Live Electrical Lineman Work

[Per Hansson]

Hi guys, time for a real technical discussion again

I've seen a documentary on live maintenance work done on 500kV lines using helicopters
The lineman sits on a small platform outside of the chopper, he wears clothing that acts as a faraday cage. (It has 20% or so of stainless steel in it.)
He uses a metal rod which he brings close to the wire, an electrical arc is created when he does this

They wait for a while then he either ground the chopper with a grounding clip/cable or he climbs over to the line and the chopper leaves

http://www.youtube.com/watch?v=Z3q9WdjD5wc

Now my question is this, why is this step necessary, what creates the arc?
We don't see birds getting an arc against their feet as they land on the transmission lines do we?

It must be something related to the copper, do the moisture in the air actually create a small amount of grounding of the chopper as the rotor blows the extreme volumes of air required to keep it flying?

Because I think we can agree on one analogy, if the lineman had used a parachute to land on the wire (taking care that the chute does not hit a neighboring powerline at another phase) then the arcing step would not occur right? Since then he really would have been like a big bird...

[Oklahoma Wolf]

It negates the electrical potential between the helicopter and the tower. If they didn't do that, the lineman would get barbecued

[Per Hansson]

Potential between the tower and chopper?
The chopper is not even close to the towers?

[Zero82z]

He meant the power lines, not the tower.

[Per Hansson]

But what electrical potential?
If a chopper is hovering a meter over the ground and I touch it while standing on the ground I wont get any electrical chock

And same thing if a bird is flying in the air and landing on the transmission line

[Zero82z]

The power lines are at a very high potential, whereas the chopper is not. The arc is created because the potential difference between the two is enough to ionize the air and cause it to conduct electricity between them. Then he clamps a wire onto the power line which equalizes the potential between the line and the helicopter. That allows the man to work on the lines, because now that he's at the same potential as they are, no electricity will flow from the power lines to him.

If he didn't equalize the potential first, he would be at a much lower potential than the power lines, so the second he touched them, it would cause a flow of electricity which would kill him.

[Oklahoma Wolf]

More info: http://en.allexperts.com/q/Electrica...gh-Voltage.htm

[Per Hansson]

Hmm, ok
But then why isn't the bird vaporized as it lands on the line?

I mean when it takes off from ground it is at ground potential, yet it still lives from landing at the distribution lines?

[Oklahoma Wolf]

It doesn't land on anything - it has to get close enough to do the bonding on procedure, and yet stay far enough away that there's no flash-over. Otherwise, it would go kaboom.

They do the bonding on procedure to equalize the electrical potential, and then the lineman steps off onto the tower so the chopper can go somewhere else.

[Per Hansson]

The lineman can also clamp the chopper to the line (but he must first use the wand to equalize the potential)
And then the chopper stays put, while the lineman sits on the platform doing the maintenance work

This was how they did it on a program I watched on Discovery

I still don't understand why it's necessary, obviously I can see when watching the movies that it is because he has a 20cm arc going to his wand, but I can't understand why something which is not connected to ground can conduct electricity