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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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  • 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.
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  • 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.
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