Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 19 replies
  • Subscribers 32 subscribers
  • Views 4979 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

Distance between other electrical systems

Former Community Member
Former Community Member
Hi all,


I've been asked this time and time again and it could be a case that I've looked at this so much that it's made me go insane, but I'd like a 2nd opinion.


When installing a TT feeder pillar on the street, does the 2.5m rule come into effect when it's located near to a PME system or does this need to be 2.5m away from all other systems which are not connected to the same earthing system (not connected to the earth mat/rod under the feeder pillar?  


My personal opinion is that they should be 2.5m away from anything that isn't on the same electrical installation (even if it's another TT system nearby) as if there's a fault with one and the path to earth is broken then the path would flow directly between the person or whatever touches it and take the route of least resistance. I have my copies of the 18th edition and the CoP for EVCEI, but I've been looking at it for so long it's made things become less clear the more questions I'm being asked about the regs!


Any help would be much appreciated,


Regards,


S
  • 0
    Reading this makes me realise just how badly thought out EV charging is (again). Typically every multiple EV charging point is defective because each has its own electrode following the regs. Two adjacent cars can be at differing potentials. Chris is quite correct here (and BS7671 a mess) because a TT installation inherently depends on the RCD to provide shock protection, and therefore ANY fault with danger must simply trip the RCD if contacted. Simple enough and has worked very well for many years. However people are now worrying about potentials rather than disconnection on contact, which is simply not sensible. TT does not work unless there is a potential and earth path sufficient to trip the RCD! My reading of multiple earth systems is a TN-S and a TT one, not two TT systems, because they are inherently protected from danger by the RCDs. TT islands do not work for car charging unless you trust the RCD. Clearly some do not, and the "what if" question immediately makes all this unworkable, particularly when the H&S police have had a go too! The answer is simple and can be fixed immediately by a simple bit of legislation; "Electric vehicles must be class 2, otherwise they may not be sold in the UK". All those not complying may be sent abroad when sold, and the owners suffer a loss like my diesel car, government about turns included! Fair enough and the proper way to sort the problem, which should never have happened in the first place if those responsible had been paying suitable attention!
  • 0

    davezawadi:

    Reading this makes me realise just how badly thought out EV charging is (again). Typically every multiple EV charging point is defective because each has its own electrode following the regs. Two adjacent cars can be at differing potentials. Chris is quite correct here (and BS7671 a mess) because a TT installation inherently depends on the RCD to provide shock protection, and therefore ANY fault with danger must simply trip the RCD if contacted. Simple enough and has worked very well for many years. However people are now worrying about potentials rather than disconnection on contact, which is simply not sensible.




    People are worrying about potentials which do exist ... although it's not "within the same electrical installation" or even the same means of earthing.


    Exposed-conductive-parts of a TT system are at the potential of the TT system main earth terminal (which is hopefully the potential of the earth electrode).


    The separation we were talking about is to prevent the "import" of earth potential rise from a PME installation, and has nothing to do with automatic disconnection within the TT installation. It's no good having requirements to prevent PME touch voltages, and returning them by inadequate (or, alternatively as I've pointed out, inappropriate) separation of the TT system from the PME system.



    TT does not work unless there is a potential and earth path sufficient to trip the RCD! My reading of multiple earth systems is a TN-S and a TT one, not two TT systems, because they are inherently protected from danger by the RCDs. TT islands do not work for car charging unless you trust the RCD.





    As above, probably nothing to do with the RCD - the issue is outwith the installation, although I do agree, even separation may not be the answer to this.



    Clearly some do not, and the "what if" question immediately makes all this unworkable, particularly when the H&S police have had a go too! The answer is simple and can be fixed immediately by a simple bit of legislation; "Electric vehicles must be class 2, otherwise they may not be sold in the UK". All those not complying may be sent abroad when sold, and the owners suffer a loss like my diesel car, government about turns included! Fair enough and the proper way to sort the problem, which should never have happened in the first place if those responsible had been paying suitable attention!





    Agreed, to a point - there may be other options than Class II, but Class II would have solved the problem ... unless of course there's an issue with static electricity and hybrids??

  • 0
    further to that comment

    Exposed-conductive-parts of a TT system are at the potential of the TT system main earth terminal (which is hopefully the potential of the earth electrode).




    I'd respectfully suggest that

    Exposed-conductive-parts of a TT system should be at the or near the potential of the terra firma earth, that is to say the stuff you may stand on, whcih is a potential aquired by connection to the electrode. If the ground voltage is bouncing up and down because of a fault on a PME network nearby, ideally you would want all CPCs and  earthed metal work to bounce up and down with it, in the same way that a fence post will.


    I see nothing special about the electrode of a TT system, it's purpose in this context is to make sure that metal objects are at the same potential as the (terra firma) ground they stand on.

    Its other purpose is to fire an RCD if there is a live to CPC fault.

     


  • 0
    Not on a caravan site or supply to a boat, as ESQCR effectively prohibits PME GPR from being present at a caravan (I know, not in so many words, but the spirit is there) ... and not to deal with EPR and other impulses due to HV faults which are present in a Global Earthing System, as might be connected to the PME earthing system ...


    And that's perhaps the difficulty with EV charging installations - it's not playing nicely with the "rules" we have in place already for TT systems.
  • 0
    I am not aware of any equivalent distance rules for caravans and boats, that prevent say a cold water supply pipe in  a marina, from being bonded to the PME supply in the building where it originates, but that is also PME earthed metal.  If you had a distribution pillar and meters in a boat yard,  if it is the end of the PME service, there will be an electrode on it, even if all the loads are wired as TT.


    In terms of is it a real problem, actually the current regs require all properties to bond cold water pipes if they are metal, and no insulation section, and I am aware of a few round here, where one house is PME and the one next door is TT. Many such earthing systems are already linked, like it or not  - that is why we do see quite large diverted currents on the pipework sometimes, but it all works OK.


    As I said above, the rules are written such that in  small space you really cannot win.

    Which probably means the rules need a rethink for this case.
  • 0

    mapj1:

    I am not aware of any equivalent distance rules for caravans and boats, that prevent say a cold water supply pipe in  a marina, from being bonded to the PME supply in the building where it originates, but that is also PME earthed metal.  If you had a distribution pillar and meters in a boat yard,  if it is the end of the PME service, there will be an electrode on it, even if all the loads are wired as TT.




    As per earlier post in the thread, see BS 7430, and GN 7 (Section 7.5.6 and Figure 7.3) ... these are not new for the 18th Edition updates either.



    In terms of is it a real problem, actually the current regs require all properties to bond cold water pipes if they are metal, and no insulation section, and I am aware of a few round here, where one house is PME and the one next door is TT. Many such earthing systems are already linked, like it or not  - that is why we do see quite large diverted currents on the pipework sometimes, but it all works OK.





    Does this satisfy the DNO's requirements for separation of TT systems from the Global Earthing System? If so, in those cases, perhaps OK and I'd agree what's the difference. But this may not be the case in every installation.



    As I said above, the rules are written such that in  small space you really cannot win.

    Which probably means the rules need a rethink for this case.



    Don't disagree ... especially where EV charging is considered.

  • 0

    My reading of multiple earth systems is a TN-S and a TT one, not two TT systems, because they are inherently protected from danger by the RCDs.



    I see it slightly differently - for me a TT system isn't necessarily just the same as terra firma or an isolated extraneous-conductive-part (e.g. metal pole stuck into the ground). Two reasons -


    1. TT disconnection times don't necessarily give protection against electric shock - for larger final circuits and distribution circuits anything up to 1s is permitted. It might make sense to make a 'special case' of where all exposed TT metalwork is protected by a 30mA instantaneous RCDs - but BS 7671 doesn't seem to consider such approaches.


    2. The resistance around the TT electrode means that any (normal service) protective conductor currents can raise the TT earthing system significantly above true earth potential - BS 7671 (RA x IΔn ≤ 50V) only requires that to be limited to 50V above true earth - so two adjacent systems fed from different phases could have anything up to almost 87V between them permanently (possibly 100V if fed from opposite sides of a split phase supply). It might make sense to be able to treat adjacent systems that achieve RA x IΔn ≤ 25V more leniently (not hard for 30mA RCD and 200 Ohm rod)  but again it's not an approach that BS 7671 seems to consider.


       - Andy.
  • 0
    Here in Worcester there are two EV charging stations on the second floor of a multi storey car park that is above the shopping centre service area, what earthing system is currently required for such an installation?


    Andy
  • 0

    Sparkingchip:

    Here in Worcester there are two EV charging stations on the second floor of a multi storey car park that is above the shopping centre service area, what earthing system is currently required for such an installation?


    Andy 




    Regulation 722.411.4.1 tells you that PME is permitted where the vehicle may only be charged indoors. However, does the site have a private transformer? In which case it's likely to be TN-S and it's OK to use that.


    Both of the above scenarios are discussed in the IET Code of Practice for EV Charging Equipment Installation.


    There is another alternative - some fast charging electric vehicle supply equipment have in-built separation to add to the mix - this is also permissible, usually limited to the supply of one vehicle from one separated source (Reg 722.413.1.2). Similarly, an isolating transformer could be used for AC EVSE on the same basis.

     

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines