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Thread: Step and Touch Potential

  1. #1
    Join Date
    Jan 2011
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    Step and Touch Potential

    Trying to get a grasp on this concept.

    Is step and voltage potential only an issue during a ground fault occurrence? A colleague tells me that even if a system is not properly grounded (say your GES produces 20 Ohms), you can be killed by step voltage even without a ground fault situation. Does an underground neutral under operating under normal conditions create step potential? This does not seem correct to me.

  2. #2
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    Dec 2012
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    Placerville, CA, USA
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    To develop a potential gradient within the dirt requires current to flow. At some level that amount of current in the dirt implies a fault, even though it will not trip ODBC.
    That fault may be a design flaw or an open or compromised metallic conductor rather than a short circuit or ground fault.

    Whether the system is grounded or not, and what the earth electrode resistance is, is not a major factor.

    One example is an open service neutral. If you load only one L-N leg to avoid voltage problems, there can be tens or even hundreds of volts across a stretch of earth near the electrode or between the GES and remote earth.
    The GES is serving as the path for "normal" current to reach the POCO transformer secondary.
    POCO primary current flowing through the earth can cause similar problems.

    mobile
    Last edited by GoldDigger; 03-08-17 at 06:14 PM.

  3. #3
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    Quote Originally Posted by strap89 View Post
    Trying to get a grasp on this concept.

    Is step and voltage potential only an issue during a ground fault occurrence?
    Most of the time, swimming pools and farms are exceptions.
    A colleague tells me that even if a system is not properly grounded (say your GES produces 20 Ohms), you can be killed by step voltage even without a ground fault situation.
    How?
    Does an underground neutral under operating under normal conditions create step potential? This does not seem correct to me.
    Doesn't to me either. We are talking about less than 1000 volts aren't we?
    Once in a while you get shown the light
    In the strangest of places if you look at it right. Robert Hunter

  4. #4
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    Step potential is a very real danger re: downed power lines and voltage gradients across water can be deadly too, havent heard of any other situations where it can happen tho Im sure a few exist.
    Electricians do it until it Hertz!

  5. #5
    Join Date
    Feb 2003
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    Minnesota
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    Quote Originally Posted by strap89 View Post
    Trying to get a grasp on this concept.

    Is step and voltage potential only an issue during a ground fault occurrence? A colleague tells me that even if a system is not properly grounded (say your GES produces 20 Ohms), you can be killed by step voltage even without a ground fault situation. Does an underground neutral under operating under normal conditions create step potential? This does not seem correct to me.
    Your thread title includes "touch potential." This can happen with a solid system of energized conductors, neutral and grounds.

    Last week I got called to a 1958 house, wood framed, ell shaped ranch on a cement block basement, that I have almost entirely rewired over the last 30+ years. It has a 200 Amp 240 / 120 Volt overhead service with a rigid metallic mast on the northeast end, and the municipal metallic water service over on the southwest end (around the ell).

    The rigid mast is original, as is a bond of the mast to the service entrance grounded conductor. This is done with #6 between a water pipe clamp on the mast and a split bolt on the service neutral. The split bolt was left exposed (no tape) and, over the decades, slowly wore through the insulation of one of the service entrance energized conductors. The first fault came during last weeks windstorms shaking the overhead drop.

    Now, the power company transformer is two full spans plus the back yard drop away, easily 300 feet of #2 aluminum ACSR and #4 hard drawn copper. There is a ground rod (one) immediately below the mast, just outside from the Service Disconnect (and the Main Bonding Jumper - MBJ) in the basement.

    The grounding electrode conductor (GEC) runs from the service disconnect to a nearby 1/2" copper water pipe and runs easily 100 feet to the water supply entrance, out to a 10" metal municipal water main, a main that also services the next three neighbors to the north, neighbors that are electrically supplied by the same transformer. This metal water pipe path back through the neighbor's electrical services to the transformer, this electrical path is going to be about the same resistance as the path from my friend's 1958 electrical mast short, and out the service drop and down the secondary running between poles.

    I've just described a simple voltage divider. Service line to neutral short at the split bolt, with about half of the fault current going overhead to the transformer, and the other half going along the GEC and along the water piping system to the neighbor's and back to the transformer.

    This results in the Main Bonding Jumper raising about 60 Volts. This shows up as a "touch voltage" on the enclosures of the service equipment, and everything connected to the Equipment Grounding Conductors (EGC). A person with bare feet, standing on the basement concrete floor in the mechanical area will likely be able to feel this voltage on any metal surface connected to the EGC.

    The insidious part of this fault situation happens over on the "other side" of the electrical panel. The branch circuits supplied by the service entrance conductor that is not involved in the fault wind up with bursts of 180 Volts on them.
    Another Al in Minnesota

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