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Induced voltages in cables in trunking

  1. Sep 12, 2011 #1
    I was carrying out a insulation resistance test on a ring main circuit at the dist board in an industrial workshop. The majority of the wiring is single core pvc cables run in metallic trunking and conduit. The majority of circuits have no cpc as the trunking/conduit is being used. When I disconnect the 2 phase and 2 neutral conductors I measure approx 50v to earth on these conductors. This voltage reduces to around 6v when I turn off the mcb feeding a fluorescent lighting circuit (9 x twin 1500mm). Is the lighting circuit inducing a voltage on the ring circuit? Is this a problem and if so how do I overcome it?
    The installation is approx 30 years old with numerous alterations and additions

    Many Thanks
  2. jcsd
  3. Sep 12, 2011 #2

    jim hardy

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    Likely capacitive coupling from conductors in same conduit?

    What meter are you using?
    A high impedance meter like a digital will scare you by indicating capacitively coupled voltage

    try an old fashioned analog meter, preferably one with 1 kohm/volt sensitivity.
    The $10 one from Walmart will do -
    Put it right in parallel with your digital (i'm guessing hat's what you are using) and see if the reading drops a LOT.

    If so that is probably your trouble, too good of a meter.
    Used to be they specified what sensitivity meter to use. Does term "Kohms per volt" appear anywhere in your test specification?

    Hopefully nobody has crossed wires on you. This test should tell.
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