Really? In all those EM fields you keep talking about, the EM stands for electromagnetic. Do you really think you can separate out the magnetic part into a nice little pile distinct from the electric part?
And if we’re getting our words right, you shouldn’t be saying that you “posit that birds indeed don’t get spontaneously fried on high voltage transmission lines, and that the majority of bird fatalities from power cables can be traced to electrocution from bridging adjacent wires.” You should be saying that you admit it. Finally. At long last.
E.M., I’ve never stated that birds are fried by EM fields; that was the question in my OP. But lots of people have been stating that EM fields have no or negligible effect, and that plain isn’t true. I did originally wonder whether the induced currents in body tissue would be enough to electrocute, and it seems that they aren’t. But it’s still possible to be harmed by EM fields without being electrocuted.
If you want to talk magnetic fields (which I don’t), then this is basically how it works: Electricity and magnetism are two sides of the same coin. You can have one without the other, and indeed most EM health experiments usually subject the test organism to one field or the other, but rarely both together, for reasons of simplicity.
When it comes to overhead power lines, the electric field is a function of the line voltage, and the magnetic field is a function of the line current. I’ve deliberately omitted talking about magnetic fields in isolation for reasons of clarity, and given the levels of confusion I’ve generated, that was probably a good thing. If I’ve used the all-encompassing term EM it was just for convenience and save on keyboard wear. In the cites that I’ve given on safety limits for electric fields, there are adjacent tables of safety limits for magnetic fields. Both are important.
Fridgemagnet,
Basically, what determines whether a bird can ‘sit on a wire’ has everything to do with the voltage vis-a-vis Corona-Discharge
At voltages around 230KV and above, the corona-discharge is such that it is extremely uncomfortable for any living thing to remain in contact with the conductor. Basically, it feels like being stuck with a bazillion needles (think about an amplified ‘pins and needles’ feeling from a waking limb). There was a recent History Channel show entitled HIGH RISK: HELICOPTER LINEMEN wherein a USA Airmobile lineman did not have his bonding jumper correctly attached to the conductor and had to call the helicopter back to pick him up.
Linemen that bare-hand energized Extra High Voltage (EHV), 230KV and above, lines always wear a conductive ‘Faraday’ suit which is woven from nomex and stainless steel threads. This effectively places them within a Faraday Cage and eliminates the effects of corona-discharge on their bodies (there is still corona discharge off the suit, but the linemen can’t feel it). There are a number of utilities here and in Canada that do EHV bare-hand work using insulated bucket trucks or insulated ladders. I have heard that Chinese linemen work EHV bare-hand off silk rope ladders but cannot verify.
On voltages below 230KV, a lineman can bare-hand without the suit as the corona-discharge is not as intense. Still, one must be isolated from a difference in potential (another phase or ground) or working out of an insulated platform.
In answer to the original post, it’s not the size of the bird (providing conductor spacing is sufficient), it’s the voltage of the line. So no, a bird can’t sit on ‘any’ high voltage line. I have never seen a bird perched on the conductors of an EHV line and i don’t believe i ever will. I have seen them perched on the static wires (those thin lines on the tops of the structures), but never on the phases themselves.
For those interested in live-line bare-hand helicopter work:
Yay! An answer! Thankyou, octothorpe, I owe you several pints of foaming nut-brown ale. Yup, I did wonder in my OP whether the size of the creature perched on the wire would make a difference, but it seems that it doesn’t.
Fascinating links, BTW. I’ve never thought of pylons as air purifers before, but the principle seems sound. It’s a larger version of the domestic ionizers that attract airborne grime to adjacent earthy surfaces. Except that power lines are AC, so they’d be alternately producing positive and negative ions. It’s another good reason not to live under a power line - a constant drizzle of airborne pollutants raining down.
A job as a helicopter lineman seems quite fun. From the photos in the links, it seems that the 'copters get a lot closer to the wires than I would have thought possible. All adds to the fun, I suppose.
Anyone curious to see the shape of electric fields for different types of conductor and particle can do some neat visual simulations on this vector field analyser applet.
[HIJACK] They also do very little perching because they only land to raise their children; they spend the rest of the year at sea, either floating or flying, and are considered pelagic birds. Ducks and geese are waterfowl, and often land in fields instead of ponds during migrations. Now, the northern spotted owl is a nocturnal raptor, and its talons, unlike the albatross’s webbed feet, are not normally covered in salty water (an electrolytic solution that decreases contact resistance). The owl’s talons have a known impedance, which, when we allow for the conductivity of mouse blood…
Reading this thread brought back memories of my experience with “barehanding” conductors.
As a student I attended lab classes at an AB Chance High voltage labs site. One of our lab assignments was to go into a four story lab room, don a farady suit, climb a 25 ft fiberglass ladder and “barehand” a 345Kv cable. The suit had a tail on it which we connected first to the conductor via a handheld pole.
I remember the feeling in my ankles and feet (not covered by the suit) which is best described as a 100mph windstorm. The instructor dimmed the lights in the lab and told me to take off my faraday gloves and extend my hand as far away from the conductor as I safely could. From the tips of my fingers I could see a faint bluish corona light emerging. My finger tips felt like they were asleep.
Before climbing down I surprised the instructor by leaning over to the cable and giving it a quick kiss goodbye. He said that I was the first out of hundreds of students to kiss the conductor. Gotta get your thrills wherever you can
In the movie Jurrasic Park, there’s a scene where a young boy is climbing over an electric fence (around 20,000 volts, IIRC) that is currently turned off. However, about halfway on his way down (while still ONLY touching the fence, not in contact with ground) the fence is turned on and he gets shocked (and thrown about twenty feet from the fence).
So in reality, would he have been shocked? I know that an electric fence is not the same as a live wire, especially since the fence is designed to shock things, but I still can’t see how he could get shocked if he was only touching the fence and not ground.
Electric fences incorporate the earth as part of the structure of the fence itself. I can’t remember Jurrasic Park, but standard electric fences have one or two strands of live wire and the rest of the fence wires are earthed. So anyone touching a live wire and any other part of the fence at all are going to complete the circuit and as a result will be shocked. Someone climbing the fence will certainly take a hit as the current travels from the live wire through their body and out through the earthed wires.
