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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
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    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?

Reply
  • 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?

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