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Safe Disconnection to Undertake Work.

Zoomup
Scene. An old farm building now used as a double glazing frame maker's workshop.


The supply comes from an old shed about 40 metres away, where the meter is. The old shed has some 70s/80s rusty M.E.M. isolators that feed several S.W.A. cables that exist the shed in many directions underground. I suspect that one feeds the main farmhouse, another a rented cottage and the others various farm buildings. There is no clear labelling of the isolators.


What is the best way to locate the frame maker's  building supply where I have to work?


Somewhere I have a circuit i/d set, if I can find it, but that will need access to the live parts and I do not wish to turn off the wrong isolator.


Z.


  • 0
    Chris Pearson:

    I still don't understand a pair of cottages with an earth adjacent to the transformer and no other. DNO says PME, so that must be what it is.


    The DNO might prefer you to treat it as if it were PME so that they are then free to alter it to real PME in the future. That seems to be the general advise these days for "presented as TN-S" supplies - treat them as if they could be PME.


    Also if there's a CNE between the first electrode and the consumer's N-PE link, then it's TN-C-S anyway and all the dangers of a broken CNE would apply just as if it were a proper PME system with multiple electrodes on the distribution system.


       - Andy.


  • 0
    Huh I don't get this surely you must have an LV earth at the transformer or at least at the first pole round here there is one or possibly 2 premises with  a earth/neutral link the rest are straight TN S
  • 0
    I think that Andy is right with the farm supply. It probably is P.N.B. as it has a single black cable running to ground at the main cutout position connected to the main metal switch fuse from a neutral block. So essentially the L.V. earth electrode is at the farm's intake position rather than at the big pole transformer which is in a field some distance away. I presume that the H.V. earth electrode is at the pole transformer, but can't be sure. So in the past, if the farm was considered just one consumer it was supplied with a P.N.B. supply. The L.V. earth electrode is situated at the farm's intake position rather than at the pole transformer. Zs at the remote building where I was working, was less than 0.5 Ohms so earthing is good. At the farm intake position Ze would be even lower. The farm has no P.M.E. label displayed.


    Z.
  • 0
    AJJewsbury:
    Chris Pearson:

    I still don't understand a pair of cottages with an earth adjacent to the transformer and no other. DNO says PME, so that must be what it is.


    The DNO might prefer you to treat it as if it were PME so that they are then free to alter it to real PME in the future. That seems to be the general advise these days for "presented as TN-S" supplies - treat them as if they could be PME.


    Also if there's a CNE between the first electrode and the consumer's N-PE link, then it's TN-C-S anyway and all the dangers of a broken CNE would apply just as if it were a proper PME system with multiple electrodes on the distribution system.




    No CNE, just a pair of aerial singles to a JB under the eaves. Supply then goes to each cottage where N splits into N and E in the service head. Originally, it was a single cottage and barn.


  • 0
    No CNE, just a pair of aerial singles to a JB under the eaves. Supply then goes to each cottage where N splits into N and E in the service head. Originally, it was a single cottage and barn.

    Isn't the supply N single then providing both N and PE to the consumer?

       - Andy.
  • 0
    Huh I don't get this surely you must have an LV earth at the transformer or at least at the first pole

    Not necessarily - see Fig B-3 in this UKPN document: http://library.ukpowernetworks.co.uk/library/asset/f6acd931-9c79-411f-a98f-f801bce6e63L/EDS+06-0017+Customer+LV+Installation+Earthing+Design.pdf


    (In that version they have the electrode connected upstream of the N-PE link and so declare it to be TN-C-S, but if they're the other way around (or both into the same terminal) it can be classed as TN-S.)


      - Andy.
  • 0
    AJJewsbury:
    No CNE, just a pair of aerial singles to a JB under the eaves. Supply then goes to each cottage where N splits into N and E in the service head. Originally, it was a single cottage and barn.

    Isn't the supply N single then providing both N and PE to the consumer?


    OK, you could look at it that way. It still appears to be PNB serving 2 customers.


  • 0
    So it seems I owe an apology to you all with regards to the neutral earth electrode not being on the Tx but on a pole further along or at the customers premises of course i now know this is fairly common practice i don't know why I was getting so confused with it all  Soz guys

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