The ancient question of how a bird can rest on an uninsulated wire still baffles me, and recent events at the Philly airport support my arguement. People claim the birds can rest on the high voltage lines since they are not grounded. Regardless, I argue that there IS still a potential difference between the bird and the wire; therefore, I would expect an arc to jump before the bird could even land.
Now to support this, recent news mentioned two planes struck by lightning upon approach to Philly. Naturally, I argue it is the potential difference in work here since the planes were not grounded. Likewise, I use cloud to cloud lightning as further evidence that only a potential difference (of some value, depending) is needed for an electrical spark to jump. Thus, I argue arcing to ground alone does not explain all cases…and the bird should be shocked.
And yet, birds DO rest on high voltage lines. How?
Your question is more about how the bird gets to the wire without being shocked than how it sits on the conductor, right? I think it’s a good question. If you watch a high voltage inspection where they tie the helicopter to the high voltage line to get them to the same potential there is an arc when they use the shock stick to reach out and touch the line at the beginning of the procedure.
My WAG is I think the size of the bird is big factor here. The current in the wire is passing back and forth (assuming it’s an ac line though high voltage dc is becoming more and more common) rapidly in the wire with ease, there is no reason to pass to the bird. With a larger object like a plane there’s enough surface area for eddy currents and the like that it would present more of a ‘target potential’ to get the charges in that area to jump to it and try to equalize. Just a WAG though, I look forward to hearing some better answers.
Damn my stupid connection! I wanted to add that I think a metal object that gets near a potential difference will be very different also in that induced current will flow in it and set up a field of potential around the object opposite to the field that created it. The difference in potential has to be great enough that the arc will overcome the resistance of the air in between the two objects and there are a lot more dynamics involved with a big metal plane than a small bird.
This one I’m more certain about although it’s still a WAG since I don’t have any cites for it. I’ve had coworkers forget to remove their wedding rings before working on equipment. The induced current in the ring will get hot enough to make the finger quite red and that’s just working near 10,8 kV and 600V feeds. One acquaintance lost his finger working in a mine when he got too close to a damaged part of a 13,6 kV cable. The ring in that case melted. Near high voltage cable I have no doubt it would either arc to that ring or heat it up to very dangerous temperature for the pigeon.
A conductor moving inside a magnetic field (the field around the conductor) will have a current induced in it with a polarity opposing the force that created it (if I remember Lenz’s law correctly).
If you were to fall from the sky and grab a high potential line, you too would survive as long as you were on only that one wire. It’s difference in potential that makes current flow, and you and the birds equalize with the conductor pretty much instantaneously. There’s no current flow.
Things are different with very high potential (hundreds od thousands of volts) distribution systems though. I don’t know that birds alight on 500kv wires and survive.
Related observation:
I’ve never lived where got freezing cold, but when I was attending school in the Navy at Great Lakes I noticed that sometimes there would be dead birds near power poles. I was told by a local that birds would line up along the crossarms and snuggle for warmth, occasiolally bridging the two wires and killing one or two of them.
Could be, but you know how those flyover folks like to bs us people who hail from out west.
BTW; Grounding isn’t neccessary, any difference in potential will do the trick.
Peace,
mangeorge
The key is capacitance – how much charge an object will hold at a given voltage. When objects at different voltages come in contact, charge will flow between them until the voltages equalize. For a helicopter, as seen in the video, there’s plenty of charge leaping from the high voltage lines. That arcing is a not insignificant current, but once the voltages equalize, everything is fine. Just don’t want the arc messing with anything sensitive, like squishy humans.
How much capacitance does a bird in free air have? Damned if I know. Probably insignificant. Capacitance of free objects, very generally speaking, is a function of surface area and materials of an object.
I wouldn’t be surprised if larger birds got a small shock when the landed on high voltage lines, but it’s probably no bigger than the shocks you get touching a doorknob on a winter day.
True. Still, doesn’t the capacitance idea hold? Over 1/120 of a second, that wire is changing from -115 to +115 volts, and so does the bird. There would have to be some current to account for the charges moving to and from the bird during the cycling. Though there would be considerable resistance between the bird and the wire.
Either way, the current passing through the bird would be about three tenths of nothing.
Here’s my take on it -
For a small object, no significant current will flow. A large object, acting as one plate of a capacitor (the ground being the other) will experience an AC current, and that won’t equalize - it will flow as long as the object touches the line.
Note that this is not a trivial problem. I have heard a [possibly apocryphal] story that this was used as Masters level oral question in EE - If you are in a hot air ballon, would you reach down and grab a High Voltage transmission line, and if not, why?
Is there a lineman in the house? I used to know some of this stuff, but I’ve forgotten.
There is, or was, a member here who worked in power distribution, but I don’t remember her name. She may have worked for TVA, or a similar organization.
There won’t be any potential difference between you and the line though as long as you’re touching the line right? So the ac voltage of the line will vary but your potential will vary at the same time.
Remove the balloon. We can’t know enough about it’s electrical properties, as that can vary tremendously from balloon to balloon.
What you need is a vertical catapult calibrated to lift you exactly high enough to grab the wire.