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I saw this video, where a guy did power line inspection from a helicopter, which is isolated from ground. When he held out a "magic wand", a small amount of arcing came off of the power line to the helicopter.

Helicopter Repair magic wand

This got me confused, though, because I thought since he was isolated from ground, the power would not want to go anywhere. enter image description here

The helicopter simply has no relationship to ground.

So, why did the Magic Want arc to the helicopter?

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marked as duplicate by placeholder, Chetan Bhargava, W5VO Apr 27 at 21:47

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2 Answers 2

There is capacitance.

Edit: In keeping with Dave Tweed's comments below, this edit applies to AC high voltage lines, which the one in the video is.

Each of the 3-phase lines have an AC potential with respect to ground. Because of the capacitance there will be a (reactive) current flowing between the lines (and anything connected to them) and earth. That current is one reason why DC lines are preferred for long distance transmission- the current causes losses in the resistance of the conductors. When the linesman bonds to the line, the current jumps through the air first, then it is tied directly to equalize the voltages. When the bond is broken, the helicopter may be left with a residual charge, as it would be with a DC line, as below.

/Edit.

The man-helicopter system is charged at about 0V with respect to ground. For the system to be charged to 500kV or whatever, that capacitance must be charged to the same potential as the high voltage line.

The IMAX photography is breathtaking, but the verbiage less so- voltage does not "flow around" the technician, and I'd like to see a reference to Michael Faraday speculating on the welfare of a man in a metal cage. His actual experiment involved a metal ice pail.

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So does this mean when the heli hits the ground, it will have to do another discharge to get back to zero volts? –  skyler Apr 27 at 12:35
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Good question.. I don't know. Air is not a perfect insulator, and it may largely dissipate before they reach the ground, also the voltage will drop as the helicopter approaches the ground since the capacitance increases. –  Spehro Pefhany Apr 27 at 12:40
    
I agree. Charge would leak in any possible way, and the C would increase as you say. Moreover it is possible that on the landing site they have a sort of pennant (forget my english) where the tech guy can safely discharge the heli through the wand, that I believe has a big R in series –  Vladimir Cravero Apr 27 at 12:58
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This answer is a bit misleading, since we're talking about AC, not DC (although the moment that the connection is broken can leave the helicopter with a net DC charge). The helicopter (and the man) is not "charged to" 500 kVAC. The arc is AC current that is the result of ongoing capacitive coupling between the helicopter and the world at large (mainly ground). The metallic suit that the man wears simply makes sure that the potential at all points of his body is the same, which insures that no capacitive current flows through him. –  Dave Tweed Apr 27 at 14:05
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I've seen the video; it's clearly a 3-phase AC transmission line. DC lines don't arc at 120 Hz. –  Dave Tweed Apr 27 at 15:47

That is because the helicopter has some sort of capacitance with respect to ground. When the operator touches the line with the "magic wand" two thing happen:

  • There is a static discharge, just as when in windy days you get "electrocuted" by touching another person or something connected to ground
  • There is a current that flows because the heli acts as a capacitive load. It is very small so the current would be very small but at such high voltages a significative voltage drop will be located between the wand and the wire instead that between the heli and ground.
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