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 4970 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
    I think you reading of the letter of the regs is correct, and two adjacent but isolated TT zones need to be out of reach of each other. (or two of any systems that are not linked for that matter)


    However, regs aside, if the terra -firma earth potential is dangerous,  short of levitation , I'm not sure how you get more than 2.5m off the ground.

    However,  if you accept the surface of the road/grass verge as safe to touch, then by extension you have to ask if  you then accept fence-posts and so on that are accidental TT electrode like structures, just with no wiring to them, and if you do,  is it any more dangerous than a deliberately TT earthed object ?

    In reality TT is different, but the rules as written do not handle it well, in the same way there is no regulatory distinction of solidly earthed (water main bonded to PME earth next door) and loosely earthed objects (that fence post in the garden), yet the dangers they pose are very different.

    I fear you cannot win.


  • 0

    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?  





    Yes, this is the case. See Reg 411.3.1.1 "Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively."


    You can't simply bond separate installations together either. Regulation 542.1.3.3 puts a stop to that if you consider you've also got to account for HV faults and impulses. In addition, DNOs may not like it, if there is a risk of you transferring fault voltages or currents from their earthing systems (again, the behavior of HV faults also to be considered).


    In the case of certain Part 7 "special locations", (e.g. Sections 708 caravan pitch supplies, Section 717 supplies for mobile and transportable units, section 722 supplies for EV charging installation), where the original supply is PME, you're often providing a TT earthing system for safety, to cover the case of broken PEN conductors - and you wouldn't want to bond in any case.


    Further, it's not the above-ground separation that's important, but also the separation below ground. And this might have to be greater.

    For example, voltage from earth potential rise on the PME system due to a broken PEN conductor can be transferred via buried metalwork connected to the PME earthing system (e.g. structural elements, metal pipes, etc.) through the ground, to the TT earth electrode.

    What separation distances are we talking about? Figure 16 of BS 7430 provides an illustration - this is presented in GN7 as Figure 7.3.
    • For caravan sites, GN 7 (section 7.5.6) recommends 10 m separation of the TT earth electrode from buried metalwork, electrodes etc., connected to the PME system earth.

    • Some DNOs require earth electrode separation of at least 3.5 m for EV charging equipment installations - others require "at least 2 m where practicable" or similar - of course, the designer of the EV charging equipment installation will have to decide whether this separation returns a hand-to-foot touch voltage, if it is likely someone will be standing above buried metalwork connected to the PME earthing system, whilst simultaneously touching the vehicle, when the earth potential on the PME system rises due to a broken PEN conductor.

  • Former Community Member
    0 Former Community Member
    Hi Graham,


    I understand you wouldn't want to bond 2 separate installations together if they were running off of different tappings or different transformers etc, but if you had two installations (both PME converted to TT) in the area, both run from the same DNO cable, both 3 phase (or containing the same phase as the other), then bonding the two installations together would be acceptable, no? As long as the conductors can carry the maximum fault current likely to flow through them, as this would fit the requirements of 542.1.3.3?


    I understand a TT electrode being an adequate distance away from exposed conductive metals connected to a PEN conductor, but if the services nearby are also converted to a TT, then this need not apply. Is this correct?


    If I'm not understanding this properly, please correct me, every day is a school day after all!
  • 0

    SScho:

    Hi Graham,


    I understand you wouldn't want to bond 2 separate installations together if they were running off of different tappings or different transformers etc, but if you had two installations (both PME converted to TT) in the area, both run from the same DNO cable, both 3 phase (or containing the same phase as the other), then bonding the two installations together would be acceptable, no? As long as the conductors can carry the maximum fault current likely to flow through them, as this would fit the requirements of 542.1.3.3?


    I understand a TT electrode being an adequate distance away from exposed conductive metals connected to a PEN conductor, but if the services nearby are also converted to a TT, then this need not apply. Is this correct?


    If I'm not understanding this properly, please correct me, every day is a school day after all!




    I'm not sure you'd want to bond two TT installations together, unless they were owned by the same organisation who is in control of both - in which case, why not simply provide a single TT earthing system for the whole lot (which could have multiple supplies provided they were from the same source).


    If you're not in control of all installations, anything could happen, including DNO connecting to a temporary supply, or one of the premises being fed from "island mode" energy generation and storage ... all very complicated.


    Although, perhaps I've misunderstood what you've got in mind?

  • Former Community Member
    0 Former Community Member


    Hi Graham,


    I've done a quick drawing on a typical site to help explain what I'm proposing, and what would essentially happen is that you could install a link between all three METs, this would also increase the surface area of the copper earthing rods/mats and overall reducing the Ze. This would require the permissions of those who own/maintain the adjacent equipment to allow the earthing systems to be combined and also the installation of a label/notice, similar to the standard "Safety electrical connection, DO NOT REMOVE", maybe "This earthing system is interconnected with adjacent equipment - Do not remove", the furniture this is connected to could be confirmed by performing an R2 test with a wandering lead.


    This would reduce the danger of having a potential between items and create a lower path of resistance to earth that the safety mechanisms of ADS and additional protection would be able to activate in an even shorter amount of time.
    c4ea1b79f30e0cb1071ad19d42b8149d-huge-20190514_094214.jpg

    It seems like a very particular situation but in many streets and side roads, there is frequently only one supply cable which would make this possible, if the other installations weren't TT, then it may be possible to convert them to TT with the owners permission or to have them converted by the owner themselves. Wouldn't this remove the need for the required  2.5m between items? I admit, there are many "if"s involved, but we need to have a sensible method of being able to install the EVCE before it becomes impossible due to adjacent street furniture. 


    If I've overlooked something, please let me know and I can have a look at different ways around the issue.

  • 0
    In practice, this is no sillier than a pair semi-detached cottages with TT supplies, each with a bond to a metal water main that effectively interconnects them. As such it is not especially dangerous, and may actually make things safer. However, I think the letter of BS7671 is that pairs of TT systems with different origins (and the origins would be the company fuses) are to be treated no different in terms of segregation rules to any other system, and the fact they share a substation is ignored.

    From first principles, I'm inclined to agree with you, as a credible fault that could make anything live for longer than an RCD tripping time is not likely; but the rules as currently written are against that interpretation.
  • 0

    SScho:


    Hi Graham,


    I've done a quick drawing on a typical site to help explain what I'm proposing, and what would essentially happen is that you could install a link between all three METs, this would also increase the surface area of the copper earthing rods/mats and overall reducing the Ze. This would require the permissions of those who own/maintain the adjacent equipment to allow the earthing systems to be combined and also the installation of a label/notice, similar to the standard "Safety electrical connection, DO NOT REMOVE", maybe "This earthing system is interconnected with adjacent equipment - Do not remove", the furniture this is connected to could be confirmed by performing an R2 test with a wandering lead.


    This would reduce the danger of having a potential between items and create a lower path of resistance to earth that the safety mechanisms of ADS and additional protection would be able to activate in an even shorter amount of time.
    c4ea1b79f30e0cb1071ad19d42b8149d-huge-20190514_094214.jpg

    It seems like a very particular situation but in many streets and side roads, there is frequently only one supply cable which would make this possible, if the other installations weren't TT, then it may be possible to convert them to TT with the owners permission or to have them converted by the owner themselves. Wouldn't this remove the need for the required  2.5m between items? I admit, there are many "if"s involved, but we need to have a sensible method of being able to install the EVCE before it becomes impossible due to adjacent street furniture. 


    If I've overlooked something, please let me know and I can have a look at different ways around the issue.

     




    I still see some form of litigation nightmare happening here, unless you can clarify who owns what - and if separate duty holders, ensure there are covenants of similar in place???


    Even if the supplied chargers and lamp-posts are operated by the same duty holder, the DNO may make changes to their arrangement at any time.

  • 0

    gkenyon:

    I still see some form of litigation nightmare happening here, unless you can clarify who owns what - and if separate duty holders, ensure there are covenants of similar in place???


    Even if the supplied chargers and lamp-posts are operated by the same duty holder, the DNO may make changes to their arrangement at any time.




    From the consumers' perspective, what we need then is for the DNOs to install these publicly available EVCPs. Perhaps some enabling legislation is required?

  • 0
    Even if the DNO's did install the charge points, I'm sure that TT-ing large swathes of them would still present problems in many situations - including:
    •  Maintaining separation below ground between earth electrodes and buried conductive parts connected to the PME earth.

    •  "returning" the touch voltage between someone's feet and the vehicle body, if they are standing over buried conductive parts connected to the PME earth, if the TT earth to the vehicle body is "separated" as previous.

    •  Dangers of excavating for, or driving, earth electrodes, when it's not clear where buried services are in many streets.

  • 0
    So before there can be widespread provision of EVCPs in cities (where, arguably they would be most useful) we need joined up thinking and not the sort of fragmentation that privatization of utilities has produced. We also need that mythical voltage sensing device so that we may use PME supplies.


    Incidentally, I have just come back from a week in France where I saw about half a dozen EVCPs in large village/small town car parks. None was in use.

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