Why do birds on power lines not act as resistors?

[cmb]

There seems to be a lot of mis-information being allowed to go uncorrected here.

A bird that flies directly to a power line and grabs it makes no circuit. Therefore its resistance is immaterial (or futile, if you want a pun).

But what does happen is that it will become charged to the potential of the line. Assume bird is at 0 potential with respect to ground (just flown off of the ground) then it will need to receive enough charge to raise its potential to the line potential.

It will have a capacitance. Let's make a guess here, I'd say it's free-space capacitance is a bit like a sphere, let's call it 1pF. To charge to 500kV requires energy in the amount of 1/2.[5e5]^2.[1e-12] = ~0.1 J.

I reckon you'd not really feel a discharge much less than 0.5 to 1 J, but you'd likely feel a bit of a spark at that level. Maybe birds do feel it, but they get their kicks from landing on such wires.

Here resistance does play a part. That capacitance represents a total of Q=CV, or ~ a microCoulomb of charge. If that discharges in a microsecond then the bird would get an Amp of current. Again, it might not notice that anyhow, because the total energy in the pulse is small. If the bird's grip and its legs are a lower resistance, then the charge-up would be slower and the current lower.

To see how this charge-up process actually works, you should go looking for youtube videos of the guys that work on these wires - live - from helicopters. The helicopter comes up to the line and the guy on the back holds out a probe to faciliate charging up of the helicopter to the line potential. You see a spark of a few seconds duration flowing between the probe and the power line, then the guy hooks a wire onto the line, to maintain the potential, then he does his work. Those guys must be on serious danger money (because of the height - the voltage is immaterial so long as you respect it), some jobs they actually clip onto the lines and crawl off the helicopter onto the lines, and shimmy up it. A hat-tip to live helo power-line workers!

 

[psparky]

Oh so these power lines are actually close? so they can complete the circuit?

Wait a second here...

 

[Bassalisk]

There seems to be a lot of mis-information being allowed to go uncorrected here.

A bird that flies directly to a power line and grabs it makes no circuit. Therefore its resistance is immaterial (or futile, if you want a pun).

But what does happen is that it will become charged to the potential of the line. Assume bird is at 0 potential with respect to ground (just flown off of the ground) then it will need to receive enough charge to raise its potential to the line potential.

It will have a capacitance. Let's make a guess here, I'd say it's free-space capacitance is a bit like a sphere, let's call it 1pF. To charge to 500kV requires energy in the amount of 1/2.[5e5]^2.[1e-12] = ~0.1 J.

I reckon you'd not really feel a discharge much less than 0.5 to 1 J, but you'd likely feel a bit of a spark at that level. Maybe birds do feel it, but they get their kicks from landing on such wires.

Here resistance does play a part. That capacitance represents a total of Q=CV, or ~ a microCoulomb of charge. If that discharges in a microsecond then the bird would get an Amp of current. Again, it might not notice that anyhow, because the total energy in the pulse is small. If the bird's grip and its legs are a lower resistance, then the charge-up would be slower and the current lower.

To see how this charge-up process actually works, you should go looking for youtube videos of the guys that work on these wires - live - from helicopters. The helicopter comes up to the line and the guy on the back holds out a probe to faciliate charging up of the helicopter to the line potential. You see a spark of a few seconds duration flowing between the probe and the power line, then the guy hooks a wire onto the line, to maintain the potential, then he does his work. Those guys must be on serious danger money (because of the height - the voltage is immaterial so long as you respect it), some jobs they actually clip onto the lines and crawl off the helicopter onto the lines, and shimmy up it. A hat-tip to live helo power-line workers!

I always wondered why there was a huge spark between that probe and the line. Is it because the probe is initial at 0 potential with respect the to power line? And whole chopper is at 0 potential?

 

[cmb]

Here's a clear show of the helo linesmen at work;

 

[psparky]

There seems to be a lot of mis-information being allowed to go uncorrected here.

A bird that flies directly to a power line and grabs it makes no circuit. Therefore its resistance is immaterial (or futile, if you want a pun).

But what does happen is that it will become charged to the potential of the line. Assume bird is at 0 potential with respect to ground (just flown off of the ground) then it will need to receive enough charge to raise its potential to the line potential.

It will have a capacitance. Let's make a guess here, I'd say it's free-space capacitance is a bit like a sphere, let's call it 1pF. To charge to 500kV requires energy in the amount of 1/2.[5e5]^2.[1e-12] = ~0.1 J.

I reckon you'd not really feel a discharge much less than 0.5 to 1 J, but you'd likely feel a bit of a spark at that level. Maybe birds do feel it, but they get their kicks from landing on such wires.

Here resistance does play a part. That capacitance represents a total of Q=CV, or ~ a microCoulomb of charge. If that discharges in a microsecond then the bird would get an Amp of current. Again, it might not notice that anyhow, because the total energy in the pulse is small. If the bird's grip and its legs are a lower resistance, then the charge-up would be slower and the current lower.

To see how this charge-up process actually works, you should go looking for youtube videos of the guys that work on these wires - live - from helicopters. The helicopter comes up to the line and the guy on the back holds out a probe to faciliate charging up of the helicopter to the line potential. You see a spark of a few seconds duration flowing between the probe and the power line, then the guy hooks a wire onto the line, to maintain the potential, then he does his work. Those guys must be on serious danger money (because of the height - the voltage is immaterial so long as you respect it), some jobs they actually clip onto the lines and crawl off the helicopter onto the lines, and shimmy up it. A hat-tip to live helo power-line workers!

Cool stuff.

 

[cmb]

Is it because the probe is initial at 0 potential with respect the to power line? And whole chopper is at 0 potential?

I think you mean to ask if the probe is at +/-500kV. Yes.

The other thing to note is that the AC potential is, well, AC. The current discharge is therefore continuous AC until they are clipped on, because the helo acts as a capacitor drawing + then - charge on each cycle of the AC on the line. Once clipped on, the helo and the guys are both bouncing up and down through 500kV AC, with respect to ground, 50 [or 60, dependent on network] times a second. That's why they have to 'probe off' as they leave the line, as well as when they come up to the line.

 

[psparky]

I'm assuming those three lines are phase A for example?

 

[Bassalisk]

I'm assuming those three lines are phase A for example?

Yes I was wondering too. Because when I saw him touching all 3 I was like :O

 

[psparky]

Yes I was wondering too. Because when I saw him touching all 3 I was like :O

CMB...I'm assuming you have a physics background. EE's don't think much in the way of charge or capacitance. We tend to run from that stuff!

Also, you explanation is clearly better, but I believe my V=IR explanation still holds true.

 

[cmb]

I'm assuming those three lines are phase A for example?

Clearly!

If I were designing power lines [I don't, but just thinking it through], I'd really want redundancy in each phase. This strikes me as being essential because if one single line were to fail mechanically, you'd still get power transmission if there were multiple lines.

However, if one phase were to drop out, then you are in biiiig doggy doo-doo, if it is a network link. This is because you'd likely get a sudden phase-change in the link and where the next link in the network meets it there would be a phase error. This would trip the next link. That in turn puts a bigger load on the next link, which changes its phase, tripping out the next link, etc., etc.. I believe this is what happened in California a few years back, one link went down from overloading and it dragged all the others it was connected to down with it.

This is one of the highly beneficial engineering advantages of HVDC transmission as network distance links - there are no possible phase errors.

The other thing to bear in mind is that the skin depth of copper at 60 Hz is 8.5mm, so there is no point in having 60Hz AC power lines greater than 17mm in diameter. So in the video you see 3 ~17mm wide conductors. Wider conductors would be just a waste of copper, as the conduction losses would be exactly the same as a 17mm diameter conductor. So, for a given power transmission (viz. a given current) you need a certain multiple of 17mm diameter copper cables, rather than one big diameter one.

 

[jim hardy]

thinking real simply

let's take your 1 pf
and charge it to 500 kv

Q = CV = 1E-12 X 5E5 = 5E-7 Coulomb

that charge moves 120 times per second (it's 60 hz ac)

5E-7 Coulomb X 120 /sec= 60 microamps

so a current on order of 60 ua /pf will flow into anything connected to a 500 kv line.
and that's why a tiny current flows into a bird's feet - to charge the small capacitance of his body up to line voltage.


In my tinkering with old analog meters i figured out that my threshold of feeling electric current is somewhere around fifty micro-amps.
a quick google gave widely varied numbers for human body capacitance, generally in tens of picofarads. How to guesstimate a bird?


Linemen i knew wore a cloth suit with conductive fibers in it and the whole suit gets charged to line voltage. That way the current flows in the suit not him, else he wouldn't be able to hold his tools.


As to those swamp birds - sorry for the lack of clarity. The wires were about twelve feet below the steel crossarm where the birds roosted, and when a big heron emits a stream that's right over the conductor - well - flash bang he completes the circuit!