Bird on an overhead line - Current flow

Fjolvar

Hi I have a basic question. When birds sit on an overhead line, they are sitting on a bare conductor which is transmitting high power, but they don't get electrocuted. I think the answer is because the bird itself doesn't complete the circuit so current won't flow through them, but I don't fully understand why. Why couldn't the current form a loop or circular shape path within the bird and then leave the bird back into cable, thus forming a closed loop and conducting current?

berkeman

The resistance through the bird foot-to-foot is much higher than the resistance of the wire between the bird's feet. So pretty much zero current flows through the bird's body instead of going through the bird.

FOIWATER

It is voltage that is the 'driving force' for current to flow. A volt is equal to a joule of energy, per coulomb of charge. When there exists a potential different between two electrically continent points, current flows.

For all intents and purposes, the difference in voltage between the birds feet is zero since the voltage on the line is ideally constant at any point.

That is why no current travels through the bird. I've read that they do enjoy sitting on the lines, though, as the high voltage provides a 'tingle'.

Fjolvar

So the current chooses the path of the wire instead of the bird since it has lower resistance, but if the bird was holding onto a second wire that was grounded, would the current still flow through the first wire as before or would it now flow through the bird and then through the grounded wire? I read somewhere that if the bird is grounded then it will be electrocuted, but I don't understand.. does the ground create some potential within the bird itself? I mean wouldn't the bird have the same resistance no matter if it's touching the ground or not? Sorry for all this confusion, I'm just trying to convince myself of this phenomena and going through the arguments in my head :)

sophiecentaur

The high voltage lines carry AC (with a very few exceptions). When an animal hangs on the line, it has a certain Capacitance (a tiny capacitance between it and Earth). The AC voltage on the cable causes a minscule charge to flow on and off the animal every cycle of the AC. The size of the animal and the actual value of the AC voltage govern whether this tiny AC current is enough to kill it, tickle it or have no effect.
Apparently, the highest voltage transmissions lines cause death or discomfort to birds that try to perch but the intermediate voltage lines (up to about 50kV) are OK. Hence you only get birds perching on them.
Maintenance staff can work on the high voltage lines, live. They get access by helicopter and they have to wear Faraday (conductive) suits so that the induced currents flow over the suit and not through their bodies. (Total nutters - I reckon - hanging up there with those lethal voltages and then having to step off and onto a hovering helicopter. Danger of death, danger of death and danger of death)

jim hardy

lineman's hot suits (a Faraday cage they wear)

http://www.hubbellpowersystems.com/l...ories/suit.asp

Sophie is right. Capacitance is proportiopnal to area/distance, and the bird's surface area gives him a small capacitance to earth. If capacitive current into and out of his feet is enough to tickle he'll probably leave to find another wire.

I recently drove across Kansas , quite a few miles along some medium voltage transmission lines. On each pole was a structure to keep birds from landing on the crossarm that supports the wires. That's because a large hawk (or buzzard) has wingspan enough to bridge the distance between the wires ..

old jim

old jim

sandy.bridge

Fjolvar

Can anyone help me with this one please?

DrZoidberg

I have a 220kV line running past my town and I have often seen birds sit on the ground wire but they never sit on a current carrying wire.

gnurf

There's a potential difference between the wire and ground regardless of the bird, but when you connect a conducting bird between the two, that entire potential will be across the bird. The bird provides a current path with whatever resistance the bird can muster. OK I just realized I picked a rubber duck. I'm not very good at this, sorry.

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Fjolvar

Thanks for the illustration! So just to clarify, when current is traveling it will choose the path of least resistance, but if there are two paths (through the bird which is grounded or continuing through the original wire) it will flow through the bird because the ground potential is present on the other side. So current always wants to make it to the ground potential even if it has to flow through some higher resistance to get there despite there being an immediate lesser resistive path such as the wire transmitting power?

DrZoidberg

No, that statement is simply wrong. The current will always! choose all available paths. And the current through each path is always! V/R. Now V may drop if the load is too high but that won't happen in this case.

Fjolvar

There seems to be a contradiction here, unless I'm misunderstanding something. Can anyone explain please?

Fjolvar

Yes but there is a voltage difference from the line to the bird right? So wouldn't this cause current to flow through the bird??

berkeman

No contradiction. There is a current division, with the ratio in the currents inversely proportional to the resistance of each path. The current that flows trough the bird's legs is infinitessimal because of the much higher resistance compared to the low resistance of the piece of wire between the bird's feet.

Look up "voltage divider" or "current division" to learn more.

sophiecentaur

Forget birds and wires. Take two (or any number of) resistors in parallel. The current will be V/R in each case - hence the lower resistance path will pass more current. The voltage across a few cm of cable will be very very very low (you may lose 100V over 100km so that would make a drop of 10μV for each cm of wire). This means that the voltage between a birds feet is no more than a few tens of microvolts.

sophiecentaur

This is where the Capacitance of the bird's body comes in. It can store charge and discharge it back to the cable as the voltage varies. The voltage on the bird 'follows' the voltage on the cable as the charge flows on and off the bird with the alternating voltage. This corresponds to a small (AC) current flowing up and down its legs. As described earlier in the thread, the Capacitance for a large bird is greater than for a small bird and the magnitude of AC current in the bird's legs is not enough to harm it when it stands on intermediate voltage cables but it is enough to be uncomfortable / dangerous on very high voltage cables - 100kV and more.

Most cables are spaced far apart enough that most birds will not bridge between two conductors. Their feathers are pretty good insulators actually and would limit the current so I imagine they would get more of a tingle than the shock that humans would get hand-to-hand. For a similar reason, gamekeepers can discourage mammal predators around nest boxes with small electric fences which affect wet, inquisitive noses but not the birds' beaks, I believe.

Incidentally, birds are often frazzled when they perch on the feeder lines connecting high power radio transmitters to the antennae. At radio frequencies, the Capacitance allows much greater current to flow and they do not get the buzzing sensation that 50Hz gives them - so no warning.