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  #1  
Old 02-10-2003, 07:00 PM
Joey P Joey P is offline
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Bouncing Telephone Lines

I was on my way home from somewhere today, and while sitting at a red light I noticed that a telephone line (between two poles about 40 feet apart) was bouncing. Like the shape of a sine wave several periods long. I at first figured that somewhere on the line, there were people working on it, but no one in sight. Then I figured maybe the wind was doing it, or the poles were shaking. But all the other lines (electrical and phone) were perfectly still. As I made my right turn, I noticed that the it was doing the same thing for about a half mile (even with all the poles in between that you'd think would dampen it). The only think I can think of is that there was exactly enough current going through it to cause it to resonate. It went the entire length of the wire, from where it came up out of the ground, around a turn and until it went back into the ground. And none of the other wires on the poles were moving at all. If it makes a difference it was one of the big one inch or so diameter phone wires. Has anyone else ever seen this happen?
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  #2  
Old 02-10-2003, 07:26 PM
Ezstrete Ezstrete is offline
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conductors

"Dancing conductors"-----a phenomenon of weather-------wind,ice ,wire length,degree of slack etc.

Can actually become so severe as to bring wires down and damage suspension insulators,

You'll see some hi-lines which have weights on each end of a conductor span to act as counterweights to minimize vertical motion.

Real trouble for long line hi-voltage disatribution.
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Old 02-10-2003, 07:27 PM
Q.E.D. Q.E.D. is offline
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This is why drugs are bad, kids.

Seriously, I've never seen this happen, and I can't even put forth a WAG as to what caused it. But, I can tell you it wasn't the current. There's no mechansism for electrical current through a wire to cause it to bounce in the manner you describe.

Thinking about it, I think I can put fort a WAG. I'm thinking it probably was the wind. The wire was probably swinging back and forth, rather than bouncing up and down. From your perspective it would look the same. And it makes sense. Larger, less tense wires would be more susceptible than thinner, more tight ones.
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Old 02-10-2003, 07:36 PM
Mangetout Mangetout is offline
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I suppose the wind blowing over the wires could cause them to oscillate (in the same way that a taut rubber band or a blade of grass can be made to vibrate in a stream of air) - the resonant frequency of such a long, relatively slack wire must be quite low; what you're seeing is probably what a violin string would look like if it were filmed then replayed as greatly reduced speed.
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Old 02-10-2003, 07:38 PM
Joey P Joey P is offline
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I don't recall it being all that windy (but I was in a car). The thing is that even the wires that were perpendicular to the first ones I saw were doing it to. This could be explained by wind moving at a 45 degree angle to the wires. BUT none of the other wires were doing it (including other telephone wires of simlar diameter). Also, when wind causes things to sway it usually somewhat irregular. This was perfectly regular motion across the enitre wire. As far as I could tell each section (between each set of poles that is) was at the same speed. I've really never seen this happen. Quite odd.
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Old 02-10-2003, 07:44 PM
Berkut Berkut is offline
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Man, I had this very same question on my mind yesterday, fellow Joey. We got a decent amount of snow here in TN, and it stuck to the powerlines. While I was gassing up my car, I was standing there watching the lines bounce up and down, with a pretty good amount of deflection. It was calm out, so I know it wasn't the wind. The lines were alternating also, the bottom one was bouncing exactly opposite the one above it, and so on.

Pretty weird. It was a snow covered night, and dead still out, except for the powerlines merrily bouncing around.
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Old 02-10-2003, 07:53 PM
micco micco is offline
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If air flows over a cylinder (wire), you get a pattern of oscillating vortices shed in the wake. That is, first the top surface will shed a vortex (axis parallel to the cylinder) then the bottom surface, etc. The asymmetry leads to an oscillating side force on the cylinder (e.g. lift) which pushes it up and down. When the frequency of the vortex shedding corresponds to a resonant frequency in the length of the wire, the oscillations build.

While the side force is relatively small, it can occur in very light wind. The more important parameter is duration of the wind, that is, if the wind blows the same speed for a long time, there is more chance for this oscillations to build to a visible strength. If the wind is gusty, the frequency of oscillations will be changing and the force on the wire may get damped by other factors. I've never seen this "in the wild", but I've demonstrated it in a low speed wind tunnel, and I think this effect could certainly be responsible for what you saw.
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Old 02-10-2003, 08:23 PM
Berkut Berkut is offline
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I don't think there was any wind, though. There was snow falling, and I could clearly see the flakes for a good 30-40 feet up with the gas stations solar-like lighting. They didn't show any wind that I could tell. Could be that somewhere down the line they were getting blown a bit.
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  #9  
Old 02-10-2003, 09:48 PM
consolid8 consolid8 is offline
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The CATV wire on the poles across the street do the same thing. It vibrates loudly sometimes. It happens whenever it's still outside and not at all it appears when much wind is present. Freezing cold or summertime hot it still happens. The power lines above it never rattle. I notice the CATV wire is quite tight, and the worst vibration is in a particularly long stretch between poles. Drives me nuts in the summer; I wish it would just rattle loose and fall already!
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  #10  
Old 02-10-2003, 10:01 PM
DaveW DaveW is offline
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What a coincidence. I saw my first bouncing line this morning while driving to work. I was stopped at a red light, and noticed a big 3- or 4-inch cable on the opposite side of the intersection waving merrily in the light snow (no winds). None of the other 8 or 10 cables were doing it, and the vibrations stopped at the poles on either side of the intersection (in other words, the vibrations were limited to that one section of cable, for as far as I could see each way).

I could understand why the vibrations stopped in one direction, since there was a little slack loop (don't know the technical term) at that pole (the tension held by steel cables, instead, and the main cable drooping perhaps a foot). But that wasn't the case at the other pole.

I was a little worried about driving under it.
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  #11  
Old 02-10-2003, 10:41 PM
Scruloose Scruloose is offline
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OK, this is crazy. CRAZY! I just saw this exact same thing for the first time on my way home this Friday. Only one "section" of wire was swinging around, everything else calm. No workers in sight! CRAZY this is, I tell ya!
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  #12  
Old 02-10-2003, 10:45 PM
DaveW DaveW is offline
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A 4-inch cable on a pole? What the heck was I thinking? It had to be smaller than that.
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  #13  
Old 02-11-2003, 10:18 AM
Nametag Nametag is online now
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I suspect that part of the answer is in the pole. Most of the support poles for power lines and such are anchored firmly, and made of a material of a certain stiffness. If one pole in the series is a bit looser or a bit more or less flexible, then wires on both sides will, at times, help propagate a standing wave that is suppressed by the other poles.
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  #14  
Old 02-12-2003, 05:40 PM
octothorpe octothorpe is offline
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The phenomenon that is being referred to here is called 'line galloping' and can occur on any overhead cable from telephone to EHV transmission lines. It is normally associated with icing and wind. The methods employeed to minimize the effects of galloping basically break down into three categories:

Dampners - designed to cancel out the oscillating frequency;
Spoilers - designed to break up the airflow across the cable;
Cable profile - cables constructed such that they provide a non-uniform surface to the wind. This can be accomplished during manufacture by making the cable oval shaped, adding an over-sized strand (for bare cables), or twisting 2 cables together (commonly known as T2 cable).

Another method, employeed in communication cables, is to run an additional messenger cable (high-strength steel) a few feet under the communications cable and attach the cable to the messanger at regular intervals.

To get a look at this phenomenon for yourself, download the mpeg here.

#
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  #15  
Old 11-18-2012, 01:21 AM
digibud digibud is offline
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video of bouncing telephone wire

Found this thread while looking into a bouncing wire I filmed. In the film I make the guess that the movement is getting its energy from the snow but my brief research points to the likelihood it was the wind over the wire, however small, that caused the wire to set up some kind of a sinusoidal bouncing in response to the breeze.
The street seen in the short clip is actually a pretty busy street but you wouldn't know it from the looks of it.

https://www.youtube.com/watch?v=rfPgj5COUNc
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  #16  
Old 01-10-2013, 03:27 PM
digibud digibud is offline
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so much for wind

watched another bouncing telephone wire (I don't know if it was power, telephone or what) bouncing along and the car exhaust showed there was virtually no wind. Most other lines in the same area (busy 4way intersection with many lines) were not bouncing.
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