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Lightning Rod attached to railing of my balcony

  1. Aug 10, 2009 #1
    Hi @ all,

    I absolutely love thunderstorms & lightning and I'm used to go out on my balcony to watch them. Our (11-storey) house is equipped with a lightning rod, which, as I noticed today, is attached to the metal railing of my balcony (on the 5th floor).

    My question now is: What happens when a lightning strikes our house at the top while I'm having my hand on the metal railing of my balcony? Is there a danger of being electrocuted by the current which discharge through the lightning rod?

    Greetings

    Till
     
  2. jcsd
  3. Aug 10, 2009 #2

    mgb_phys

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    Does the lightning rod only attach to your balcony, or does it continue to the ground and makes a connection with your balcony on the way down?

    If it just connects to your balcony - that is bad, it's not going to help and may slightly increase the chance of your balcony being hit.

    Assuming it is fitted properly, the lightning rod is connected to ground and just hooks in your balcony as well, this should reduce the chance of your balcony being hit.
    Even then in the extremely unlikely chance of the lightning rod being hit you can still be injured by the lightning rod. It's not unknown for the ground wire to get hot enough to start fires, boil surface water or for corroded sections to melt.

    But I wouldn't worry about it unless you are on the very top of the only tall building (or if you have annoyed any major gods recently )
     
  4. Aug 10, 2009 #3
    Indeed, I have... But I hold them all in equal contempt, thereby hoping to play off one against the other.


    But, back to the question:

    The lightning rod simply has a connection to my balcony. It continues to the ground.

    But my question was not what happens if my balcony gets a direct hit (however improbable that may be), but rather what happens to me when the top of the building gets hit and the lightning is -as expected- discharged into the ground through the lightning rod while I'm having my hands on the railing of my balcony.

    If I remember my school physics classes correctly, then there should not be any current flowing through the balcony's railing and through my body into the (concrete) balcony floor, right?
     
  5. Aug 10, 2009 #4

    PhanthomJay

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    From a practical point of view, stay in an enclosed shelter during lightning storms! Stepping onto an open or partially enclosed balcony during a lightning storm, with or without a lightning rod, even a well grounded one, doesn't sound like a good idea. And I wouldn't consider concrete as an insulator against lightning. If you are touching the railing and the lightning hits the rod, all bets are off regarding your safety, unless the gods are with thee.
     
  6. Aug 10, 2009 #5

    russ_watters

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    It is often said that electricity finds the path of least resistance. This isn't correct. What actually happens is that electricity follows all available paths in proportion to their resistance. What this means is that because the grounding cable has vastly lower resistance than your body, it will pass vastly more of the lightning than your body. If you're standing in water and soaking wet due to the rainstorm, it could become problematic, but if you are dry and wearing rubber shoes, very little current will pass through you. However, I wouldn't recommend trying to find out just how much will pass through you via experimentation.
     
  7. Aug 10, 2009 #6
    If it were me, I would not be holding on to the railing during a thunderstorm. Even though it seems logical that the lightning would hit the rod and continue to ground without affecting you, I would not bet my life on it. Lightning can be unpredictable. It could for example strike the top of your head, travel through your body to the railing and then through the rod to ground.
     
  8. Aug 10, 2009 #7

    mgb_phys

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    In theory the lightning conductor connection to ground keeps the balcony at 0v and the metal of the balcony is a at a single potential so there is no potential difference to cause a current through you.

    But when you are talking about millions of volts and 1000s of Amps then all those little - "assuming a perfect X" bits in high school physics can come back and bite you.

    If the lightning is near enough to be striking the building I woudln't be going out onto a metal balcony 11 floors up.
     
  9. Aug 11, 2009 #8
    That's the answer I was waiting for! Thanks for the clarification.

    I'm well aware of the danger of going out during a thunderstorm in a 11-storey building. However, my grandmother could have given me that advice. The reason why I posted the question in this forum is that I wanted to hear a physical explanation.
     
  10. Aug 11, 2009 #9

    vanesch

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    Indeed, I find it even strange that they connected the balcony to the lightning wire, because I would presume that that makes things worse. I think it has been a quick trick to "ground" your balcony (normally all metal structures should be grounded, whether they are part of an electrical equipment or not).

    But in the event of lightning striking the lightning rod (which it is designed for, and hence it will *attract* lightning), I would prefer to be on a balcony (if I have to be on one that is - the advice not to be there during a lightning storm is good advice) that is NOT connected to the rod!

    The reason is, as others said, the very large current that has to flow through such a rod. Now, if the potential on top of the building is 20 000 V, then you can assume this potential uniformly distributed along the cable, if the impedance is uniform. The impedance is not just the ohmic resistance! For short times, it is more likely to be the self-inductance of the cable. It means that your balcony will be, for a short time, at a potential which is 20 000 V times the ratio of your height to the building's height. This is why it is strange to understand that the balcony was connected to the lightning rod. I think there's double usage, and that they just did this to "ground" the balcony (when there are no lightning storms).
     
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