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Jinx
05-16-2009, 07:40 AM
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?
- Jinx

HongKongFooey
05-16-2009, 07:57 AM
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.

HongKongFooey
05-16-2009, 08:14 AM
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.

ivan astikov
05-16-2009, 08:23 AM
What would happen to a homing pigeon or other bird with a metal ring on its leg?

HongKongFooey
05-16-2009, 08:32 AM
What would happen to a homing pigeon or other bird with a metal ring on its leg?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.

RickJay
05-16-2009, 09:19 AM
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.
But your friends completed a circuit to the ground. The pigeon doesn't.

Unless your friends are birds, in which case I take that back.

HongKongFooey
05-16-2009, 09:29 AM
But your friends completed a circuit to the ground. The pigeon doesn't.
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).

beowulff
05-16-2009, 09:54 AM
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).

True, but the amount of current induced in a ring-sized conductor at 60Hz. with a single-turn primary (outside of the ring) would be insignificant.

mangeorge
05-16-2009, 09:57 AM
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

lazybratsche
05-16-2009, 10:07 AM
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.

beowulff
05-16-2009, 10:10 AM
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.

But it's AC - the voltages will never equalize.

lazybratsche
05-16-2009, 10:18 AM
But it's AC - the voltages will never equalize.

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.

mangeorge
05-16-2009, 10:26 AM
Woo Hoo! (http://205.243.100.155/frames/longarc.htm)

beowulff
05-16-2009, 10:28 AM
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?

mangeorge
05-16-2009, 10:51 AM
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.

HongKongFooey
05-16-2009, 10:56 AM
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.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.
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?That's an interesting one to ponder.

beowulff
05-16-2009, 11:06 AM
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.

Yes, but there will be a current flowing through your hand as your "body capacitor" charges and discharges.

mangeorge
05-16-2009, 11:08 AM
If you are in a hot air ballon, would you reach down and grab a High Voltage transmission line, and if not, why?
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.

HongKongFooey
05-16-2009, 11:32 AM
Yes, but there will be a current flowing through your hand as your "body capacitor" charges and discharges.Gotcha. What do they do about that for the HV line inspections? Does the current flow through their suit rather than the lineman?

friedo
05-16-2009, 04:19 PM
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?

That's easy: No, because grabbing onto it would surely yank me right out of the basket.

Chronos
05-16-2009, 06:07 PM
That's easy: No, because grabbing onto it would surely yank me right out of the basket. Or alternately, No, because I have no good reason to grab it, and I try to stay in the habit of not grabbing high-voltage wires.

UncleRojelio
05-16-2009, 06:13 PM
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.

This is because helicopter rotors produce hella static charge. In fact, if you are ever in need of helicopter rescue, don't reach out and grab the cable until it hits the ground or you will receive a nasty surprise. I don't think birds have the same problem.

mangeorge
05-16-2009, 06:22 PM
Are you saying, beowulff, that if I stretch a conductor out and connect it to a transformer at 12kv, current will flow back and forth at 120 times per second even at no load? The potential would go +60v then -60v, but there would be no flow.
If that was a 480v line, and I clamped an ammeter around it, I would read zero amps.
I'm not argueing. Something is missing here, and I wonder what it is.
If that 12 kv wire had a high resistance, higher that my body, and I grabbed it with both hands at different points on that wire, current would definitely flow through me.
This effect is similar to step potential (http://industry.flexiblelearning.net.au/learning_objects/GippsTAFE_e_learning_siteV4/Pages/examples3.html), as seen in this cute animation.

HongKongFooey
05-16-2009, 06:49 PM
I might be wrong but I think he meant a person touching the line wouldn't change potential across their entire body instantaneously as the wire changes potential so there would have to be a sort of charging current for lack of a better term as they charged and discharged. I'm used to being wrong though. :D

Biffy the Elephant Shrew
05-16-2009, 07:01 PM
So when will we revisit Drunk in a Midnight Choir?

lazybratsche
05-16-2009, 07:02 PM
I vaguely remember that you could set up equations for capacitors of arbitrary shape, but my physics is far too rusty and I don't even have my old textbooks to look at.

If you assume that the bird is a parallel plate capacitor (in the same magic physics land that cows are spherical...), you can calculate a capacitance. For the hell of it, assume the largest possible bird, the Andean Condor (wing/plate area of 1.5 m^2), on a really low power line (3 M above ground), that happens to be really high voltage (100 kV). This should give an absolute upper bound of how big the capacitance and currents could be. This calculator (http://www.daycounter.com/Calculators/Plate-Capacitor-Calculator.phtml) gives the bird a 4.43 pF capacitance.

The bird-capacitor will be charging from -100 kV to 100 kV, and back again, 60 times per second. 200 kV on a 4 pF capacitor gives a charge of 443 nC. Now, assuming constant current from peak to peak (essentially a sawtooth wave), 443 nC/(1/120 s) = 53 microamps. Which is pretty much nothing. I don't think anyone can feel anything less than a milliamp directly.

And that's a theoretical maximum. A smaller bird, on a higher wire, at lower voltage, and non-zero resistance will see much smaller currents. A parallel-plate sparrow, on a 120V line 30 m above the ground will have currents on the order of a few picoamps. Adding the effects of resistance will reduce the current by another order of magnitude or more. I leave that as an exercise to the reader, since I've past the limit of physics that I comfortably remember. RC circuits, anyone?

In other words, if you want to measure this, you're going to need a pretty damn good oscilloscope.

HongKongFooey
05-16-2009, 07:23 PM
Either way, the current passing through the bird would be about three tenths of nothing.Looks like you're right. Damn, I was going with the square root of fuck all.

mangeorge
05-16-2009, 07:30 PM
My memory of a capacitor is of two conductors seperated by a dielectric (insulator). Clean dry air is a much better insulator than the body, and might work as the dielectric, but where's the other conductor?
That's kinda what lazybratsche said, ian't it?

HongKongFooey
05-16-2009, 07:36 PM
The HV cable -- air -- bird. The bird is the other 'plate' of the capacitor. It's not a good conductor like the wire is but it can still hold a charge.

mangeorge
05-16-2009, 07:36 PM
So when will we revisit Drunk in a Midnight Choir?
You wanna pass that around, Bogie!

lazybratsche
05-16-2009, 07:39 PM
The other conductor here is quite literally the ground beneath the bird. The ground itself is what completes the circuit in power transmission. I think that even that's not necessary, as objects floating in free space will still have a capacitance.

ETA: Each side of the capacitor is directly connected to the power transmission circuit. The bird is directly connected to the wire it stands on through its feet.

mangeorge
05-16-2009, 07:44 PM
The HV cable -- air -- bird. The bird is the other 'plate' of the capacitor. It's not a good conductor like the wire is but it can still hold a charge.
But the bird is "common" to the wire. It changes potential with the wire at light speed.

HongKongFooey
05-16-2009, 07:45 PM
Ahh, I thought you meant the bird in flight before it reaches the line. Isn't that what we were trying to figure out? Me confoozled.

beowulff
05-16-2009, 07:47 PM
I would like to point out that I have personally seen hummingbirds that have been killed by landing on the hot wire of an electric (pulsed) fence. They were not in contact with any other conductor. The high DV/DT of the pulser may make this a worse-case scenario, but it certainly shows that it's possible to kill a small bird through capacitance effect alone.

mangeorge
05-16-2009, 07:59 PM
The ground itself is what completes the circuit in power transmission.
I think that's a misconception. The circuit is completed by the other wire, or wires in a three-phase circuit. Even in household circuits the circuit is completed by the neutral wire, which is indeed grounded. But not part of the circuit.
The ground (actually "earth") is a pretty poor conductor, and varies widely in resistance.

mangeorge
05-16-2009, 08:27 PM
I would like to point out that I have personally seen hummingbirds that have been killed by landing on the hot wire of an electric (pulsed) fence. They were not in contact with any other conductor. The high DV/DT of the pulser may make this a worse-case scenario, but it certainly shows that it's possible to kill a small bird through capacitance effect alone.
I'd understand this if the bird were near an insulator and the post were at ground potential. Air is a good insulator, but if the potential is high enough, and/or the distance is close enough, current can flow. The bird, a relatively good conductor, could shorten the distance between hot and ground.
I'm still pondering the capacitance idea. :confused:

HongKongFooey
05-16-2009, 08:49 PM
I'm still pondering the capacitance idea.Does it help if you think of the plate of the conductor as a charge collector rather than a conductor? The ground can store a lot of charge as does the wire.

lazybratsche
05-16-2009, 09:16 PM
I think that's a misconception. The circuit is completed by the other wire, or wires in a three-phase circuit. Even in household circuits the circuit is completed by the neutral wire, which is indeed grounded. But not part of the circuit.
The ground (actually "earth") is a pretty poor conductor, and varies widely in resistance.

I could be wrong here. As I said, this is right up against the limits of my physics knowledge. The vast majority of my hands-on experience is with electronics where the chassis is the ground and most definitely completes the circuit.

HongKongFooey
05-16-2009, 09:26 PM
I could be wrong here. As I said, this is right up against the limits of my physics knowledge. The vast majority of my hands-on experience is with electronics where the chassis is the ground and most definitely completes the circuit.Yeah, in power transmission there is a separate neutral or return line or separate phase conductors as mangeorge said. Sometimes they run overhead grounds for lightning strikes or safety purposes but the ground still acts as a capacitor plate just not a conductor in the circuit. It can become a conductor in the circuit if something goes wrong, that's why things like neutral ground resistors (http://www.avtron.com/neutral_grounding_resistors.htm) become necessary.

mangeorge
05-16-2009, 09:39 PM
I could be wrong here. As I said, this is right up against the limits of my physics knowledge. The vast majority of my hands-on experience is with electronics where the chassis is the ground and most definitely completes the circuit.
Chassis ground, yes. Bad naming. It should be called "common", imo. It may be connected to the grounding (green) plug wire. I don't know.
The other (more appropriately named) ground is earth ground. It's used as a safety. Some other countries call it "earth", which makes sense.
Grounded conductor = neutral, usually white.
Grounding conductor = safety ground, usually green.
This has nothing to do with the OP.