Surge suppression by tieing knots in the line

Discussion in 'Highly Technical' started by rimshaker, Jan 22, 2004.

  1. rimshaker

    rimshaker Senior member

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    Never thought about this one. We all know that current is what causes damage, not voltage. And tracing back to basic theory, a changing magnetic field induces current. Power surges can be avoided by simply disrupting the huge instantaneous magnetic fields created in the first place.. even by lightening strikes. How? Tie a knot in the line? I can see how this simple fix might work, never tried it before.

    Linky

    Induced currents travel through a relatively straight line along with the magnetic field according to the right hand rule. So a looped knot would disrupt this field, effectively cancelling most of it since the field loops around itself. Is this line of thinking correct?
     
  2. Mday

    Mday Lifer

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    it depends on how you do it though. it's not just like looping like they do with cable tv lines (that's a drip loop). you have to consider what the induced magnetic field will be doing. and yes, the indirect effects of a bolt of lightning is worse. the induced magnetic field can rip out rebars from thinner reinforced concrete.
     
  3. PowerEngineer

    PowerEngineer Platinum Member

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    Hmmmm.... That's a new one on me, but I can see where it might have some merit.

    The surge impedance of any line is the square root of its inductance divided by its capacitance, and electromagnetic waves travel most readily down a line where that surge impedance doesn't change. A point of changing impedance is a discontinuity that causes a partial reflection of the wave back towards its source. As an example, the end of the line is a surge impedance jump to infinity and the whole wave is reflected back (which means the wave voltage at the open end doubles!) This is also the reason why you want to use terminators on the ends of coaxial cables. Open-ended cables will reflect back the signal causing poorer picture quality and ghosting (and similar things happen for poorly made connections that have higher impedances than the surge impedance of the coax).

    Knotting the line gives that part of it a higher inductance (think of the knot as a coil with a couple of turns). That means two surge impedance discontinuities (from line to knot, and from knot back to line). It seems to me (too lazy to resort to doing the math) that this is bound to reduce the magnitude (voltage and current) of a surge passing through the knot because some will be reflected back. However, I'd guess that the reduction would be small.

    No harm to knot the power cord, but don't get rid of your surge protector! :)
     
  4. Geniere

    Geniere Senior member

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    Many audiophiles have known that balling up their power cords will decrease or prevent local powerful radio stations from being picked up and heard on their stereo systems. As Powerengineer stated, it?s the inductance that does the job, and a knot would not be the best way to do it. As far as protecting against lightning strikes; fagedaboudit.
     
  5. rimshaker

    rimshaker Senior member

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    Well.. yes, but that's in terms of picking up EM waves through unintentional antennas (long wires). This is about current directly traveling through a line. I dont' understand why people just seem to give up when it comes to lightening. I mean, true, it's immensely powerful, but it doesn't contain infinite energy or anything. It CAN be stopped in its tracks. Maybe by something as trivial as a big knot. Guinea pigs anyone? :p

     
  6. ZeroNine8

    ZeroNine8 Member

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    I've seen an 8 inch coil of probably 8 gauge wire (maybe 1/10 inch thick) with extra heavy insulation (3/8" overall dia) wrapped a dozen times and zip-tied to a mounting board in 4 places by heavy zip-ties completely rip through the ties and explode off the mounting board due to a lightning strike. The electric fence charger it was connected to also exploded and sent pieces of it a good 30 feet across the barn. While in theory this use of coils can help mitigate excessive current, something like a direct lightning strike to the line would pretty much just overwhelm such a simple surge suppression method. In the case I mentioned, the lighting had already traveled through probably 1/2 mile of electric fence from the point where it struck to the charger and past several other lighting arrestors.
     
  7. rimshaker

    rimshaker Senior member

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    Yea, for all practical purposes, lightening can still eat through any device man has created thus far.

    But I predict that even direct lightening will finally be kept in check sometime in the future.... perhaps by technology and physics borrowed from superconductors.
     
  8. glugglug

    glugglug Diamond Member

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    Doesn't this increased inductance mean more power will be disappated in the cord since you are using AC not DC power and the cord will get hot?
     
  9. Mark R

    Mark R Diamond Member

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    Not to a significant extent - Inductance stores energy (to be given back later), whereas resistance converts the energy into heat. The inductance won't actually dissipate a surge, but will slow it down, so that the surge is less intense but lasts for a longer time. This may be all that is required (if you can bring the surge voltage down from 10,000 V to 1,000 V then your equipment has a much better chance of survival).
     
  10. Mday

    Mday Lifer

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    i'd just use a ferrit choke. =P
     
  11. drag

    drag Elite Member

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    The way I figure it lightning travels a mile or so with only wet air as the conductive medium.

    A simple loop or knot of wire isn't going to stop it, however if you make it so that the lightning might have a easier path thru somewere else, then that may work.

    Like have a few coils of wire, and directly before the loop have a collar of metal or bunch of loops of bare wire wrapped around the wire and have that go directly to a ground (or into the ground or whatever).

    That way it would be a easier path for the lightning to blast thru the insulator then to fight the inductance of the coil loop. And since lightning will follow the path of least resistance then that may have a good chance to protect your equipment.
     
  12. glugglug

    glugglug Diamond Member

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    It's not the surge that I'm thinking is a concern with the inductance. It's the steady state (well relatively steady, it is AC after all...) that is the issue.

    If your AC current function is say 10Asin(t * 60/2¶ /sec), the rate of change in current is
    300A/¶ cos(t*60/2¶ /sec) /sec. This is what I am thinking would melt the cord if you actually managed to give it a significant inductance.
     
  13. NeoPTLD

    NeoPTLD Platinum Member

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    Obviously they're no electrical engineers. How much inductance did these people think they'll get out of a single turn air-core coil?

    Small enough to be negligble for a line frequency circuit.

    I read the article and it's nothing but speculations over coincidental event with no factual support.

    Air cores and ferrite cores are effective at snubbering high frequency(in MHz range) interference. It doesn't touch 60Hz.

    Some appliances and power strips already have a knot tied inside, but not for some speculated purpose. It's simply to prevent cord from being yanked out easily.
     
  14. CTho9305

    CTho9305 Elite Member

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    Some quick googling tells me that power surges last microseconds, so they are in the MHz range.
     
  15. PowerEngineer

    PowerEngineer Platinum Member

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    Both are true.

    The inductance added by the knot is very small and therefore negligable for the 60 Hz waves we intend to be there. None of those claims that the knot would cause additional heating or somehow affect the quality/quantity of power delivered to a load make any sense.

    Lightning (either through a direct strike or through induction) superimposes a very short but very large spike on top of the 60 Hz wave. The spike has a very steep wave front (virtually vertical compared to rate of change on the 60 Hz wave). If you break the spike down into the equivalent series of sine waves (Fourier transform) you'll get lots of very high frequency ones. The higher their frequency, the more impedance they'll encounter due to the knot's small inductance. So the knot should tend to make the wave front less steep and reduce the maximum height of the spike. This is just another way of looking at what I described in my earlier post.

    So (IMHO) knotting the cord doesn't hurt (normal 60 Hz use) and should theoretically help reduce spikes... but the help is almost certainly negligable. You still need that surge protector.
     
  16. NeoPTLD

    NeoPTLD Platinum Member

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    You already have coil w/ inductance grater than stupid knot.

    When the power comes in, it goes through a set of knot and those are coils in main 150 or 200A circuit breaker for magnetic trip.

    These coils have several turns around some type of core and these have inductance far greater than a knot in the cord. Before this breaker, you have another set of coils in the power meter.

    And... most computers and electronics have toroidal filter coils in the input.

    This stupid knotting thing is redundant at best.

     
  17. arcas

    arcas Platinum Member

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    There are surge suppressors on the market which are based on large inductors rather than MOVs. I believe surgex.com sells these kinds of suppressors but there are no doubt others (in fact, I recall reading about one named "The Brick" in the back of old 'Computer Shopper' magazines years ago).

    I've never used one though it would be tempting to install something like that at the breaker box.

     
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