Voltage Appearing Between White Neutral And Gnd Wire ?

Discussion in 'Computer Support' started by Robert11, Apr 27, 2005.

  1. Robert11

    Robert11 Guest

    Hello:

    Have been trying to get some smoke detectors interconnected, and
    during the trouble shooting of the problem, I measured (with an analog
    voltmeter) the
    voltage between the white neutral, and the bare copper ground wire in the
    box.

    Was very surprised to see that it was about 2 V AC.

    Other than the fairly obvious reasons, such as bad ground connections in the
    service box for the neutral or gnd, or within the wiring chain itself, was
    wondering if anyone might have any other thoughts or opinions on this.

    Might as well add this: The smoke detectors were on line, and functioning,
    when I measured.
    The interconnect for the smoke detectors (the third, red, wire ) uses the
    white neutral (also)
    as it's return. And, measuring a few outlets around the house showed 0
    voltage between the neutral and gnd as one would expect.

    But, even if the smokes were dumping something on the white neutral, it
    being at gnd potential, would "sink" these voltages immediately, I would
    think, if the neutral was grounded well.
    So, what might be happening ?

    BTW: How "common" is it to see voltages of this magnitude between the white
    neutral and ground ?

    Thanks,
    B.
     
    Robert11, Apr 27, 2005
    #1
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  2. Robert11

    Guest

    On Wed, 27 Apr 2005 11:13:59 -0400, "Robert11" <>
    wrote:

    |>Hello:
    |>
    |>Have been trying to get some smoke detectors interconnected, and
    |>during the trouble shooting of the problem, I measured (with an analog
    |>voltmeter) the
    |>voltage between the white neutral, and the bare copper ground wire in the
    |>box.
    |>
    |>Was very surprised to see that it was about 2 V AC.
    |>
    |>Other than the fairly obvious reasons, such as bad ground connections in the
    |>service box for the neutral or gnd, or within the wiring chain itself, was
    |>wondering if anyone might have any other thoughts or opinions on this.
    |>
    |>Might as well add this: The smoke detectors were on line, and functioning,
    |>when I measured.
    |>The interconnect for the smoke detectors (the third, red, wire ) uses the
    |>white neutral (also)
    |>as it's return. And, measuring a few outlets around the house showed 0
    |>voltage between the neutral and gnd as one would expect.
    |>
    |>But, even if the smokes were dumping something on the white neutral, it
    |>being at gnd potential, would "sink" these voltages immediately, I would
    |>think, if the neutral was grounded well.
    |>So, what might be happening ?
    |>
    |>BTW: How "common" is it to see voltages of this magnitude between the white
    |>neutral and ground ?

    Smoke detectors do loop back to the panel to alert if there is a
    problem, you might be seeing this voltage.


    --
    The Eagle Nebula image release on Hubble's 15th birthday
    http://tinyurl.com/982nm (space.com)
     
    , Apr 27, 2005
    #2
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  3. Robert11

    Shel-hed Guest

    On Wed, 27 Apr 2005 11:13:59 -0400, "Robert11" <> wrote:

    >Hello:
    >
    >Have been trying to get some smoke detectors interconnected, and
    >during the trouble shooting of the problem, I measured (with an analog
    >voltmeter) the
    >voltage between the white neutral, and the bare copper ground wire in the
    >box.
    >
    >Was very surprised to see that it was about 2 V AC.
    >
    >Other than the fairly obvious reasons, such as bad ground connections in the
    >service box for the neutral or gnd, or within the wiring chain itself, was
    >wondering if anyone might have any other thoughts or opinions on this.
    >
    >Might as well add this: The smoke detectors were on line, and functioning,
    >when I measured.
    >The interconnect for the smoke detectors (the third, red, wire ) uses the
    >white neutral (also)
    >as it's return. And, measuring a few outlets around the house showed 0
    >voltage between the neutral and gnd as one would expect.
    >
    >But, even if the smokes were dumping something on the white neutral, it
    >being at gnd potential, would "sink" these voltages immediately, I would
    >think, if the neutral was grounded well.
    >So, what might be happening ?
    >
    >BTW: How "common" is it to see voltages of this magnitude between the white
    >neutral and ground ?


    It is very common to see a couple volts on the neutral. Often it is induced by
    nearby wires. High loads can cause voltage drops as well.
    Turn off the breaker and find out what is on the circuit. Unplug it and turn
    the breaker back on. Re-measure the voltage. The difference will probably
    disappear.
    Turn off the main breaker and tighten all connections on the breakers and
    neutral bars.
    Never use those push-in quick connect things on the back of wall plugs. They
    can often cause problems.
     
    Shel-hed, Apr 27, 2005
    #3
  4. Robert11

    Keme Guest

    Robert11 wrote:
    > Hello:
    >
    > Have been trying to get some smoke detectors interconnected, and
    > during the trouble shooting of the problem, I measured (with an analog
    > voltmeter) the
    > voltage between the white neutral, and the bare copper ground wire in the
    > box.
    >
    > Was very surprised to see that it was about 2 V AC.
    >

    [...]

    Don't be!

    It could be the smoke detectors, or just about any other appliance on
    the circuit.

    When you have load on the circuit (current moving), the voltage drops
    gradually from the power sink (electrical appliance) to the power inlet
    (where the circuit breakers are). Ideally this drop is linear along the
    power line, so if you are far away (along the copper) from the circuit
    breakers, a voltage drop of as much as 2-3 % of household voltage (i.e.
    7V on 230V or 3.5V on 115V) is not worrying. Still, if you're on 115V
    and this is the sole cause for the voltage, you may need to have your
    circuits checked by a professional. (If the drop is 2v on the neutral
    line it is likely to be the same on the live one. That makes a total of
    4V).

    Inductive loads (transformers, electrical engines) complicates this even
    more. They can both generate voltage (thats the inductive part) and
    shape the alternating pattern (away from perfect sine wave) so that some
    meters will give faulty readings. The latter applies more to digital
    meters, though. With an analog meter, be sure to set it to AC metering.

    There may be ground faults in the proximity (even in a different house)
    that lifts the inlet voltage. (I don't know if that's possible with
    fixed ground and neutral line. I live in an area where there's "floating
    ground", i.e. no "neutral" end, both power lines are live).

    The gist of it:
    Don't use the power lines to carry a controller signal, unless the
    signalling appliances are specifically designed for that. You risk
    overloading your detectors, getting electrical shocks (startup currents
    on e.g. washing machines can give a high voltage pulse on the neutral
    line) and creating ground faults.
     
    Keme, Apr 29, 2005
    #4
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