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WHy Can Birds Sit On Powerlines?

by Line
Tags: birds, powerlines
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Line
#1
May14-06, 03:31 AM
P: 218
Why is it that birds can sit on powerlines without being fried? We learned something about this in my high school Physical Science class. Something about there being no danger unless they touched another line.

Also how do linemen do it? They repair highvoltage lines in helicopters. Before they get out the touch the line with a wand. Then they are free to walk on the line all they want.
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Azzy42
#2
May14-06, 04:53 AM
P: 2
I am not sure about this wand that they use but they rest is pretty simple.
If you touch the other wire you complete the circuit. If you just connect the postive end of a battery to a light bulb it does nothing. You need the other wire to connect back to the battery. Same principle.
Mk
#3
May14-06, 07:08 AM
P: 2,056
I thought there was insulation around those wires?

I saw a discovery channel show about helicopter linemen. Wow. Dangerous. Ow. And the helicopter goes down as well as the lineman if he messes up... or the helicopter pilot.
http://en.wikipedia.org/wiki/Lineman_%28occupation%29

euler_fan
#4
May14-06, 09:06 AM
P: 22
WHy Can Birds Sit On Powerlines?

Simply put, No potential (voltage) difference. Voltage measurements taken throughout a circuit are measured between two points - lets call them A and B. Now, if a bird on a power line is point A what is the birds connection to point B. Next, what do you think would happen if the bird were in contact with 2 power lines instead of the one, remember potential difference!
dimensionless
#5
May14-06, 09:45 AM
P: 464
The situation is similar to a balloon dragged 100 meters underwater. If it is filled with air there will be a massive pressure difference and the balloon will pop. If it is filled with water, there will be almost zero pressure difference, and the balloon will not pop. The pressure of the sea water and the pressure inside the air balloon can be thought of as the positive and negative terminals of a battery. The pressure of the sea water and the pressure inside the water balloon can each be thought of as positive terminals on two different batteries.
rcgldr
#6
May14-06, 10:22 AM
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But the bird's feet, legs, and body to provide a parallel path for the current to flow. Fortunately for the bird, it's resistance is much higher than the wire the current is flowing through.

The issue is different when cutting branches from a tree off a high power line. A human body conducts electricity better than a tree, so a chain or wire is used to provide a parallel path for the current to flow so the tree climber doesn't get electrocuted. The chain has to be connected to point beyond where the human is (one point above, one point below).
vanesch
#7
May14-06, 10:54 AM
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Bird claws must be exceptionally insulating. I once tried to file a research project to use bird claws as insulators on high-voltage lines based upon this argument, but the grant was refused



No, seriously, what kills birds, or you, is ELECTRICAL CURRENT, and for the current to flow, it must have two points of contact (entry and exit).

One point of contact will never make current flow. That's why the bird, or the line man, can touch ONE line (in a point, or nearly so). The current can't get out again (because the helicopter is in the air = insulator).
But of course, once there is an exit point (like touching the SECOND line, or ground, or any other slightly conducting path) you're fried because now the current can enter AND get out again.

Something a line man shouldn't do, is to pee down from the line on the floor
rcgldr
#8
May14-06, 11:24 AM
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Quote Quote by vanesch
for the current to flow, it must have two points of contact (entry and exit). That's why the bird, or the line man, can touch ONE line (in a point, or nearly so).
Assuming that there's a load on the line, current is already flowing through the wire. My point was that the bird is touching the line in two points, and does provide a parallel path for the current, so some very small amount of current does flow through the bird, it's maybe be nearly zero current, but it's not zero current.

For a real world example, imagine a busted light with the elements exposed to the water in a swimming pool. The water acts as a single line of conductance, but if a human in the water get's close enough to the busted light, the human conducts electricty better than the water and can get electrocuted.

For an imaginary case, replace a segment of the high voltage wire with a (very pure) water filled bird bath, so that the current is flowing through the water. If a bird lands in the bath, it will get a serious shock.

So the bottom line is that the line is a good conductor of electricty, there's very little voltage drop across the small segment of line that the bird touches, and the bird has a much higher resistance to current than the wire.

Obviously there's a huge potential voltage difference between a pair of high voltage wires or a single high voltage wire and and grounded source (including the air). If a bird touches both wires, or closes the gap enough that a spark travels through the air, the bird get's cooked. To reduce the odds of this happening where there are large birds like eagles, perches above the wires are made to attact large birds to the higher perches.
rcgldr
#9
May14-06, 12:27 PM
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Here's a link to a device to scare off birds. The clever part is that it gets' its power from a single high voltage line using a transformer effect.

http://www.wired.com/news/business/0,1367,48514,00.html

Based on some web sites, losses in transmission lines are only about 3% to 4%, so the voltage drop across a short distance, like between the feet of a bird would be very small, nearly zero.
Gokul43201
#10
May14-06, 01:18 PM
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(To clarify and add to jeff's point...)

There is no such thing as a single point of contact. Even if you grab a power line between two fingers, you are providing a parallel path for current flow (even if it's only a few millimeters long). If this alternative path is suitable, it will draw a large current and burn all the cells in its way (ie:the cells in the fingertips). The reason this doesn't happen is that this alternate path is not a suitable path. What determines the unsuitability is the electrical resistance of the path compared to the atlernative.

The resistance (from one foot to the other) of a typical bird is large compared to the resistance of the power line between the bird's feet. The resistance of the cable is way smaller than a micro-ohm, while the resistance of the bird is at least several kilo-ohms. On average, the bird will draw about one part per trillion (10^12) of the current flowing through the power line, which is something of the order of nanoamps. That is an incredibly tiny current. Bigger currents flow throught the body all the time - when we touch objects on a dry day - but we don't notice them.
blacklilly
#11
May14-06, 01:41 PM
P: 1
I suppose it's because they only touch 1 wire, so they don't complete the cirquit...
But I've never really thought about insulation...
vanesch
#12
May14-06, 01:46 PM
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Quote Quote by Jeff Reid
Assuming that there's a load on the line, current is already flowing through the wire. My point was that the bird is touching the line in two points, and does provide a parallel path for the current, so some very small amount of current does flow through the bird, it's maybe be nearly zero current, but it's not zero current.
Yes, that's correct. In fact, nearby where I live, there are some high voltage power lines, and there are some red lights on them (for low-flying airplanes I guess), which are exactly powered that way: they are claimped onto the wire, and then there's a wire on it clamped about 2m further on the same wire. The voltage drop over this distance is apparently enough to make them light up.
So indeed, the spatial extention that's to be considered as "one point" is dependent on the relative impedances of the two bodies (wire and bird) as well as the current flowing and the maximal "safe" current through the body.

I would think that for all things metallic, though, compared to a human body, this will be more than a few meters...
And even this is not true in the case of lightning impact.
vanesch
#13
May14-06, 01:54 PM
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Quote Quote by Gokul43201
(To clarify and add to jeff's point...)
There is no such thing as a single point of contact. Even if you grab a power line between two fingers, you are providing a parallel path for current flow (even if it's only a few millimeters long). If this alternative path is suitable, it would draw a large current and burn all the cells in its way (ie:the cells in the fingertips). The reason this doesn't happen is that this alternate path is not a suitable path. What determines the unsuitability is the electrical resistance of the path compared to the atlernative.
Yes, agreed. A cow can get electrocuted, for instance, when standing near a defective high-voltage pole (when one of the phases touches the pole), because the potential difference induced in the ground near the pole over the distance between her legs is deadly.

In fact, when in "power electricity" class, one of the rules was to keep always your left (or right) hand in your pocket. Of course, it doesn't protect you from being electrocuted through ground, but normally the differential switch should protect you then, and you're protected from putting yourself "in" the circuit (which would NOT be detected by the differential switch).
arunbg
#14
May14-06, 02:30 PM
P: 600
A similar question regarding this topic, suppose you have a metallic and an insulating rod outside on a stormy night. Which one will probably get struck by lightning ?
I am inclined to think that it will be the metal rod but I am not sure of the reason.How does the lightning strike realise the easier discharge? From potentials ?
rbj
#15
May14-06, 02:44 PM
P: 2,251
it's just a little off topic, but here are some incredible pics (view them in order). it's amazing that this guy lived.

http://members.tripod.com/~StormTrooper_2/burn1.gif
http://members.tripod.com/~StormTrooper_2/burn2.gif
http://members.tripod.com/~StormTrooper_2/burn3.gif
leright
#16
May14-06, 02:55 PM
P: 1,194
Quote Quote by vanesch
Yes, agreed. A cow can get electrocuted, for instance, when standing near a defective high-voltage pole (when one of the phases touches the pole), because the potential difference induced in the ground near the pole over the distance between her legs is deadly.

In fact, when in "power electricity" class, one of the rules was to keep always your left (or right) hand in your pocket. Of course, it doesn't protect you from being electrocuted through ground, but normally the differential switch should protect you then, and you're protected from putting yourself "in" the circuit (which would NOT be detected by the differential switch).
I was also told (I think it was by a professor) that allowing a path of electricity to go into one arm and out of the other is worse than allowing a path of eletricity to go in one arm and out your leg for physiological reasons. If you allow a path to ground from the circuit by touching the circuit with one hand all electricity goes from your arm to your feet. If you touch the circuit with both hands current will flow through both arms and across your chest region (your heart), which is a more deadly scenario.

Also, the path between hand to hand is shorter than the path between hand to leg, which means less resistance so here is more motivation to only use one hand.

And also, like you said, the ground fault protection circuit doesn't work when you are connected into the circuit with both hands because there is no difference in current at the ground connection of the circuit (not much current that entered you takes another path to ground). However, if you are only connected to the circuit with only one hand there is a lot of current that takes another path to ground, so there is a LARGE current differential.

However, even if both of your hands are connected to the circuit some of the electricity will take an alternate path to ground, (through your feet and into the floor) and the GFP will still kick in. This would depend on the difference in the resistance in the path between hand to hand and hand to foot, however.
Line
#17
May14-06, 07:28 PM
P: 218
So I can touch a high voltage powerline without getting fried?

The current has an entry and exit point. it can go in one part of your body and out another back into the wire. DOesn't some current go through your body?
Cyrus
#18
May14-06, 07:31 PM
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No, you better keep your hands to yourself please, unless you have a death wish.


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