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Job of Lightning Rod (arrestor) in buildings, power stations, towers

Discussion in 'High Voltage' started by Willen, Apr 16, 2015.

  1. gary350

    gary350 Well-Known Member

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    If you set up an experement 2 toy houses on a table with a tesla coil to produce lightning. It is a game of chance that either one of the houses will ever get struck by lightning. But if one house gets struck by lightning it will most certainly catch on fire and be destroyed. Do the same experement again with a lightning rod on only 1 house the house with the lightning rod will always get struck by lightning first but there will never be any damage. With that in mind if your neighbor has lightning rods most likely his house will get struck by lightning and your house wont. Some people claim lightning rods do not attract lightning but experments prove a table full of toy houses the house with the lightning rod always gets struck first. Experements are not always realistic because neighbor hoods have hills some houses are higher than others, some houses are 2 story and some are 1 story, some houses have trees and some dont. A table full of toy houses and only 1 with a lightning rod the rod is definitely and attraction to lightning only because the lightning rod it the tallest conductor on the table of toy houses. A 2 story house with no lightning rods next to a 1 story house with lightning rods the 1 story house will get stuck because copper wire is a better conductor than wood unless the 2 story house has water pipes in the upstairs bathroom or a metal vent pipe on the roof. Lightning comes from the clouds and the closest CONDUCTOR to the clouds will get struck by lightning.
     
  2. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    Sonic boom of lightning is from the detonation of air faster than the speed of sound.

    Lightning rods do attract more lightning due to the higher E-Field levels due to geometry which in turn reduces breakdown threshold. But they do not generate any charge by themselves.


    I wonder how a sharper point makes it quieter than a smooth surface. Perhaps from surface area of sound pressure.

    Just recall the sound of 2 wire tips vs 2 smooth balls with equal charge size, discharge current and compare discharge noise levels.

    There is a chain of partial discharges in air which is the precursor of the major discharge. This is usually called the streamer.

    The purpose of a lightning rod is to raise the chance of discharge (above the surroundings beneath it) and to conduct and divert the path of current safely; when it does strike, to prevent further ignition of materials and safeguard inhabitants.
     
    Last edited: Apr 18, 2015
  3. Willen

    Willen Well-Known Member

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  4. dave

    Dave New Member

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  5. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    That only because you live in wooden houses :D

    It's pretty rare over here for houses to catch on fire and be destroyed by lightning, as they mostly aren't wood.

    Does make a mess of them though! :D
     
  6. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    It may not be a contradiction.

    I think Franklin is still correct for near field effects and the article is correct for far field effects.

    A large surface area has more electrostatic force on the flow of charges for far field but the breakdown voltage for a sharp tip is lower and thus greater gaps at the point of breakdown for a smooth surface. For this reason, EHV insulators use large smooth to torroids on top to lower the E-Field at the surface which they say would attract discharge more but the breakdown occurs at a lower voltage with the sharp discharge tips located above the wires and insulators.

    Experiment yourself. Using a key or wire with an old TV set you can start an arc at a greater distance with a sharp conductor tip than a smooth flat surface.

    Do not attempt unless you know what you are doing.
    If you have a spark plug wire handy on a V6 or V8 , remove a plug and insert a small bolt with a wire wrapped around it both pointing away from the socket. then if you have 30kV transient you should be able to get an arc with 20mm gap with the wire... but less ... maybe only 10mm or so with a smooth round bolt pointing towards smooth chassis.

    Sparkplugs must arc under 8~10 atmosphere's of pressure so breakdown voltage increases rapidly.
    The best quality will have a pointy platinum or tungsten tip to ensure reliable arc.

    I have done more sophisticated lab measurements, but these are just home experiments.


    FWIW, I have found 1kV/mm to be a reliable metric for wire arc for DC or ac-pk in air but for large distances BDV is more proportional to ‚ąödistance.

    Power meters have a blunt gap for 6kV breakdown inside.

    Arcs on AC have the same type of negative resistance as Triacs Diac's or gas tubes with a hold current threshold. but air or gas has a much slower rise time before ionization or turn-on. But the difference in AC is the grid follow-on current has a much greater potential of damage than the surge, so current limit or fusing is critical. I have seen huge 50mm holes burnt in PCB's because the Design Engineer was not aware of this effect using a gas tube.
     
    Last edited: Apr 18, 2015
  7. shortbus=

    shortbus= Well-Known Member

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    What hasn't been mentioned in this tread is the lightning rods "cone(zone) of protection". A single lightning rod on a building may not protect the whole building. I have a old barn with lightning rods, three of them, on the roof's peak. The roof is ~45 feet long. The cone of protection extends at a 45 to 60 degree angle from the tip of the rod.

    This link explains it, about 1/3 of the way down the page. http://www.atmo.arizona.edu/student...0/lect20_lightning_protection_structures.html
     
  8. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    Yes, Although I am not sure what affects the range from 45 to 60deg cone variation. My observations are consistent with this. e.g. 3m above the highest insulator spaced every 3m would be a 45 deg cone.
    Rising altitude is one factor that reduces coverage and breakdown levels as the CHinese have discovered with their 1.8MV transmission lines at different altitudes and also found the IEC recommendations were too conservative. ( The Chinese have the highest voltage UHV lines in the world)

    It seems there are also some streamer mitigating patents using torroidal wound protection at the top of the insulator ( a.k.a. bushing) in HV lines. using conductive polyester Litz wire bundled from ~0.1mm strands by Dr. Farouk at Quebec Hydro and also verified by Manitoba Hydro which have many variations such as those commercialized by Dr Farouk et al. http://www.lightningelectrotechnologies.com/Files/inhibitorbrouchure.pdf

    I'm not sure how to apply this to a barn, but you could always write to Dr. Farouk and ask.

    Unfortunately the exciting video by Quebec Hydro's Lab tests on You Tube is privately owned.
     

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