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Per Hansson
10-23-2009, 05:09 PM
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

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? :D

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
10-23-2009, 05:20 PM
It negates the electrical potential between the helicopter and the tower. If they didn't do that, the lineman would get barbecued ;)

Per Hansson
10-24-2009, 01:43 PM
Potential between the tower and chopper?
The chopper is not even close to the towers?

Zero82z
10-24-2009, 04:28 PM
He meant the power lines, not the tower.

Per Hansson
10-24-2009, 05:12 PM
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
10-24-2009, 05:51 PM
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
10-24-2009, 05:52 PM

Per Hansson
10-24-2009, 06:53 PM
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
10-24-2009, 09:31 PM
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
10-25-2009, 05:23 AM
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

HOOfan_1
10-25-2009, 10:18 AM
I guess it is like lightning hitting a plane. There is so much difference in potential there that the electricity can flow into the helicopter for as long as it takes to equalize the charge.

davidhammock200
10-25-2009, 10:23 AM
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.You have grown, your skills as a Jedi are now complete!

Congratulations,
Dave:beer:

The helicopter also produces a strong static electrical field of its own,
this is a very big deal when a Navy or Marine choper lowers equipment or personal on to a ship at sea.

Travis
10-25-2009, 11:58 AM
I=C\frac{du}{dt}, Helicopter has a much larger capacitance than a bird, so it get more current on contact.

mdk777
10-25-2009, 12:27 PM
Also, birds get fried all the time on high tension lines.

There's just nothing left to worry about after.:lol:

370forlife
10-25-2009, 01:39 PM
I can attest to that.

Where we live out in the country there are some big high voltage wires coming from in the woods in a corn field across the street. Big birds just get zapped and burnt, and fall to the ground. All the little birds just disappear.

Per Hansson
10-25-2009, 02:09 PM
Oh, so you are saying that at a certain transmission voltage birds are not able to land on the transmission lines without being barbecued?

shinji2k
10-25-2009, 02:36 PM
Oh, so you are saying that at a certain transmission voltage birds are not able to land on the transmission lines without being barbecued?
Yup. I would imagine the EMF would dissuade most animals from going near the high voltage lines like the ones in the video. The local transmission lines in the US are something like 4 or 13kV but I think it varies. Those are the ones feeding each neighborhood where is it is then jumped down to 120/240V for each house. I know the high voltage lines are uninsulated, but I'm not sure about the local transmission lines. I doubt any animals would want to be on even a 4kV uninsulated wire. The 120/240V going to each house is insulated though.

370forlife
10-25-2009, 03:50 PM
The smaller lines that branch off of the transformer station down the road are insulated and birds love to sit on those because they get a bit warm.

RagingDragon
10-25-2009, 06:19 PM
Oh, so you are saying that at a certain transmission voltage birds are not able to land on the transmission lines without being barbecued?

Sounds like the big birds get cooked, and the small ones vaporized (either that or their body fluid flash boil and blow them to little gory bits...).

RagingDragon
10-25-2009, 06:23 PM
Yup. I would imagine the EMF would dissuade most animals from going near the high voltage lines like the ones in the video. The local transmission lines in the US are something like 4 or 13kV but I think it varies. Those are the ones feeding each neighborhood where is it is then jumped down to 120/240V for each house. I know the high voltage lines are uninsulated, but I'm not sure about the local transmission lines. I doubt any animals would want to be on even a 4kV uninsulated wire. The 120/240V going to each house is insulated though.

The local transmission lines in my area are insulated. I imagine the 500KV lines are uninsulated, because at those voltages insulation light enough for the wires to support wouldn't make a significant difference, while insulation that would make a difference would be *far* too heavy for the wires to support.

davidhammock200
10-26-2009, 12:59 PM
The local transmission lines in my area are insulated. I imagine the 500KV lines are uninsulated, because at those voltages insulation light enough for the wires to support wouldn't make a significant difference, while insulation that would make a difference would be *far* too heavy for the wires to support.Correct. The "big ones" are not insulated.

gdjacobs
07-05-2018, 12:15 AM
Correct. The "big ones" are not insulated.

Most primary overhead distribution lines (2400V residential, 12.5kV, and 14.4kV -- all phase to ground) use non-insulated Al, Al-Steel, or all steel wire. That way, the lines are a bit more performant as there's no outer coating to retain heat and line workers can perform the bulk of routine operations live with hot sticks, plus the installed cost for wire is less.

Buried primary cable is, of course, insulated and overhead insulated wire does exist in the form of polyethylene insulated "tree wire" and other specialized application conductors.
https://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet42

ashiekh
07-05-2018, 10:26 AM
Static electricity from a helicopter

http://realitypod.com/2011/03/29/helicopter-static-electricity-phenomenon-explained-corona-effect/