HV transmission lines - UK practice.

I've recently noticed something on overhead HV transmission lines that I can't think of a purpose for. Close to each tower, there is
something hanging down below each conductor, including the Earth conductor at the top of the tower. Where the conductors consist of bundles of cables, each one will have these things fitted. They are shaped like a letter 'L' turned on its side, with the short part vertical and attached to the cable, and the horizontal longer part running parallel to the cable, and below it by a few cable diameters. the horizontal part looks like a foot or two long, but it's difficult to judge from ground level, some distance away, These things are fitted in pairs, pointing in opposite directions, and are fitted a few metres from where the conductors are attached to the insulator strings at each tower. There is usually one pair on each conductor on each side of each tower, but occasionally a second or even third pair can be found, at roughly equal spacings. They look almost like small radio dipoles, but that wouldn't make any sense. Does anybody know what they are?
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I've recently noticed something on overhead HV transmission lines that I can't think of a purpose for. Close to each tower, there is something hanging down below each conductor, including the Earth conductor at the top of the tower. Where the conductors consist of bundles of cables, each one will have these things fitted. They are shaped like a letter 'L' turned on its side, with the short part vertical and attached to the cable, and the horizontal longer part running parallel to the cable, and below it by a few cable diameters. the horizontal part looks like a foot or two long, but it's difficult to judge from ground level, some distance away, These things are fitted in pairs, pointing in opposite directions, and are fitted a few metres from where the conductors are attached to the insulator strings at each tower. There is usually one pair on each conductor on each side of each tower, but occasionally a second or even third pair can be found, at roughly equal spacings. They look almost like small radio dipoles, but that wouldn't make any sense. Does anybody know what they are?
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Sound like vibration dampers. Helps to keep the conductors from galloping when the wind blows.
Charles Perry P.E.
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     snipped-for-privacy@mail.croydon.ac.uk writes:

They are called Stockbridge Dampers.
The ones you saw are a horizontal tube which is a weight, which is attached to a sprung metal support bar which runs through the hollow and attaches to the tube at the far end. The sprung metal support bar is clamped on the cable. A google search turns up a nice picture halfway down this article, although the construction is not clear: http://www.theharrowgroup.com/articles/20020513/20020513.htm This was the classic design, but there are other designs to be found now, including simple spirals around the cable end as can be seen in a picture I took some time back when I first saw them:
http://www.cucumber.demon.co.uk/lights/pole.jpg
Dampers prevent vibration traveling up the cable and putting excessive stress on the insulator and supports. Vibration comes from wind passing over the cable, and also if the cable ices up in freezing rain or snow, and large chunks of ice break away and have an effect like someone plucking a guitar string.
The distance from the insulator and the resonant frequency of the damper are critical for proper protection of the insulator and support structure.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
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