A couple of days ago I was watching a crew of linemen working on the high tension distributor cables running down the street on overhead poles. Evidently, the cables were live while they were working on them.
Two of the linesmen were in a cherry picker, and were dressed in a black body suit that covered them entirely, including hands and head.
From where I was standing it seemed that there was a cable with one end attached to their body suits and the other clipped onto the high tension cable that they were working on. There also seemed to be a second cable attached to the body suit, but I could not see where the other end was attached to.
Given that I was some distance away, and am not sure of what I think I was seeing:
what was I seeing? Why were they wearing these suits? What is with the cables connecting the suits to the high tension cables? What was that all about?
(1) The worker’s electric potential is first raised to the line’s electric potential and then maintained that way while the work is being done, and
(2) At no time during this process does the worker touch anything that might conduct electricity to ground.
If these conditions are met, then no current flows through the worker’s body and the worker is safe from electric shock.
The suits are most likely protective insulating gear. Even with such a suit, the worker still has to meet the conditions above.
There are several methods of safely establishing the connection between the worker and the line. Once that’s done, the cables between the protective suit and the line keep the suit’s potential and the line’s potential the same. When the work is done, the procedure has to be reversed so that the worker can safely return to ground.
The suits serve two purposes. First, they are usually made out of a metal mesh that conducts any electricity over their surface, protecting the lineman from any electrical discharge that would otherwise go through his body. Second, the suits are designed to protect against an arc flash, which also includes protection against burns.
The bucket in the cherry picker is insulated. One line attaches the lineman to the line being worked on, which brings him up to the voltage of that line. The second line goes to the bucket so that it is also at the line voltage. Once all that is done, the lineman can safely pick up tools and work on the line without getting shocked (it’s the same reason birds don’t get zapped when they stand on power lines).
The suit is made of a fine metal mesh sandwiched in plastic, making it a wearable Farday Cage.
In simple terms, you get electrocuted because there is more “electricity” at one point on you body than there is at another. The electricty then tries to disperse by flowing from the high point to some low point, through your body. It’s the electricity flowing that kills you, not it’s mere presence.
The point of the Faraday suit is that it keeps the level of electricity “even” all over your body. In oversimplified terms, because it’s metal, the current runs *around *the suit near enough instantly, so a “high electricity” area can never develop. With no high or low points the electricity never needs to run through your body, so you can remain safe in contact with a live wire.
The whole point of clipping the suit onto the wire is ensure that the current flows through the suit constantly, so the suit is always at the same potential as the wire. If it weren’t clipped on, then any time they had let go of the wire they would get zapped when they touched it again. The result probably wouldn’t be fatal as long as the cherry picker remained insulated, but it would be the mother of all static discharges, like the zap you get leaving a car on a dry day multiplied by a hundred.
The second cable would have been clipped onto the cherry picker. It stops them falling out, nothing fancy but kinda important.
There’s a bit more about this on the wiki under “Live Line Working.”. In fact, it says:
But if an approaching worker is liable to get a debilitating shock as he approaches… why don’t birds? I know that once they are sat on the wire they are at the same potential, and therefore no current flows, but why don’t they get the same shock human workers get as they approach?
Yeah ? So it protects his body when the metal in the suit turns into plasma, and the body is thrown 100 yards down the street ? That would a pretty amazing plasma at work, how do they get it to do all that ?
Well no, those suits don’t protect if hit by the full current of a short to ground.
The power (and energy) of the shock the suits can prevent must be quite small… IE it can only prevent from static electricity, the accumulation of charge near the high voltage.
Also, the talk of insulators is too loose. If its insulated, it resists all electricity ?
No, insulators hold off a certain voltage per millimetre… the best plastics are around 1000 volts per mm. But Don’t trust your life with this figure, find it out for your material yourself.
Air also has a maximum voltage over a distance… you can get shocked for being just too close to a conductor… The current can jump over a meter for 300,000 volts…
It sounds like if the workers were just in this cherry picker and went up to the wire, the lines were off. The protections are just against residual/induced voltage. (like static electricity).
If they only worked with poles, of a few meters length, then perhaps the lines were still on.
Main point of my post was to say that None of : metal suits, thin layers of plastic (and any improvided “insulator” ) , and “Not touching” are not protection against high voltages… Also you do need to discharge the wires to ground, even when the wires are off.
They do.
I have seen hummingbirds that were killed by landing on the charged wire of an electric fence. Normally, one can only be shocked by bridging ground and “hot” on an electric fence, but this fence charger had enough voltage to shock a bird to death just by touching the hot wire alone. The capacitance of the bird was enough to cause a fatal current.
In general, power transmission lines are lower frequency than fence chargers, so it’s less of an issue.
It’s not strictly true that no current flows when you or a bird is attached to the wire. The voltage is going up and down 60 times per second, and to match that voltage, your total charge needs to also go up and down 60 times per second, so there’s some current flowing, but it can be a very small amount. You are much larger than a bird, so it will take more charge to raise you to the right voltage, so you will have more current flowing. I’d need to think about whether the difference is proportional to length or to volume. A larger line voltage obviously makes this current flowing larger. A higher frequency also makes the current larger, since the same charge needs to be built up in a shorter time.
Also, before you or the bird touch the wire, you are shorting out the electric field in your body by having some charge flowing to the part of you near the wire, and opposite charge flowing away from the wire. Since you are larger, you are going to have a larger charge separation, and so a larger charge near the wire before any arc happens. A larger charge means a larger voltage difference, and the arc can happen at a larger distance from the wire. So your being larger than a bird makes it worse for you in two ways.
There is a most excellent video on Youtube of a live-line worker being conveyed by helicopter to the line.
He’s dressed in a magic suit so that the varying voltage in the electric field around the wire can’t hurt him (otherwise having part of his skin a few thousand volts above some other part would be really dangerous although not for long), but being earthed would kill him anyway. As the chopper approaches, he uses a wand to conduct charge to it and not through him as it has enough capacitance for a respectable current to flow at that voltage - the crew inside the heli are in a Faraday cage so they have nothing to fear.
He then clips himself to the line and he, the helicopter and the wire are all at the same potential at the same instant. Then as the heli pulls away he again uses the wand, now connected to the wire, to equalise the helicopter with the wire safely (i.e. not through him) until it’s out of range and he’s good to go. He’s now attached to a wire carrying 400 kilovolts but it has nowhere to go through him so he’s safe; he can shuffle along and make his checks and repairs.
He adds in voice-over “I’ve only ever been afraid of three things: electricity, heights and women. And I’m married.”
They have all kinds of things to fear. Helicopters flying that close to big cables are kind of asking for trouble. That’s a lot of mechanical energy, height, wind gusts, and human skill tangling together. They don’t have to be worried about the capacitive high voltage effects, but they’re still way braver than I’ll ever be.
No, the workers use these suits to work on lines that are still carrying full current. That’s the whole point of them.
No, the workers use these suits to work on lines that are still carrying full current. That’s the whole point of them.
And yet the workers use these suits to work on lines that are still carrying full current. That’s the whole point of them.
No, you don’t.
If you attempted to earth the discharge the wires when they were on by earthing them, then you would trip every circuit breaker in the system, maybe before you blew every component in the system.
And when the wires are off, any residual charge will be instantly dissipated by whatever they were supplying power to. A couple of TV sets are enough to dissipate the current from a wire that is off. No need to discharge.
So noted, but I believe you understood what I meant. It looked like a lovely calm day anyway - I don’t see linemen wanting to work when they’re swaying a few feet back and forth with every gust…
At the time I was rubbernecking, I was told that the lines were “live” because turning them off would have shut down the entire downtown area, and the electricity company would have been sued out of existence.
But this raises two questions: who was the first guy to attempt to work on live wires; and how long did he live?
Also, does a Faraday cage have to be mesh? Can you have the same effect using a sheet of conducting material; ie: is a box lined with aluminum foil the same as one lined with copper mesh?
A solid sheet is better than one with holes, but at these frequencies, it won’t matter if the Faraday cage is a mesh or a solid sheet. At higher frequencies, the holes have to be smaller, much smaller than a wavelength. A good example is a microwave oven (although there the waves are kept in, not kept out). The wavelength is about 5 inches, and the holes for you to see through are much smaller.
I’ve realised that I have a question after watching that video again, which isn’t answered in it: What exactly is the maintenance that they are doing on the lines? I see them crawling back and forth with a bunch of tools, but no explanation of what they actually do. Is it testing for faults? Welding cracks?
Also, do they have to take special precautions around the insulators that hold the wire onto the pylon? Or is the presence of the insulator enough to keep them safe if they, for example, stand up on the wire?
If the insulator’s not letting the line current leak to earth, chances are they’re not going to earth the lineman either. But they wouldn’t want to make themselves a conducting path that offered a possibility of arcing to the pylon, I’m guessing.
That was my guess too, but I wasn’t sure. I was thinking of a situation where the wand that they carry to equalise the voltage before stepping onto the wire causes an arc to the pylon, or it arcs through the suit because they get closer to the pylon than the wire does.
