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Where does lightning want to go?

  1. Jul 6, 2011 #1
    Where does lightning "want" to go?

    I work on navigation equipment at airports, and we've had a few lightning storms recently... Every time there is lightning on (or near) the airport, we end up replacing circuit cards in our equipment. These are obviously not direct hits, because the damage would be much more evident.

    I understand that lightning occurs when a build up of potential (between cloud and the earth) becomes great enough to ionize the air and produce an arc to "equalize" itself.

    We've been having conversations about what happens when lightning hits the earth... An idea has been put forth that since our terrain is clay and is usually wet, that the lightning (or current, or energy, or whatever you want to call it) kind of travels around through the ground and might actually be using the grounding system to travel from the ground and enter the buildings housing our equipment. In other words, we're thinking that the nearby lightning is traveling through the earth, finding the copper and traveling through it and up into the building and taking out some of the circuit cards...

    Is this possible?

    One guy said that he has heard of people at other facilities (with similar terrain) who have actually disconnected their equipment from the grounding system because of this theory, and it has helped reduce damage to their equipment. It was also stated that places that have sandy soil (like Florida) don't have this problem because the lightning dissipates quicker since the loose, dry, sandy soil doesn't give a good conducting path for it to travel around in the earth and find its way to the equipment.

    Hence the question... Where does lightning "want" to go? Is it "satisfied" as soon as it hits the ground? One suggestion is that is it trying to "get to" the core of the earth (seems pretty unlikely to me since there would be too much insulation between the core and the earth's surface)?

    Disconnecting the grounding system seems like a bad idea since a direct hit would obviously turn all of our equipment into charcoal, but would it help with these transient surges that are happening when lightning strikes the vicinity? It's obviously a bit of a gamble... but the cost of replacing dozens of circuit cards over a season is quite high. If this works, the risk might pay off in the end. Especially since a direct hit would likely cause catastrophic damage anyway...

    Any lightning experts out there?
     
  2. jcsd
  3. Jul 6, 2011 #2
    Re: Where does lightning "want" to go?

    Keep in mind that there are charges flowing in that ionized column of air. They will dissipate chaotically throughout the ground as they interact with each other and the earth.

    It is certainly possible that some are dissipating through the grounding equipment. Its also possible that the electric field is inducing some currents in the equipment also.
     
  4. Jul 7, 2011 #3
    Re: Where does lightning "want" to go?

    Yes I've seen this happen in marl (which is like clay) areas.

    At the instant of strike and for a very short period after, the local ground potential (dare I use that word without a long argument?) is raised substantially by the strike.

    I no longer have pictures but one particular incident comes to mind of an ISDN line blown off the wall by the strike at an isolated farm.
    A large screen TV was also damaged at the time, although disconnected except for the commn ground with the phone system.

    go well
     
  5. Jul 7, 2011 #4
    Re: Where does lightning "want" to go?

    Exactly right. In addition, your system acts like a shortwave antenna with a ground connection. The EMP wave from lightning is being received by this "antenna," and if you disconnected the ground, the peak voltage/current received by the "shortwave radio" would be much smaller.

    But then you have a problem. With disconnected ground, if lightning actually struck your building directly or within a few tens of yards, you'd get a lethal hazard. Without grounding, the lightning discharge can leap out of the electrical system and pass through human bodies during it's "search" for a ground path. Parts of the grounding system are provided for human safety, not for circuit protection.

    If you can figure out which grounds are irrelevant for safety concerns, then perhaps you could install a spark-gap in series with that conductor. Put a ~100K resistor across it to drain any electrostatic buildups. If the spark gap is wide enough so it doesn't fire during distant lightning strikes, then your boards might not fry so often. But if your building gets struck, the gap provides a short circuit to ground, and the potential of your disconnected "grounding" network would only rise to ~couple of kilovolts before the spark gap grounded it out again.



    If they do it wrong, and their building gets struck, someone could be killed. It's probably a low probability scenario. But imagine the lawsuits involved! Me, I wouldn't touch lightning-safety grounds even with somebody else's pole!
     
  6. Jul 7, 2011 #5
    Re: Where does lightning "want" to go?

    Thank you for the helpful feedback. I hadn't thought too much about the safety bit. Good point. Discussions are ongoing, so we'll see what they decide to do. Nice to know that this does in fact happen other places...
     
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