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TNS and PME connected]

lyledunn

I would appreciate a wee bit of advice on this one. Quarry with everything supplied by one transformer. There is an overhead bundle to the office block which DNO notice says PME. The quarry supply is TNS and the workshop is also supplied via the main panel in the quarry intake. There is a gen set that feeds into a changeover switch in the quarry intake that offers a back up to the quarry and to the workshop. A new SWA cable was installed from the panel in the workshop to a metal clad changeover switch in the office block to allow the office to benefit from the back up generator. The PME earth is now exported to the quarry. Issue?

one.

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    I presume. like the quarry supply  the genset is supplying TNS and so , when running on genset, is the supply to the workshop. The problem then, if there is one, is the NE link at the office. How is the change-over from the PME supply to generator power managed ? if it is done in a way that breaks the potential current path through this link, then there is no issue. (though that does mean that simply strapping all the earths together at both ends won't quite do)
    I'm kind of assuming the generator is only used when the transformer and it's hybrid earthing is not. Of course if both ever run together that is different.
    One could of course TT the offices and workshop, but I can see in a rocky environment why that may not be desirable. (though there must be an electrode in the vicinity of the generator already)
    Mike.

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  • 0

    I presume. like the quarry supply  the genset is supplying TNS and so , when running on genset, is the supply to the workshop. The problem then, if there is one, is the NE link at the office. How is the change-over from the PME supply to generator power managed ? if it is done in a way that breaks the potential current path through this link, then there is no issue. (though that does mean that simply strapping all the earths together at both ends won't quite do)
    I'm kind of assuming the generator is only used when the transformer and it's hybrid earthing is not. Of course if both ever run together that is different.
    One could of course TT the offices and workshop, but I can see in a rocky environment why that may not be desirable. (though there must be an electrode in the vicinity of the generator already)
    Mike.

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  • 0 in reply to mapj1
    mapj1 said:
    if it is done in a way that breaks the potential current path through this link, then there is no issue. (though that does mean that simply strapping all the earths together at both ends won't quite do)

    BS 7671 does permit transfer switching to also transfer the means of earthing.

    BUT ... to completely isolate one earthing system from another fully in these circumstances, and maintain safety, is not easy.

  • 0 in reply to gkenyon

    Agree, its a funny one, and the fact the other end is a quarry is a complication.  If it wasn't it would be similar to a street with a mix of TNS and TNC-s service heads on the same substation with the earths linked by common water pipes etc. That common situation does introduce currents in the bonding as sort of 2nd parallel neutral,  but we don't normally worry unduly. I also suspect that in reality the additional risk would be very low. 

    If it was a farm it would be all TT.... 

    It does beg the question of how hard it would be to separate N and E between transformer and site offices - but I assume its not a private transformer and the metering is in the wrong place so that is a DNO owned cable.

    Not for the first time the PME approach solves one problem and introduces another.


    Mike

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