Most recent answer: 10/01/2012
Why can a bird stand harmlessly on a high potential wire?
There are two reasons --
[Our first reason here concerned insulation, and was incorrect: see below.]
The second reason is that you need a complete circuit to flow current through the bird to cause it damage. Even if there's no insulation, electricity cannot flow through the bird unless the bird is also touching another wire. "Bug-zappers" have closely-spaced uninsulated wire meshes where a bug can touch both one mesh at one voltage and another at another voltage, conduct electricity, and burn. One mesh is completely encircled by the other mesh, and if you touch just one, you don't make a complete circuit and are safe. The outer mesh should have the same voltage as the ground, so that if you touch it and the ground, you still won't conduct electricity. This reduces the hazards to humans who may blunder accidentally into one. It's still not a good idea to touch them (or even go near when they are on).
But have a look at this interesting news item from CNN, July 19, 2004 about a wildfire near Los Angeles which is claimed to have been started by a red-tailed hawk that hit some power lines, caught fire, fell, and ignited the dry plants below. I can only surmise that the hawk must have touched two conducting elements, completing a circuit.
Actually, this is a fairly common problem for larger birds of prey such as red-tailed hawks and even bald eagles. They are big enough to stand with one foot on each of two different power lines. If the two lines are at different voltages, there is a voltage difference across the birdís body and they can be electrocuted.
(published on 10/22/2007)
Follow-Up #1: bird on a wire
Actually, when birds are standing on a single phase power line with both feet, they do get electric shock...in the nano volt range so they don't feel anything. Even though they do not complete a path to ground, they are in fact in parallel circuit with the power line. Since the power line has not 0 ohm of resistance, even between the bird's legs, some electric current is deviated into the bird. That is Ohm's law! To prove it, remplace the part of wire between the bird's leg with a Gigaohm resistor, and see what happens to the bird. Electricity will always follow all the given paths, but how much current flows through those paths depends of their resistance.
(published on 08/28/2010)
Follow-Up #2: Birds on a wire
Since birds can sit on one wire without getting electrocuted, because they are not completing the circut, can they touch another bird or animal sitting on the same wire without getting fried?
- Rachael (age 17)
Brooksville, FL, US
Sure they can, because they don't complete a circuit. But they shouldn't bill and coo with another bird on the other return wire, else, fried bird for dinner.
The reason is that there is a negligible voltage difference between two birds sitting next to each other on the same wire as compared with the other other wire.
(published on 03/22/2011)
Follow-Up #3: 3-phase return current
This question is for Vickie Backman. In long-range very high voltage transmission lines (500kV +), is there any return by earth path for the current between the power plant and the step-down transformers or if the 'return' is made between the phases only? I ask this because it seems to me that a return path by earth throughout thousands of kilometers seems very impractical since soil has much higher resistance than aluminum wires.
In the unlikely event that Ms. Backman happens to read this, we'd welcome her comments.
So far as we can tell from our intensive study of Wikipedia http://en.wikipedia.org/wiki/Three-phase_electric_power
(you've got some real experts here) the low- power end of a 3-phase system does usually include a neutral wire to return any current needed in case the loads on the three phases aren't equal. They say that in the high-power parts, there may be a convention of only using two-phase loads, so that no neutral return is needed. In either case, as you surmised, the ground itself is not used as part of the circuit.
Strictly speaking, your comparison of the resistance of the earth and the wires isn't appropriate. The ground has much, much higher resistivity
than the metal wires. Therefore you can't make good wires out of it. How high the resistance of the path through ground is depends on what sort of geometrical shape is available to the current. With a sufficiently large effective area, that resistance can also be quite low.
(published on 03/25/2011)
Follow-Up #4: a bird on a wire
Hi :) I'll like to ask why there is a negligible potential difference between a bird's feet when a bird is standing on the same power line. According to Ohm's Law, this would mean that the bird has negligible resistance, such that it functions just like a normal wire. Is this correct? However, if the bird has negligible resistance, as current takes the path of lower resistance, doesn't it mean that the current will flow through the bird and then kill it?
Very confused D: please help, thanks!
- Melanie (age 14)
It's not the bird that has negligible resistance, it's the wire. The current keeps going through the wire. Almost none of it detours through the bird.
(published on 10/01/2012)
Follow-Up #5: birds on wires- correction
The bird on a wire answer in the electrocution section is incorrect. I used to work for an electric utility, and Im an electrical engineer. Now I teach physics.
The power lines on poles are NOT insulated wires, i.e., they do not have insulation wrapped around them. They cant! They would overheat under high load (current carrying) conditions. Overhead power lines are generally made of stranded aluminum conductors with steel reinforcing core wires (ACSR), although some older conductors are still copper. The insulation comes into play at the ceramic insulators that hold the wires away from the wooden or metal poles/towers. As long as the insulators are intact and relatively clean, the voltage potential is maintained and there is no path to ground. Birds are safe on electified conductors as long as they are only in contact with a single phase. Often, the larger birds, such as hawks and owls, will sit on pole tops. When they take off, their wing tips may brush across two phases, and this is rather deadly for them. It explains the cause of the California fire, and its common. The feathers can catch fire if the bird gets hung up in the wires. Ditto for branches-- they can hit one phase (one wire) only and be OK, but if they short across two phases, they provide a short circuit path and may even cause the wires to melt open (burn down).
All those insulated wires you see on poles are cable and telephone wires, and possibly 120V house drops. The higher insulated lines run 12 to 21 thousand volts from phase to phase, and need 6-12 inches of insulating material. Transmission lines on towers may have 3-12 feet of ceramic insulators to insulate up to 500,000 volts.
- Vickie Backman (age 46)
San Luis Obispo, CA
(published on 10/22/2007)
Follow-up on this answer.