HV Transmission lines & Helicopters

Discussion in 'Physics' started by RichieG, Jul 19, 2010.

  1. RichieG

    Thread Starter New Member

    Dec 9, 2008
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    Hi all. Concerning repair of high voltage transmission lines using a helicopter, could someone explain why there is an arc from the transmission line to the wand held by the repairman on the helicopter platform?

    I understand the helicopter is not at the same potential as the line and apparently that is the reason for the arc current flow, but I feel the real science is a bit deeper than that.

    Birds, for example, are not at the same potential as the transmission line but they don't get zapped when they land on one. Yes, I know it's because they are not touching ground when they land on it and so, do not complete the circuit for current to flow through them, but - neither is the helicopter!

    Perhaps it is not an AC arc at all, but a discharge of static electricity generated in the helicopter?

    Thanks!
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    The voltages on those transmission lines is tremendous, often well over half a million volts. There is still the potential of CORONA even without a complete circuit. This is caused because the voltage gradient on any object (like the wand they use) creates a "seed" for corona.

    Though corona discharges are very low current...they can be very unnerving for the personnel. Of course flying a helicopter near a high voltage line would be unnerving in itself for any NORMAL person, but these are NOT normal people doing this! :)

    Eric
     
  3. davebee

    Well-Known Member

    Oct 22, 2008
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  4. Markd77

    Senior Member

    Sep 7, 2009
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    Nice, did you google it and end up back here or just an excellent memory?

    Nothing much to add that wasn't in the original thread.
     
  5. sceadwian

    New Member

    Jun 1, 2009
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    Richie, how can you compare a small bird with a several thousand pound helicopter? The capacitance difference is HUGE. Sure the helicopter will generate a static charge, but not enough to create an arc for seconds,
     
  6. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    the bird, due to it's low mass, will take on the charge as it enters the electrostatic field, and will be at equalibrium as it alights. I'm sure it feels the tension however. The helicopter begins to take on the charge, but due to it's mass, would not reach equlibrium for a considerable time, and never will as it does not actually touch the wire.
     
  7. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Maybe not for several seconds but I've seen guys get knocked out cold from the 20-kilovolt+ discharge.
    I worked in combat cargo in the Navy loading gear on helicopters hovering above the flight-deck on a ship at sea. You never never ever want to touch one without first grounding it first. X2 when loading weapons or fuel on a sling or bladder.

    http://en.wikipedia.org/wiki/Helicopter_support_team

    [​IMG]
     
  8. mrmeval

    Distinguished Member

    Jun 30, 2006
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    I was pulled over by a cop. As he exited his car he got the crap knocked out of him. I asked him what happened and he said he got a bad shock. I told him about the straps you can put on cars that will discharge the static when stopped but which otherwise will be pushed off the ground when moving. He thanked me, warned me and left. :-D
     
  9. RichieG

    Thread Starter New Member

    Dec 9, 2008
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    Wow - you are absolutely right, that was me! It seems I started that thread 4 years ago, then promptly forgot about it and never checked back. A mind is a terrible thing to waste. I'm just now going through all the replies there. Thanks!
     
  10. b.shahvir

    Active Member

    Jan 6, 2009
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    I think it is leakage currents combined with capacitive currents in AC. But if the line were to be HVDC, capacitance current will cease after some time if the Helicopter (plate 1) is fully charged w.r.t to earth (plate 2)

    In spite of DC voltage, if the arcing is continuous, then leakage currents exist in the ambient atmosphere (air) surrounding the HV line might be the only reason. The leakage currents would exist due to the presence of ionised particles due to contaminants in the surrounding air.

    I just cannot comprehend one thing...why is there no flashovers (or line to earth faults) when heavy rain is incident on the HV lines. Are rain drops electrically neutral??... case in point is electric trains operating in heavy rains without electrical faults or power trips!!
     
  11. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Helicopters are notorious for static buildup because them whopping rotors make a perfect Van DeGraff generator with the air!

    Eric
     
  12. sceadwian

    New Member

    Jun 1, 2009
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    Sure the rain drops are conductive, there's also huge space in between them even in the most torrential downpours, no conduction path ever exists between the wire and ground.
    Mythbusters did an episode about this testing the myth that if you pee on the third rail of an electric train you'd get shocked which can't happen, the liquid stream isn't solid enough, and they tested it with a much thicker liquid flow than rain could ever produce. Sure at higher voltages the distance the arc can go is longer, but have you ever seen how high those HV towers are? I would say even during the heaviest downpour ever recorded that not even 1% of that space would be filled with water.
     
  13. Markd77

    Senior Member

    Sep 7, 2009
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    They do make a funny noise when it rains. I get a tingling when I am under them.
    Have you seen the videos of flourescent tubes with one end planted in the ground under power lines? It's good to see them lighting up with no visible power.
     
  14. beenthere

    Retired Moderator

    Apr 20, 2004
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    In San Diego, the insulators get a salt spray coating and do a continuous corona discharge with frying bacon noise.

    I was out last evening trying to chase people out of the yard in the path of a thunderstorm. One cloud-to-cloud discharge caused the hair on my head to twitch just a bit.
     
  15. b.shahvir

    Active Member

    Jan 6, 2009
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    Kinda hard to believe..since I've heard college rookies are ragged by their seniors by asking them to pee on live phase conductor..no proof of the rookie gettin' zapped...but what does 'solidness' of liquid stream got to do with elec. conduction??..urine contains salt & should conduct electricity since phase to earth circuit is completed thru you know what & earth if one is peeing , say, barefoot!! ;)

    There's an article by Mr. Beaty which says rain water cannot conduct electricity as rain drops are water vapors which are nothing but neutral molecules! :confused:
     
  16. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Don't try it. (peeing on power line)
    http://seattletimes.nwsource.com/html/nationworld/2011224615_apusurineelectrocution.html
     
  17. b.shahvir

    Active Member

    Jan 6, 2009
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  18. Markd77

    Senior Member

    Sep 7, 2009
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    Rainwater contains dissolved carbon dioxide and other gasses and so is slightly conductive. It's easy enough to test with a battery and a multimeter.
     
  19. beenthere

    Retired Moderator

    Apr 20, 2004
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    There are many articles by Mr. Beaty that do not hold up to examination. As Markd77 says, they are conductive. Downwind of a coal fired power plant, there may be dissolved sulfuric acid in addition to the CO2.
     
  20. Wendy

    Moderator

    Mar 24, 2008
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    Deionized water is absolutely nonconductive, which is probably why Mr. Beaty said what he did. Water, being a really excellent solvent, is never found in pure form in nature though, it will dissolve something, anything. DI water is an artificial condition. If you drank DI water it would not be good for you, you body is geared to always having some electrolytes in the water. I don't know if it is true or not, but someone told me DI water would actually dissolve the enamel out of teeth in a very slow kinda way.

    Many brands of purified drinking water is made into to DI, then they add a mix of electrolytes such as potassium chloride and others to make it taste good.
     
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