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VRI insulation failure

Discussion in 'Electrical Engineering' started by Beemer, Mar 26, 2007.

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  1. Beemer

    Beemer Guest

    I have just seen a domestic earth fault involving a cotton-braided
    Vulcanised Rubber Insulation (VRI) cable. A 60A branch fuse had blown and a
    friend asked me to look at the problem.

    The live cable and its neutral had passed through a brass gland but to me
    the insulation appeared undamaged. I did not have a 500V insulation tester
    with me but I could find no low resistance to earth with an ohm-meter.

    I replaced the fuse and when I closed the isolator the fuse again blew but
    this time the fault revealed itself as the live cable had shorted to earth
    on the inside of the brass gland.

    The cable and fuse were was renewed and the problem was cleared.

    My question is with 40 year old VRI cable what changes take place in the
    insulation? I know it gets brittle but assuming the cable is undisturbed
    and not under any vibration why would it fail like this? Could there have
    been a chemical change going on which gradually carbonised the insulation?

    Beemer
     
  2. Well the VRI does get sorta fragile with age, any chance of condensation?
    Usually the carbonisation is the result of the arcing and heat and dampness
    in the porous old VRI sure helps with arcing :).
     
  3. Ozone is another detrimental factor. Over a period of time, it
    affects rubber near switch contacts, motor brushes, and anywhere
    else near a source of sparks. Any accidental use of VRI in old
    electromechanical telephone exchanges was quickly a disaster.
     
  4. Beemer

    Beemer Guest

    |
    | | >I have just seen a domestic earth fault involving a cotton-braided
    | > Vulcanised Rubber Insulation (VRI) cable. A 60A branch fuse had blown
    and
    | > a
    | > friend asked me to look at the problem.
    | >
    | > The live cable and its neutral had passed through a brass gland but to
    me
    | > the insulation appeared undamaged. I did not have a 500V insulation
    | > tester
    | > with me but I could find no low resistance to earth with an ohm-meter.
    | >
    | > I replaced the fuse and when I closed the isolator the fuse again blew
    but
    | > this time the fault revealed itself as the live cable had shorted to
    earth
    | > on the inside of the brass gland.
    | >
    | > The cable and fuse were was renewed and the problem was cleared.
    | >
    | > My question is with 40 year old VRI cable what changes take place in the
    | > insulation? I know it gets brittle but assuming the cable is
    undisturbed
    | > and not under any vibration why would it fail like this? Could there
    have
    | > been a chemical change going on which gradually carbonised the
    insulation?
    | >
    | > Beemer
    | >
    | >
    | Well the VRI does get sorta fragile with age, any chance of condensation?
    | Usually the carbonisation is the result of the arcing and heat and
    dampness
    | in the porous old VRI sure helps with arcing :).
    | --
    | Cheers .......... Rheilly P
    |
    |
    No dampness or sunlight and normal domestic inside temperature band. I
    guess it must just have been long term pressure of the cable against the
    brass gland unless of course the guy had been moving the wires about without
    telling me.

    thanks,

    Beemer
     
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