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Can't explain

Discussion in 'Electrical Engineering' started by Den, Apr 13, 2004.

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  1. Den

    Den Guest

    Group:

    I have un ungrounded metal lamp. I have no reason to believe that there is a
    problem with it, but with the lamp turned off I am seeing about 2V between
    the lamp frame and ground, and with it turned on about 4V between the lamp
    frame and ground. All as measured with a digital multimeter. Can anyone
    explain why to me?

    Cheers

    Den
     
  2. SQLit

    SQLit Guest

    Digital meters depending on the type are notorious for low voltage readings
    that are false.
    Try turning the meter on and letting the probes be in free air. Take a
    reading, now wave them about..

    Try an RMS meter if you have one.
     
  3. "False" is not the word I'd choose. DMMs have a very high input
    impedance - as a result, even a few picofarads of capacitance between
    the power circuit and the lamp frame could provide a high impedance
    that would allow this reading to appear on the meter. 120 V / 2 V = 60,
    typical DMM input is 10 meg ohms, and 600 megohms is the impedance of
    about 4.5 picofarads at 60 Hz.

    In my old steelmaking days we had a mimic panel for a dust collector
    that was quite hard to read - it used neon lamps to indicate damper
    positions, and since the lamps were connected to a couple hundred
    metres of cable, it was very easy for stray capacitance to provide
    enough voltage to allow the neons to light.

    RMS isn't so much the problem as the high impedance. If you're looking
    for hazardous leakage current, use a low-impedance tester (a Wiggy or
    Square D) or a purpose-built leakage current test set.

    Bill
     
  4. Guest

    Incorrect measurement technique. Put a load - like a
    100 watt bulb - across the meter leads, then take your
    measurements. A digital meter doesn't put enough load
    on branch circuit wiring to avoid erroneous low voltage
    readings.
     
  5. Dave M.

    Dave M. Guest

    The static voltage or minute power is either capacitively coupled or induced
    from conductor to frame. The power should be canceled in theory, with the
    zip cord conductors direction of current being 180 degrees out of phase.
    Perhaps one of the conductors is longer at the beak-out point or one
    conductor is closer to the frame. Either way, it would appear to involve
    asymmetry.
     
  6. Dave M.

    Dave M. Guest

    "static voltage" Sorry, not the case unless measuring in D.C.
     
  7. jriegle

    jriegle Guest

    It is all relative. One of the conductors is usually tied to ground. If you
    measure voltage relative to ground, the induced voltage, due mainly to
    capacitive coupling from each conductor, is somewhere between the hot and
    ground.
    John
     
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