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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
    I would be inclined to remove the trunking lid VERY gently and whilst wearing leather gloves, a full face visor, and a flame resistant overall coat. AFTER explaining to the customer what might go wrong, and that you are not replacing it all FOC if it all goes orribly wrong.
  • 0
    broadgage:

    I would be inclined to remove the trunking lid VERY gently and whilst wearing leather gloves, a full face visor, and a flame resistant overall coat. AFTER explaining to the customer what might go wrong, and that you are not replacing it all FOC if it all goes orribly wrong.


    Yes, that may be necessary. My customer is only a tenant. He seems reluctant to speak to the owner of the installation. Just a feeling I have.


    Z.


  • 0
    Zoomup:

    Just a feeling I have.


    "Go with your guts" was an aphorism which I learned many years ago. Your gut feeling is, in fact, a sub-conscious analysis of the situation. Zoomy, you are clearly experienced enough to have plenty of guts. ?


    When I did my training, we were given the acronym "SILT" for safe isolation - switch off, isolate, lock off, test. I reckon that it should be "PSILT" with a silent P as is psychiatrist. The P stands for "permission".


    Removing trunking lids sounds as dodgy as removing the wings off a TR2. If the bolt head shifts at all, it simply shears off. Then where do you stand?


    Afraid that I must still advise trial and error.


    ETA: put yer tin 'at on. Isolate one by one and see who comes at you with a loaded 12-bore. ?


  • 0
    Are all the SWA cables the same diameter and make ? if you can see the jackets you may be able to reduce the scope for error.

    If you are brave you could try injecting a modest current onto the armour - ideally you'd need to lift the earth at the load end and that may not be possible or safe.

    However,  assuming the armour is the CPC, or at least part of the CPC, then  pulling an L-E fault current of a few amps ought to allow the correct SWA to be isolated with nothing more than  a clamp meter over the jacket of each SWA in turn to identify the one that reads more when the 'fault' current flows. Please check Zs first of course.


    Caveat, I may not have the right mental picture of the installation .

    It is of course silly to ask why are they not labelled, and what the official emergency isolation process is.

    Mike.
  • 0
    mapj1:

    -----

    If you are brave you could try injecting a modest current onto the armour - ideally you'd need to lift the earth at the load end and that may not be possible or safe.

    However,  assuming the armour is the CPC, or at least part of the CPC, then  pulling an L-E fault current of a few amps ought to allow the correct SWA to be isolated with nothing more than  a clamp meter over the jacket of each SWA in turn to identify the one that reads more when the 'fault' current flows. Please check Zs first of course.


    Caveat, I may not have the right mental picture of the installation .

    It is of course silly to ask why are they not labelled, and what the official emergency isolation process is.

    Mike.


    Not certain that this will work. A clamp meter applied around the outer jacket of a SWA should read about zero. If the current flows "out" along the phase conductor, and "back" via either the neutral conductor or via the SWA, the NET current is still zero, excluding any minor leakages from other circuits.


  • 0
    So let's see if I understand correctly you can't switch any of the switches off because it will disconnect the farmers equipment?  OK so why not arrange withe the farmer to turn power off at an agreed time after all if you agree it with him it can be arranged to not interfere with milking etc. I understand the switchgear in the shed is a bit mingy which makes it difficult you'll find a way though
  • Former Community Member
    0 Former Community Member
     I do not want to turn off trial and error style as I may cause problems for other people and the farm. 


    Can't be too much of a problem because you can at least give them notice of a potential loss of supply and if it were really a problem, they'd have an automatic changeover generator installed anyway................


    How much notice do you think the yellow digging machine would give the users when ragging up their supply?


    People have got to find something to whinge about............


    I'd really have to ask myself, do I want or need to work on an installation in that condition and if I were to, what would people think of my ethics and furthermore, guess who'd be taken to task if something goes pear shape in the future?


    Regards


    BOD
  • 0
    Humm, what about a transformer generating a small current connected between your chosen building's earth terminal and a temporary electrode - the circuit should then be completed by the supplier's electrodes and the supply PEs - so should be flowing (unbalanced) in the SWA to that building, A clamp meter around the entire cable should then be able to detect your test current.


    (non-conducting helmet on)


        - Andy.
  • 0
    I think I was not clear in my texplanation. there are two approaches I can think of  to 'unbalance' the  SWA, then a clamp meter will see it. By trying to be terse, I have not separated them.

    Either you need a fault from the far end live to earth - current into either  an electrode or better a wander lead that is going back to supply earth but not via the SWA, or a similar wander lead loop, but to inject current into the armour that way with a small transformer or sig gen.

    M.
  • 0
    Chris is correct, safety first. The farmer is not there Kelly, he is a gentleman farmer and is noticeable by his absence. If there are chicks being reared under heat lamps somewhere they may suffer if the mains is disconnected. I have a feeling that the supply to the workshop does not come from the damp shed because the three phase supply to the workshop has a separate C.P.C. running alongside it and none exists in the damp shed with the main fuses and meter. I suspect that the workshop supply comes from a distribution board in a farm building somewhere. This idea came to me in the night.


    I will not work live under any circumstances on this job. I do not wish to remove the covers of the old rusty metal trunking or bus bar chamber as that would mean turning off the main switch.


    I will try to ask the farm manager, if he is present, about distribution board locations and access. Or I will try to connect up the new 4 way three phase board in the workshop to the outgoing side of the existing switch in the grotty old three phase board in the workshop.


    I will report back later. (Hopefully in one piece with all of my natural hair).


    Thanks all,


    Z.

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