It's common (for me) to hear that a HV power line can kill a person without touching it "if they entered its field." I'm not convinced by this reason and I think it quite naive and that it doesn't clarify anything.
Warning: Video shows people dying. Video with information about the accident(longer than the YouTube version)
Here's a video of some workers killed by HV but it's not clear if the scaffold touched the line or just "entered the field of the lines."
Yes. The higher the voltage, the larger an air gap is needed to keep it from jumping or arcing between conductors. You, a wet fleshy human, can provide an ideal path between two high voltage conductors if you get in the middle. You do not need to touch one.
This is basically what allows lightning, Tesla coils, and Jacob's Ladders to exist.
As pointed out in the comments, the higher the voltage carried, the taller its transmission towers will be and the further each conductor will be from the others.
Gas is not a very good insulator, so enough voltage will jump an airgap. Once an arc is established between two points, the gas between then is ionised and becomes a lower impedance, meaning that current will continue to flow - and in fact that you need to move those two points further apart than their initial separation to break the arc. Paschen's Law describes the breakdown voltage of a gas, based on gas pressure and voltage across them.
A fluorescent tube relies entirely on this principle. The starter ionises the gas so that the voltage will jump the gap initially, then turns itself off because the spark will stay established. If the starter or tube are faulty so that the gas in the tube is not fully ionised and the spark does not sustain itself, the starter will keep turning on, the spark will keep briefly jumping without properly "taking", and the tube will flash annoyingly.
Fun fact - it's called an "arc" because if you have two points in free air with a horizontal air gap, as this principle was originally demonstrated, the air along the electrical path is heated and tends to rise. The two end points are fixed, but the path of lowest resistance rises with the hot air, so the glowing line of the electrical path starts at the fixed end points and curves up in the middle. The heated air quickly rises out of the electrical path, but of course more comes in behind it. In a sealed tube it naturally doesn't have those convection currents, but the term remains the same.
It is possible.
Another way to see it: a metal, like the one used to build the cables of a power line, is used in engineering just as a tool to simply and reliably control the flow of electrons from one point (at a higher electric potential) to another (at a lower electric potential).
The air we breathe can also act as a conductor if the potential difference between two points (wherever they may be) is high enough to overcome the electrical breakdown voltage (think of phenomena like lightning).
See also: https://en.wikipedia.org/wiki/Electrical_breakdown
The reason "entered its field" is pretty much mumbo-jumbo, but the effect it warns against is very real and very life-threatening, and the phrase probably makes much more sense to somebody without a high-level education than talking about potential gradients and insulation breakdown mechanisms, which are fairly irrelevant to the business of keeping oneself alive.
A human being is a very good conductor of electricity compared to air. At ordinary mains voltages, the plastic soles of one's shoes can offer some insulation against a lethal shock coming in via one's hands or head and out to earth through one's feet, but they are not nearly thick enough to present a significant barrier to tens or hundreds of kilovolts.
So, that human gets too close to a very high voltage cable. He's effectively earthed. The air distance between him and the cable becomes too short. The potential gradient becomes too high. The air breaks down. An arc forms, connecting the human between the cable and earth. He probably dies. If he doesn't, his injuries may well be such that he wishes he had died.
So stay out of that "field".
Well, yes. The field is where the energy travels. In between, and around, the conductors, a kind of electromagnetic whirlwind transports energy through space, neatly coralled by the slight movements of charges in the sea of free electrons in the wires.
Unfortunately, bring another body with charges that are free to move, (eg a sack of water with free ions), near the cable/s, and the field gets distorted and diverted, increasing the potential differences in the field. This eventually results in the intervening air gap ionising and further diverting the field energy in runaway manner.
That is very bad, on the whole, for the water sack:(
We used the term "Strike Distance" in the Utility for which I worked. On a new project, I arrived at work to find that the concrete poured only two days before (for a pair of bus supports) was being broken out. When I asked our Design Engineer "Why?" I was flabbergasted to learn that the Strike Distance for 345KV was (I think) FORTY-TWO INCHES and the height of these supports put grounded steel only 36 inches away. Yes, a very-hot line will reach out and grab you!
A few years later, I was in a Land Survey team and needed to raise the target/prism on my rod somewhat higher for the shot. I was not within 15 feet of the conductors, but the rod had enough antenna effect that I felt a strong, uncomfortable buzz due to current flowing through me to the ground below me. The moral of this second story is: even if you aren't close enough to cause an arc to appear, you can, at much greater distances, be uncomfortably affected by the high energies present in the vicinity of high voltage.
The answer is a definite yes, it is possible. If there isn't enough "safety margin" (height of the line above ground), the height of the person will be enough to shorten the "arching length" of HV wire. The result is that the voltage will arch from the line to (and through) the person, and to ground.
Whether the person gets killed, depends on how long the arch is maintained.
There's plenty of very serious risks involving arcing. Everyone is correct to point out that this is the main issue. However, there is another piece of the puzzle that would point directly to the argument that it can kill "if they enter the field."
Once you're in the field of a high voltage AC line, you have to start worrying about new effects that you didn't have to worry about before. For example, anything metal that you're holding has just become an antenna. Anything that can act like an inductor starts producing current. All sorts of surprising interactions can occur in a powerful EM field.
I would feel comfortable saying that the arcing is the really big hazard, but if you're in a strong EM field, you do have a host of unintuitive things that you have to pay attention to as well. But like all high voltage topics, the best rule of thumb is "If you have to ask Stack Exchange, don't do it!"
Having had the dubious pleasure of working on fences running below high voltage transmission lines I have been seriously zapped by inductive voltage on a number of occasions. I was once told by a transmission engineer that one of the reasons that high voltage substations have an earthing mat is for safety reasons. A 800KV transmission line can produce lethal inductive voltages in ungrounded metalwork sitting below the line. At the normal working height this poses no threat but even here you can take a neon tube,stand under the line and it will glow.
If the cable is on the ground and live then you have a step distance problem that if you take a large enough (normal sized) step, the potential between your feet can be such that your resistance is less than the ground and it goes through you. For places like substations where this is a problem there is a ground plane such that at least for humans this wont happen, nor when you touch a fence or other metal post (hand to foot distance).
You should shuffle or hop with your feet together (and dont fall) rather than walk if in this situation. Also note that power distribution systems might retry a few times if there is a fault, line goes down, power goes out, retry lines on the ground are energized, fault, off, retry a minute or two later...So dont trust a newly fallen line to stay off.
