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A voltage to ground mystery.

Discussion in 'Electronic Basics' started by Glenn Ashmore, Mar 28, 2005.

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  1. This is more of an electricity question than electronic but: We just
    finished running water and power to a new construction site. Water is in a
    copper pipe run about 1,000' through the woods burried about 2' deep that
    just ends in a 2' vertical piece with a faucet on top. Power runs parallel
    to the pipe about 100' away to a transformer on a pole at the construction
    end.

    The mystery is that there is a 25-30VAC difference between the faucet and
    the mud puddle below it. Touch the faucet without wearing rubber boots and
    you get shocked! I can see how a current might be induced into the pipe as
    it runs along the power line but how can there be a potential between the
    ends of a 24" copper pipe???

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  2. John Fields

    John Fields Guest

     

  3. Given the physical situation you describe and the way
    power *should* be connected and grounded, the AC
    voltage you report should not occur. Something is
    very wrong. I suggest you call whoever is responsible
    for that power installation to fix it, and if they will not or
    are unable to discover what is wrong, get an independent
    qualified electrician to look at it.

    There may have been another power line contacted by
    the pipe when it was laid. The situation could become
    more dangerous.

    Do not use your hands as "got AC volts" testers anymore.
     
  4. There are two difficulties with that analysis.

    1. The currents flowing in the power cable should be somewhat
    well balanced and they flow within an inch or two of each other.
    The conductors carrying those currents will have some degree
    of twisting, tending to cancel when considering the net coupling
    to any single parallel conductor. Magnetic coupling coefficients
    sufficient to effect a 30/240 stepdown are highly improbable.

    2. The copper pipe may be in the same trench, but it should be
    separated by some distance. (I forget the code requirement,
    but enforcing it is part of the reason an inspection is required
    before the trench is filled in.) Unless the pipe is jacketed, (a
    condition not stated by the OP), the fill around it will further
    serve to reduce the coupling from nearby power conductors.

    As I stated in another post, the OP's observations could easily
    be due to an unsafe condition that should be investigated by
    a competent person on site, who can discover the additional
    facts necessary to explain and correct the problem. This is
    true regardless of any debate that might occur regarding the
    magnetic coupling. I should hope that Mr. Fields is able to
    concur with that view, even if he disagrees with my points
    about magnetic coupling.
     
  5. John Fields

    John Fields Guest

    ---
    If it's not magnetic, then it's either capacitive crosstalk or
    leakage.
    ---
    ---
    _Able_ to concur, you pompous ass? I'm able to do whatever I _choose_
    to do, with or without any prompting from you. As a matter of fact,
    I'd prefer not to be associated with you at all so, in the future,
    stand on your own two feet and don't involve me in posts which require
    rubber-stamping validation of your views. Glenn Ashmore isn't stupid,
    and whether I _choose_ to concur with your view or not, I'm sure he'll
    consider your view and do whatever's required to keep from killing
    himself or someone else.
     
  6. Chris

    Chris Guest

    I had a similar problem once with a ground level bathroom. When starting the
    shower, no problem but when you turned it off you got a slight tingle
    (shower floor then being wet). Rang the local electrical contractor who said
    it was not an uncommon problem for that area and there was no answer for it.
    He mentioned that it may have something to do with the clay soil - if that
    is of any help.

    I as well, couldn't see the difference in potential between the taps/faucets
    and ground.

    Cheers.
     
  7. Well this is in the Piedmont of Georgia so the pipe is definitely burried in
    1,000' of red clay. :)

    The thing I can't figure out is the copper pipe comes out of the ground
    right at the puddle so it should be well grounded but if you use a VOM
    between the faucet and a nail stuck into the mud a foot from the base it
    reads 25V!

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     

  8. To determine what is going on from a distance is difficult,
    without knowing a few more specifics, so maybe you could
    answer a few questions to help narrow it down.

    1. Does the voltage you measure between that faucet and
    "earth" (the puddle, for instance) vary significantly as you
    add or remove load from your construction pole? The no
    load case is best for distinguishing magnetic coupling, being
    pretty much ruled out it the 25-30 VAC occurs without any
    load at the far end of that power run.

    2. Does that voltage vary if you load it, for example with an
    low wattage incandescant lamp? (You would connect the
    lamp between the faucet and a good local ground. Under
    ordinary circumstances, that faucet should be a good ground,
    but you will have to find another.

    3. I presume there is a power pole at your site, and that you
    have a small breaker panel there, and that there is a staked-
    into-earth ground connection right there. Is that true? If not,
    what is at the site end of that 1000' power line?

    Some people here have implied that what you see is normal
    or harmless. In the face of uncertainty about what is really
    going on, and not knowing whether the coupling mechanism
    could get better or not, I believe prudence calls for leaving
    that exposed faucet alone, perhaps even covering it, until a
    solution or convincing and harmless explanation is found.

    By the way, my hope regarding concurance, (in my 8:57
    post, poorly received by Mr. Fields) is about avoiding
    needless confusion about the hazard you face. Of course
    you can and will make your own decisions, but I assume
    your question appeared here because you felt you could
    use or needed some technical advice. If that means you
    must choose and pick what to believe without knowing
    for sure, I urge you to err toward safety.

    What is most odd about your situation is that buried
    conductors in contact with earth are often considered
    to be good ground connections. Seeing 30 VAC on
    a conductor with 1000' of earth contact means either
    a very dry, high resistance connection or something is
    driving it with some ability to drive power into it. If
    that something is an errant and now poor connection,
    it could become a better connection in the presence
    of water or after some motion, say a truck running
    over the trench where the illicit short is. If there is
    such a connection, able to deliver power to an earthed
    copper rod, it takes little imagination to to see either it
    getting better or the earthed rod getting more poorly
    connected so as to increase the AC voltage. It takes
    only 5 to 10 mA in the right place to upset a normal
    human heart rhythm, (and less for some people), so the
    situation you describe is one dangerous enough that I
    would not tolerate it if anybody I cared about was to
    exposed to the hazard.
     
  9. And the copper pipe is not isolated from the clay in any way?
    Is it covered with some kind of laquer which may be invisible?
    You could do some experiments to find out more about this problem.

    Disconnect the cable from the mains so it carries no voltage anymore,
    then measure again. If the voltage on the pipe disappears we know that
    the voltage comes from the cable somehow.

    Try connecting a 1k resistor between the pipe and the nail in the ground,
    then measure the voltage again. If it is still 25V you have a dangerous
    pipe there, capable of delivering current as well as voltage.

    Check up the possibility that the pipe is making contact to some mains
    wiring under ground or before it goes into the ground. The voltage must
    come from somewhere. And it is very unlikely that it is done through
    induction only, for reasons others have explained.
     
  10. John

    John Guest

    Right on!
     
  11. Rich Grise

    Rich Grise Guest

    Don't forget the 900' of pipe running parallel to the hiline upstream
    of the transformer.

    Cheers!
    Rich
     
  12. Glenn Ashmore wrote:
    (snip)
    But that reading tells you nothing about the isolation between the
    pipe and the point a foot away. By ohm's law, if there were a
    milliohm if resistance between the two points, that voltage would
    imply 25 amperes passing between them. If the resistance were 1 meg
    ohm, that voltage would imply 25 microamperes between them. The real
    resistance is almost certainly between these limits, so the current is
    somewhere between those values. Your problem is not only to find a
    source of 25 volts AC, but a source that can support a significant
    current load.

    In the mean time, you may be able to lessen the shock symptoms while
    you search for the cause by placing a few short ground rods around the
    puddle, and tie them all to the pipe. This will increase the load
    current on the pipe, but it will lower the drop across that particular
    segment of the circuit.
     
  13. Chris

    Chris Guest

    Another idea.

    Have you tried to connect a wire between the faucet and the base of the
    pipe?

    You may have plumbers tape insulating the faucet from the pipe.

    Cheers.
     
  14. Kinda frightning when you do the math! That could kill! There can't be
    more than a couple of milliohms resistance between the ground and the
    faucet. That means there must be a lot of current.

    The pipe is brand new 1 1/4" soft copper tube running from the meter. I
    burried it myself with a Ditchwitch and put a tee with a 1/2" temporary
    stand pipe at the end for use during construction. All the joints are
    soldered. No problem until the power company came in last week to plant
    their poles and hang line. The line is about 50' to one side and the
    transformer pole is back about 100'. Single phase service so there is one
    insulated wire and a bare cable to the transformer.

    I can see how that arangement can induce a current in the pipe but I would
    think it would bleed off into the ground. I will call the power company
    tomorrow. It does not sound like anything I want to fool with.

    --
    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at: http://www.rutuonline.com
    Shameless Commercial Division: http://www.spade-anchor-us.com
     
  15. Steve Dunbar

    Steve Dunbar Guest

    Glenn Ashmore wrote:

    ....
    ...

    Ouch! Try doing a Google search for "stray voltage."
     
  16. Rich Grise

    Rich Grise Guest

    Yes, this is the best suggestion yet. :)

    I was getting my brain all in a bunch here: "What's the resistivity
    of "Good ol' Georgia Red Clay" and stuff -

    But "check with somebody who knows what they're talking about" is
    probably the wisest option in this case.

    Good Luck!
    Rich
     
  17. A couple milliohms between that bit of ground and the faucet? What
    makes you say that? I have worked very hard with a couple thousand
    feet of buried copper to get the ground resistance (whole Earth to
    copper resistance, not resistance from some small patch of ground to
    copper) below 5 ohms.
    Are you saying that this service buried a bare conductor and an
    insulated conductor along side the pipe? If the service has a high
    leakage current from the primary side to the supposedly isolated
    service, it may be dumping quite a bit of current into the Earth
    through that bare neutral wire. The power company should check that
    out at the pole, to determine how much AC current it is passing to the
    neutral with no load connected at your end. This may well be the
    problem. And if it is, they would want ot know about it, because it
    is an unmetered power consumer they are carrying.
    Agreed. It ain't normal.
     
  18. You meant ohm instead of milliohm here, didn't you?
    I saw it before but didn't say anything because it had a positive effect,
    he realized how dangerous it is. (25V=25A*1Ohm)
    Maybe you made him think and say that yourself, see above.
     
  19. Fred Bloggs

    Fred Bloggs Guest

    The power grid is circulating current through the GND returns at the
    two different points of connection to the grid- one at the origin of the
    copper pipe and the other at the construction site. You have created a
    nice short-circuit with that pipe and the circulating current can be
    very large. You will have no option except to install a section of
    non-conducting plastic pipe at the construction site, make the faucet
    and its T-section riser plastic also.
     
  20. My readings of the OP's description gave either 1000', 100' or 900'
    of parallel pipe and power. My remarks apply to any of them. For
    sake of discussion, suppose the power flows thru a pair of wires,
    one located 11 inches from the pipe and the other 12 inches. (an
    assumption corresponding to Romex with about 2 inch diameter)
    For the moment, assume they maintain these distances along their
    length. (no twisting) Assume a current +I at 11 inches, -I at 12.
    The magnetic field in free space encircling one wire at radius r is:
    B(r) = u0 * I / (2 pi r)
    for an infinite straight wire where u0 = 1.26e-6 and r is in meters.
    (Except near the ends of the run, this *closely* approximates the
    situation given the assumed geometry.)
    Evaluating for +I at 11/39.37 m and -I at 12/39.37 m, yields
    B = (1.26e-6 * I / (2 * pi)) * (39.37 / 11 - 39.37 / 12)
    B = I * 5.98e-8
    Considering I to be the peak value of a 60 Hz AC current, and
    so taking B(t) = sin(2 * pi * 60 * t) * I * 5.98e-8 , then
    d(B(t))/dt = 2 * pi * 60 * cos(2 * pi * 60 * t) * I * 5.98e-8
    = I * 2.25e-5 cos(...) Tesla/Second
    This converts directly to Volts/Meter induced along the pipe.
    (Doubters can start from F = q V x B, noting that the B field
    changing in and out w.r.t. the wire pair is equivalent to the
    V x B term. This is too basic to be spelled out here in detail.)
    Taking the parallel run as somewhere between 30.5 and 305
    meters, the induced peak voltage is 685 uV to 6.85 mV per
    Amp of current flow.

    With any twisting, that number will go down. If a twist were
    applied to one end and the intermediate cable were follow in
    proportion along its length, the coupling would be multiplied
    by sinc(a), where a was the twist in radians.

    Unless they are using thousands of Amps at that construction
    site, the 30 VAC observed by the OP clearly has to be due
    to some other effect or a markedly different geometry.

    Some may opine or proclaim that only the integral form
    of the induction law can be validly applied, or that twist
    could be non-uniform, or the spacing may vary in that
    trench. Except for the latter, none of it changes the
    result because my use of the differential form already
    assumes the worst possible return path (whereas the
    real return path is coincident with the pipe, reducing
    the net coupling between earth and pipe). Twisting
    of any kind can only reduce the coupling; uniformity
    simply causes more reduction, on average. If the OP
    buried separate conductors willy-nilly with respect
    to the pipe, that could be a problem. In Washington
    state, that would never pass muster with the inspector
    and I doubt local codes on this issue vary much.

    If the OP's problem arose from mixing the power and
    pipe badly, the result shows (in part) why that code
    exists, and he has some digging to do. But I think it
    much more likely that there is an exposed portion of
    a hot power conductor leaking *a lot* of power into
    ground and the pipe. That is an unstable situation,
    dangerous as I've stated, and one the power company
    will want fixed, if only because their meter is going to
    be downstream from a ground-heating fault.

    Given the OP's assertion that the 1000' pipe is buried
    in red clay, I don't see any way for it to be anything
    but a fault. Driving a well grounded conductor like
    that to 25 VAC would take more power than even
    an accidental air core transformer could deliver.
    You bet!
     
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