Grounding Wires

Discussion in 'General Electronics Chat' started by goddard1824, Jun 22, 2006.

  1. goddard1824

    Thread Starter Member

    Jun 17, 2006
    10
    0
    Hello again everyone,

    I was reading up on the electrical safety section of the site and I became a bit confused with the science behind the grounding wire of a telephone pole. Technically, if I were to touch the grounding wire, wouldn't the voltage also use me as a connection between the wire and the ground? Apparently not according to the site. Could someone please ease my confusion?

    Thanks again!
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    When you grab the grounding wire, you - and the that section of wire between your hand and ground - form two resistances in parallel. Resistance value of the grounding wire is a fraction of an ohm. Resistance value of you is somewhere between 5000 and 20000000 ohms. Most of the current will go through the grounding wire, and almost none through you.

    Note also that there is no voltage across, and no current through, the grounding wire in normal operation. The grounding wire only conducts fault current.

    Many folk will confuse the grounding (earthing) and grounded (neutral) wires.
     
  3. dougp01

    Member

    Dec 6, 2005
    27
    1
    There is a wire strung between tops of poles and there is a ground tied to wire at every pole. An old app note I have for MOVs states that a typical lightning strike can travel down 10 power poles before all the ground wires can successfully dissipate the voltage of one lightning strike.

    In product safety, the goal is to keep the leakage (touch) current on a chassis below 3.5 mA. You know about this if you ever got a little buzz from touching the electric range and the kitchen sink at the same time. This test holds true even when there is a single fault located anywhere within the product. There is a generalized "human body model" which is approximately 1500 Ohms placed in series with the test. Just as thingmaker3 said, the parallel ground path must be very low, typically less than 1 Ohm. Not only that, the ground wire must able to withstand the maximum fault current possible without opening.

    -Doug
     
  4. bob/bergelectric

    New Member

    Jul 7, 2006
    7
    0
    thats exactly it. The grounding conductor is not a current carrying conductor except during a line to ground fault.Ex:lightning strikes or hi voltage phase conductor contact.
     
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