for everything, the situation is in a right pickle. There will be other areas where for one reason or another soil resistivity is far lower than average too.


    To be honest, I'm most worried in all this how electricians are supposed to work out what's going on with other services ... in many places, no-one knows where they are. And to boot, none of it is really directly related to the consumer's installation, it's all outside influences driving a range of, what some may say are, less-than-satisfactory solutions at the moment.
  • Former Community Member
    0 Former Community Member
    We can only do as much as feasibly possible to do everything correctly and by the book - but this doesn't prevent someone who isn't electrically trained from coming along and installing something in the vicinity that then puts the original electrical install into a position which would be against the regs.

    What with the multitude of services located underground which aren't always recorded properly (or at all)... can we realistically expect every electricians (and even other trades) to find out the eastings/northings for where they need to dig and then contact every supplier to ask if they have services in the area? It's very messy indeed... If only we could rip it all up and start again! ?
  • 0

    Yes, you can't legislate for everything, the situation is in a right pickle. There will be other areas where for one reason or another soil resistivity is far lower than average too.


    To be honest, I'm most worried in all this how electricians are supposed to work out what's going on with other services ... in many places, no-one knows where they are. And to boot, none of it is really directly related to the consumer's installation, it's all outside influences driving a range of, what some may say are, less-than-satisfactory solutions at the moment.



    Indeed. I've a feeling that all this trying to create an "equipotential zone" out of doors is rather letting the perfect become the enemy of the good.


    There's a long standing exception to bonding extraneous-conductive-parts within reach of lighting columns etc. (currently 714.411.3.1.2 but has been there in a similar form for decades I'm sure) - which doesn't seem have have caused any problems whatsoever in practice. At the moment I'm thinking that exposed-conductive-parts of a TT system - especially ones that'll probably easily comply with RA x IΔn ≤ 25V (let alone 50V) and satisfies 0.2s disconnection time and 40ms @ 150mA) doesn't pose any more of a hazard w.r.t. an adjacent PME's lighting column than any other bit of metalwork that's in contact with the general mass of the earth (such as a fence or cycle stand) - or indeed the ground surface itself (which usually seems to get overlooked in equipotential zone discussions). In these cases the TT'd item won't be at a hazards voltage above true earth for a hazardous duration - so it seems to me we should be able to treat it as simple extraneous-conductive-part as far as neighbouring systems are concerned - and so apply 714.411.3.1.2 (even if the EVSE happened to be installed after the lamppost). The hazard posed by the PME lamppost is what it already is and isn't made unacceptable by installing another bit of metalwork nearby (whether a fence or a cycle stand or a TT'd EVSE).


    Indeed I might question how "conductive" to touch all these items are in practice - steel lighting columns are usually heavily painted, likewise car bodywork, car door handles and charging connectors tend to be plastic, outdoor footwear usually insulating, posts for roadsigns plastic coated and so on. Nothing you'd want to rely on in lieu of basic insulation of course, but nevertheless they must reduce the chances of a shock in practice. What is the real probability of someone coming to harm even with a broken CNE?


        - Andy.
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    I like your straw man  idea of an equipotential zone outdoors - the idea is of course beautifully ludicrous (rather like including the floor area served by a garden socket in the loading calculation), and highlights the problems in the regs we do not really distinguish that  cycle rack from a cast iron water main, and the both the Ze, and the chances of being bonded to the local PME CPC at an adjacent property are very different, and therefore so is the associated shock risk.  I think any form of words  that really says 'don't bother if its just an isolated metal thing planted in or the ground' is reasonable.

    New water pipes and gas mains are plastic, but round here the ~ 50 year old iron pipes are very much still in service, and to meet the 2020 no cast iron gas main target, we are having yellow plastic liners pushed and inserted inside the old mains, and the old pipe is not actually being dug up, and as they do not even leak yet, I suspect they will remain electrically interconnected between properties and to ground for probably another 20 -50 years before rusting through to the point of being electrical islands  .
  • 0

    The hazard posed by the PME lamppost is what it already is



    Thinking a bit further - the hazard from a PME'd lamppost suffering from a broken CNE is often mitigated by the supplementary electrodes that DNOs (if not BS 7671) have been insisting be installed -  and given the modest load can be reasonably successful at keeping touch voltages below hazardous levels.


      - Andy.


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