Birds on High Voltage Lines
Most recent answer: 10/22/2007
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 theres 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 dont 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 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 bird’s body and they can be electrocuted.
(published on 10/22/2007)
Follow-Up #1: birds on wires- correction
- Vickie Backman (age 46)
San Luis Obispo, CA
(published on 10/22/2007)
Follow-Up #2: bird on a wire
(published on 08/28/2010)
Follow-Up #3: Birds on a wire
- Rachael (age 17)
Brooksville, FL, US
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 #4: 3-phase return current
So far as we can tell from our intensive study of Wikipedia (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 #5: a bird on a wire
- Melanie (age 14)
(published on 10/01/2012)
Follow-Up #6: What about capacitance?
- Ted (age 26)
Stanford, CA, USA
Yes, It is not strictly true that there will not be any current at all. There are currents, but they are really small, and this not limited to landing only. Perhaps most negligible of all, the humid air is not a perfect insulator, so there will be losses from the body of the bird. But as you also point out, a bird can be considered a (roughly spherical) capacitor with second shell infinitely far away and at 0 potential. Therefore the bird will be charged and discharged at f=60Hz (50 Hz in Europe), because the power lines carry AC, not DC.
Lets make a rough calculation considering the bird as a sphere with 20cm diameter, the capacitance C should then be ~10pF. The rms current is then 2πfVrmsC f. Say there is 100kV on the wires, these parameters give about 400 µA for the rms current. For comparison, for a human being AC currents of around 10 mA start to become dangerous.() For a bird, somewhat lower currents presumably can be dangerous. It sounds like even for the high voltage line, however, the purely capacitative current isn't quite a problem.
Tunc + Mike W.
(published on 12/05/2014)
Follow-Up #7: Man on a wire?
- praveen (age 23)
I do not think there will be a significant current in the human case, either. Let's have a look at the post #5. Humans are significantly bigger than birds. Say a pigeon in is 0.5 kg whereas a human is about 70kg. Unless the wire breaks, there will not be a steady current as the circuitry will not be completed. But using the crude sphere approximation, human's radius (r) should be 5 times larger. As the capacitance is linearly proportional to r, so 50 pF. This gives a current of 2mA, which is still lower than the cited threshold. I do not think the total count of ions in the body is relevant in this respect, but an increased ion density would mean reduced resistance of your hands, so your hands will heat up somewhat less due to resistive current flow [I2R = (ωVC)2R ]. However you will still be at a greater risk of skin burn, unless ion concentration is >25 fold larger. But a comparison of blood titers of the two species does not to reflect such a fact (compare vs. [see Table 1]).
(published on 03/18/2015)