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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. Willen

    Willen Well-Known Member

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    Before I thought that clouds are positive and earth is negative always. And a lightning rod (connected to earth) is negative always. And I thought when positive cloud tries to discharge its charge around earth near of building, towers etc then Lightning Rod will bypass the surge, because the rod is a good earth (negative) than building or towers. And also because it is near to cloud than earth.

    But I read in a school book and according to the book- clouds can be positive or negative both. If positive cloud came to near of earth then trees or buildings produces negative charge. If negative could came to near of earth then trees and buildings produces positive charge. Then lightning strike will happen between the positive and negative. Then how the Lightning Rod acts? Confusing!
     
  2. nsaspook

    nsaspook Well-Known Member

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  3. JimB

    JimB Super Moderator Most Helpful Member

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    The lightning rod is connected to earth, it just produces the opposite polarity of the cloud.

    JimB
     
  4. dave

    Dave New Member

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  5. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Lightning rods hopefully provide a preferred path to earth due to their pointy shape.
     
  6. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Why do you care which direction the 40 to 150KA flows during the actual strike? Do you think that the lightning rod cares?
     
  7. atferrari

    atferrari Well-Known Member

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    Maybe to decide quickly on what direction to run away? :p :D :p :D
     
    • Funny Funny x 2
  8. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I seem to recall it goes both ways anyway :D

    Something like a small flash going up, then the main charge coming down?.
     
  9. gary350

    gary350 Well-Known Member

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    I saw a nature show on TV about 2 years ago about a group of people hiking in the mountains high elevation about 10,000 feet plus. Thunder storms would come up fast and it was very important to stay dry and not get struck by lightning. When they saw a storm coming their way they put up their 1 man tent and a small lightning rod about 6 feet tall. Video showed 75 mph wind, rain and lightning that lasted about 5 to 10 minutes then it was over. Wow pretty darn intense. Even though it was summer the rain was ice cold and the tiny lightning rods would take several lightning strikes. I dont remember the details of the TV show how the people manage to lay, sit, squat, curl up in a ball right next to their lightning rod and not get fatally killed to death. :) The guy with the camera asked them questions about did they get shocked even a little bit and they all said no. Interesting, good example of faraday cage working.
     
  10. atferrari

    atferrari Well-Known Member

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    My closest experience: crossing what is known as the Drake Passage, South of Argentina, in our journey to Antártida, PM hours in stormy weather, I was on watch in the bridge when lightning struck our vessel, directly on the foremast on starboard side.

    The sudden and tremendous noise plus a thick "vein" of blinding light running from its top to main deck, got me frozen and unable to react for a several seconds.

    Every time I read or hear about a SWAT group using stun grenades to disorient the bad guys, I recall Nature doing that to me.

    Lucky crew, lucky vessel, lucky me; we were in open sea. Oh sí!
     
  11. Willen

    Willen Well-Known Member

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    Experiences of Gary350 and Atferrari are amazing! In some small house they have metal roof. There's zinc coted galvanized thin metal as a roof. In case of my house all the metal sheets are electrically connected to each another (I tested continuity) and now I am using the metal roof as a batteryless crystal radio to listen AM station. I also can light a bright red LED little from the metal roof antenna.
    Once I thought to connect the metal roof with thick wire to the earth to make house safe from lightning. Would it be a good idea?
     
  12. gary350

    gary350 Well-Known Member

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    It sound like your metal roof is producing a small voltage. That small voltage might be enough to keep away termites and ants. A wire to ground would be good for lightning but it will short out your battery roof. If it were me I would run insulated wire from roof to ground but I would leave the insulation on the wire at the metal roof so not to short out the battery roof. If the roof gets struck by lighting the high voltage will have no trouble jumping through the wire insulation. You dont need a large wire, #12 copper will do fine. Electricity always takes the path of less resistance.

    Real lightning rod wire is TEN #18 copper plated steel wires woven in a hollow tube shape. It looks like 5/8" garden hose from a distance. It runs across the point of the roof to all the lightning rods, down the side of the house to ground. When lightning strikes 1 of the lightning rods the woven wire turns bright WHITE it looks like a photo flash and looks like it is 4" diameter solid LIGHT. It is amazing there is never any damage to the wire.

    You do not need a heavy wire for lightning. I use to have a lot of fun attracting lightning to my back yard during thurderstorms. I set up 3 Estes stick rockets with dead man switches to be launched 100 ft away at the house. I attached 500 feet of #30 copper wire to each rocket. During a thunderstorm count the seconds between lightning strikes. As the storm gets close lightning strikes might be 4 minutes apart but during the very worse part of the storm lightning strikes might be 20 seconds apart. Use a watch time 15 seconds then fire off rocket #1. The wire is pulled up into the sky and it discharges the cloud and the lightning strike comes right down that tiny wire, BOOM it is so loud it shakes the house and sounds like a bomb exploded in the back yard. After the next natural lightning strike then time 15 seconds again and launch rocket #2. BOOM again another big explosion in the back yard. After the next natural lightning strike fire off rocket #3. BOOM another big explosion sound in the back yard. My neighbor use to say, why on earth are there so many lightning strikes in your back yard I can stand at my window and watch them it is so loud it shakes my house and knocks the pictures off my wall.
     
    Last edited: Apr 17, 2015
  13. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    Lightning in air is a form of Avalanche effect when dielectric strength of a high E field is exceeded. The negative resistance allows conductance rate limited by the effective L/R nonlinear time constant of the path. For ESD this can be as low as 10 ps from a finger or <1 us from are a large cloud. Faraday discovered the inverse relationship of ESR and the Q charge where ESR has an inverse relationship with current. An elevated lightning rod has at least two benefits of raising the E field gradient vs a smooth surface and diverting to the conduction path. As we would expect skin effects in conduction a braided wire would have lower ESR than a solid wire of same resistance but same ESL. Thus lightning surge tests for grid mounted insulators must pass a Basic Insulation Level (BIL) of surges with crest and tail times of ~1, >=10 us respectively (typ).

    However, dynamic surge withstanding of say 200kV BIL may usually not survive a static, DC or AC level of only 25% of this BIL level but if perfectly clean , may withstand 50%.

    Power distribution stations are usually protect by sharp antenna 3m (?) above the highest insulator spaced every 3m (?).

    I had several near lightning strike experiences in my youth.

    1) winding up a 100m seismic wire reel on a small boat with seismic sensors on an island and a lightning flash several km away, gave a gentle zap , like a van den-graph generator.
    2) lightning strike few hundred metres away, on a lake surface , while capsized after an overnight storm , in a 5m Hobicat , mast down in water, with a hull leaking. No sensation from strike. But the black flies were terrible while resting on a rope hammock between the Hulls.
    3) 3m away bright blue corona inside cottage , at same time a strong flash 1 block away damaged a tree. The corona seemed to last several seconds between a metal Chinese Lantern style Swag Lamp and a metal rimmed & legged but Formica plastic topped eating table , while I was near an ungrounded metal chimney fireplace and a grounded yagi antenna was higher up the roof . Fortunately no one was near the 1m long blue corona. That was either a branch lightning or combined with ground shift induced field in the wires.

    Lately I have tested 5MVA transformers up to 100kV dc arc level and 150 KV impulse passed to locate PD events in oil that detonate many H2 invisible molecules above 20kV primary and result in epidemic liability problem of many new units , some on wind farms with resulting DGA combustible gas levels reaching the LEL 5% lower explosive limit in oil within months instead of 30 yrs. I found the solution,using a spark plug immersed in oil but Greek owner didn't accept it.

    Silicate dust with iron dust from sharp edges broken on CRGOS lamination process. "We always build this way for 30 yrs!" .. But the steel and the silicatecoatings are now thinner.
     
    Last edited: Apr 17, 2015
  14. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Yes. You run a heavy conductor from the roof, on the outside of the house to ground rod(s) driven into the earth outside the house.
    In the event that your roof is struck directly, or even if there is strike near-by, providing a low-resistance path on the outside of the house prevents the current pulse from flowing in the wood or masonry inside the house.

    The wood or masonry absorbs moisture from the air, and absent the conductor, the current pulse goes through it , the moisture is flashed to steam which blows the wood or masonry apart, creating a fire or much structural damage.
     
    Last edited: Apr 17, 2015
  15. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    Here the cable TV installer added a long 14 AWG solid wire 30m from water tap outside to the RF coax entry and disconnected it from Gas meter underground supply for safety.

    My OTA rooftop antenna is safely grounded to same. You want to divert the surge conductance path safely between potential risks. A blunt > 3m antenna above ground plane roof and diversion braid wire would be advised. I am not sure if they use Tungsten tips in power substations, but it may be a good idea, ... and I see now it is already patented with silver alloys.

    The effect of a sharp tip increases breakdown threshold for 1us rise time, so it demands greater height above nearest ground plane or smooth conductors . Long wires parallel to earth for distribution use a safety ground above the 3 phases , does not need as much gap to attract lightning, but may need the gap to reduce H field coupling.
     
    Last edited: Apr 17, 2015
  16. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Tony:

    I don't have an outside antenna mast connected to a ground rod and the logistics of doing so bother me. Now, I do have a grounding block about 10 feet inside on the Coax, so that will remove static build up. There is a HUGE tree with like a 5' trunk with a branch within like 15' of the antenna. The tree is at least 30' higher than than 35' antenna. I suppose that the tree would get hit first.

    There is a ground strap around the rotor and the mast is attached to aluminum siding. The issue is that the driveway is under the antenna and there is a porch on the back side of the house. Running a ground rod close to the house on the driveway would create a tripping hazard and could possibly crack the footers. The only other choice is the front yard, but there would have to be some bends. e..g. Along the roof line or below the siding then around a front of the house about 2' and down.

    The house is brick with aluminum siding. The porch is brick. The shingles are asphalt.

    I really don't see any fundamental issues besides the NEC.

    Are there any other alternatives?
     
    Last edited: Apr 17, 2015
  17. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Using a water pipe or gas pipe as a ground (or even the electrical panel ground rod) for lightning protection is a BAAAD idea, and is counter to code in most places...
     
  18. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    In our land, water pipes are at least 6~8ft down to be well below the frost line ( although Winnipeg had some frozen pipes in 2013/14. Since the soil is predominantly clay which retains moisture, the water pipes provide good earthing to housing.

    this is not the case in regions where sand is the dominant subsoil as dry sand with low water table can create a poor ground. generally <5 Ohms is worst case for power lines and residence alike, but usually much less.

    Check your local building codes for preferred method. I might opt to use outdoor water tap and compare resistance with outdoor outlet Supply ground
     
  19. Willen

    Willen Well-Known Member

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    How to keep away termites and ants using small voltage of metal roof? Did you mean the voltage of the roof itself keeps away termites and ants?

    Your breath taking experiments about attracting lightning sounds VERY dangerous to me! Due to lack of 'lightning safety knowledge' I cannot handle such dangerous thing. You said- your back yard stroke by lightning much than other place, because you had installed Lightning Rod there. Did it mean- actually a lightning rod 'attracts' lightning? I mean- Now I am seeing almost just 2 or 3 very nearby lightning around my neighbour village area. If I installed a lightning rod, then will I get MORE strikes than before?

    BOOM is VERY frightening thing to me and during every thunderstorm, I close my two ear holes with my fingers, till the end! Believe me, I am a boy! :)
     
  20. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Lightning rods do not "cause" lightning strikes; they mitigate the damage that would otherwise be caused if they were not there.
     
  21. gary350

    gary350 Well-Known Member

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    I am saying the tiny voltage produced by the roof could help keep away termites and ants.

    There was a company many years ago that came out with metal rod stuck in the ground all the way around the house, with voltage applied it will keep away termites, ants, bugs. BUT the consumer product safety agency made them stop selling them. It worked. That was 60 years ago. Interesting thing about business these days no one wants to sell a produce that is 100% effective. If the termite company has to return every few years to spray again it makes them $$$$$ and keeps them in business. That is way every produce you buy eventually has to be replaced it keeps all the companies in business. That is also why Wal Mart sells the most worthless products available when it breaks you have to return and buy another one.
     

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