# Why weights on cables between utility poles?

Yesterday I saw what look like weights hanging from a cable between utility poles. This was in northeast Arizona on Route 60 between Show Low and Springerville. Here is a picture of a whole span:

The white blobs are the weights. Here is a closeup of a single weight:

They appear to be just concrete blocks with no dashpot or any other apparent way to dissipate power. These weights seem to be at 1/4 and 3/4 of each span, which would be the nodes of the first harmonic standing wave. They would add inertia and affect the frequency, but is that really their purpose? I can guess some possibilities, but I'd like to hear what these things are for from someone that actually knows.

Due to being a single cable, it must be for communications, not power, but I'm guessing that doesn't matter.

Sorry, I added this note earier, but apparently somehow aborted the editor so that it wasn't posted.

To answer PlasmaHH's question, yes, these were on just about every span. The bottom of the cable was quite high, and a little lower didn't look like it would have been any problem. The bushes you see in the first picture are well in front of the cable. There is a lot of clearance below the actual cable. Even if there was a tall bush there, there would still be a lot of clearance.

• Perhaps it prevents the lines from blowing around as much in the wind? – Brendan Simpson Aug 22 '16 at 16:22
• I can pretty much guarantee they're used to prevent vibrations and cable "galloping" caused by wind that could disrupt the connection. I'm afraid I don't really have any details at this point, though. – DerStrom8 Aug 22 '16 at 16:25
• Are those on every cable, or just one section? The concrete block doesn't look like it is several decades old, so for dampening I would have expected something more modern. A far fetched guess would be to raise the lowest point of the cable, so it would probably be only applied where necessary – PlasmaHH Aug 22 '16 at 16:41
• The resonant frequency $\omega$ is inversely proportional to the mass, so maybe it's intended to attenuate standing waves between the supports. – Spehro Pefhany Aug 22 '16 at 18:46
• They may just be used to weigh down the line to reduce most of the oscillations that are created within a certain wind speed range. But, then again, given the apparent surface area of the concrete blocks, it's hard to imaging they wouldn't have some kind of pendulum affect in the wind--but then that oscillation may not be as stressing on the lines as the ordinary wind induced oscillations... If they've installed a lot of them, someone probably modeled & tested it to ensure it's a wise implementation. – zeffur Aug 22 '16 at 19:24

There are several modes of vibration on conductors between poles. Different devices damp different vibrations. These weights are intended to primarily dampen torsional vibration.

Torsional vibration is more closely linked to low frequency high amplitude oscillation - conductor gallop - versus the high frequency low amplitude flutter, which is more commonly tamed with tuned mass dampers.

These weights, in other words, are more effective for the low frequency vibrations linked particularly to torsional vibration expected at this specific location than stockbridge dampers would be. Stockbridge dampers are more useful for high frequency vibrations (flutter, 10Hz or so).

As such I expect these are pendulum detuners:

Pendulum detuners.

These anti-galloping devices are based on the fact that the torsional movement of the bundle interacts dynamically with the vertical motion. Wind energy is injected to the vertical motion through torsional movement. The control of torsion can control the vertical movement. This occurs only when the torsional movement is close to the frequency of the vertical motion, which is valid for bundle conductor lines. To avoid frequency coalescence between torsion and vertical movement, which is at the basis of the instability, it is necessary to separate the frequencies one from each other (so called detuning). So, the principle of detuning is the avoidance of such frequency coalescence due to torsional stiffness increase.

Thus, while the despacering technique relies on changing the ice shape from a potentially unstable shape to a stable one, detuning accepts the ice shape but modifies the conductor dynamics in order to prevent the potential aerodynamic instability.

Some of the testing realized with detuning pendulum on bundle conductors gave satisfactory results. Their negatives impacts on the lines are quite small: some tests showed that the mass of the pendulums can lead to high values of conductor dynamic bending strain at the fixing clamps from aeolian vibration. An appropriate design (weight, arm length, location) is imperative.

There are newer devices that better control torsional vibration, each with distinct advantages (usually less weight) but they are also more expensive, and require new engineering work to determine the correct parameters, so you'll still see a lot of simple weights such as those you've pictured.

• Adam Davis had it right. Fascinating stuff; start here tdee.ulg.ac.be/doc-25.html and read thru for more info. You have to increment the page number in the address each time. – user121586 Aug 23 '16 at 21:08
• I've seen these in California as well, and had always been told they were to damp vibration in the cable. – Doktor J Aug 24 '16 at 14:23

I think it's unfinished business. I'll get to that lower down.

When you have a pole-to-pole run of delicate communications cable it needs to be supported in several places from a "structural" steel wire. The steel wire is suspended between the poles and is needed because the comms cable's own weight would inevitably cause it to stretch and fail. Installation engineers involved in wiring these cables up use things like this to support delicate comms cable: -

Now consider the scenario where it is expected to add more cables at some later date like this maybe: -

The concrete weights are there to be taken off (or made smaller) during the process of adding more cables so that the supporting steel cable (sometimes called a messenger wire) remains under the same stress i.e. it doesn't elongate due to the extra weight of new cables. If it did elongate then it could either stretch the original comms cable (if it were hard fixed to the messenger wire) or reduce the support to the delicate comms cable and damage it.

• I upvoted this because it sounds like the most reasonable, but now I'm not sure.. You start with "I think", and your answer is different from the other answers. Do you have anything to point to? – pipe Aug 22 '16 at 19:02
• @pipe well, the images are kind of a reference: We hold these images to be self-evident… – Marcus Müller Aug 22 '16 at 19:45
• I did research when I saw the question that led me to believe this is a likely reason but, given you can find pictures of sheep hanging dead from overhead cables, to get into the mindset of a random person hanging concrete blocks from a wire is a tall order. – Andy aka Aug 22 '16 at 19:47
• I don't really buy this answer. If the weights, wiring, and cables were all installed at the same time in the order: cable -> weights -> wiring, then there would be no reason to leave the weights on. If they were installed in the only other relevant order: cable -> wiring -> weights, then the weights would stretch the wiring. This answer really needs either citations, or at least verification from an engineer who's installed them. Wind / harmonic dampening makes a lot more sense and is the only explanation you find when you research these on the internet. – Jason C Aug 23 '16 at 21:03
• This is a very interesting hypothesis that noone else considered, +1. However, I'm accepting the answer that they are pendulum detuners since I think that is more definative with examples of very similar devices known to be used for that purpose. I also remember driving the same road maybe 10 years previously and wondering about the weights then too. They have been there for a while. This time I stopped to take pictures so that I could ask here. If anyone here is from around Show Low and Springerville, they can check them out for themselves any maybe give us a history. – Olin Lathrop Aug 24 '16 at 1:33

I'm pretty sure they serve a specific mechanic purpose, that nowadays is usually solved with Stockbridge dampers:

They absorb the energy of mechanical oscillations in the line.

• I expect you're right in that they have something to do with avoiding certain mechanical oscillations, but I don't see how a dumb concrete block absorbs any energy. If a dashpot or damper was between the block and the line it would, but there appears to be nothing in the linkage that dissipates mechanical energy. That's why I'm asking. I was expecting something dissipative, but there doesn't appear to be anything like that. – Olin Lathrop Aug 22 '16 at 16:52
• My understanding is that simply by adding weight, you change the intertia of the system, making it harder to excite under normal wind conditions – Marcus Müller Aug 22 '16 at 16:59
• ... Also, it's not the steady sideways force from the wind that causes problems, but the "flutter" effect, as with the Tacoma Narrows Bridge collapse. A couple of relatively heavy weights, that stop the cable flapping up and down freely in the wind, kill the problem at its source. – alephzero Aug 23 '16 at 3:05
• When protecting cables from winds, the last thing you want is to add more wind-exposed surface and create a potential pendulum with a concrete block. – Dmitry Grigoryev Aug 23 '16 at 8:46
• @DmitryGrigoryev unless the mechanical forces you fear are high-frequency rather than low-frequency. – Marcus Müller Aug 23 '16 at 9:01

They are dampeners. They are there to prevent the line from whipping up and down so much during high winds. Those oscillations can put more tugging force on the line than the line can handle. Putting weights on the line does slightly increase the tugging force on the line, but it prevents that force from becoming too large.

• FYI, dampeners are those things which keep potted plants moist. And why would you assume the middle of the line remains stationary during oscillations? That is certainly not true. – Dmitry Grigoryev Aug 23 '16 at 9:39
• @DmitryGrigoryev No, dampener can mean a lot of things. A dampener in the way I am using it is referring to something that deadens vibrations. Yes, a dampener is also something that can be used to keep potted plants moist, but that's not how I'm using it here. And I'm not assuming the that point in the middle stays stationary. I'm not sure how you got that from the drawing aside from you just read too much into it. I know it moves... – Chris Aug 23 '16 at 17:57
• I think you mean damper, not dampener. – markrages Aug 23 '16 at 19:31
• Seems like damper (a device that damps, meaning to make (something) less strong or active) and dampener (noun form of dampen, to check or diminish the activity or vigor of) are both right, though "vibration damper" is historically quite a bit more popular on ngrams search. No big deal. – Jeff Bowman Aug 23 '16 at 20:52
• I think we all know what Chris is describing, because we have brains, and it doesn't matter if there's an extra "ne" in there. Just call them dampificators and move on. – Jason C Aug 23 '16 at 21:07

I believe it does have a static harmonic balancer effect on fundamental standing waves.

THis may be an economical solution for single cables made from copper that are prone to vibration fatigue as the steel cable reduces the axial stress on the coax.

This method appears to give more sag and not used for AC phase lines as the sway might create gap issues and is just a band-aid to a long span single phase drops in rural areas.

I would never expect this method to be used by Quebec Hydro with Ice weight load risks compounded by weighted solutions such as this with 3 phase lines, but ok for Arizona rural users who expect Cable TV and internet.

From working in Telecom/Cable industry, and lack of HV insulators. I would expect this to be approx 3/4" solid copper coax with foam core. A "Hibachi" like alum case should be inline every few miles or so, to boost and equalize the video signals and 2 way internet modem links. Low voltage AC would be included to power the inline repeaters.

• Not all of Arizona is hot desert. The area of the picture is at about 7000 feet altitude, and would get snow and ice in the winter. – Olin Lathrop Aug 22 '16 at 16:50
• Yes but its a dry climate unlike Quebec's winters lowestrates.ca/sites/default/files/eastcoaststorm.jpg Even here in Toronto we had 10 % of certain trees fall in areas with ice accumulation. – Sunnyskyguy EE75 Aug 22 '16 at 17:17
• Half the displacement at double the frequency sounds like roughly the same amount of fatigue to me. Extra surface and extra weight only increase it further. – Dmitry Grigoryev Aug 23 '16 at 8:52
• Perhaps need a better model like an electrical transmission line with weighted impedance stubs to attenuate standing waves from pole reflections with a lossy line. – Sunnyskyguy EE75 Aug 23 '16 at 15:51
• As anecdotal evidence of rough winters in this area, the local high school football stadium is an enclosed dome. – Michael Richardson Aug 24 '16 at 18:38

You are right, they are for altering the self-resonant frequency. I can see this type of supressing the standing waves also in my country at regions with strong wind. Actualy the wind comes in bursts, it's not a constant wind. The stockbridge dampers are mounted as well, but I guess they can't avoid the resonance.

• Of course, altering the resonant frequency only helps agains winds which blow with constant frequency, which is very polite of them. – Dmitry Grigoryev Aug 23 '16 at 8:58
• They blow turbolently, so adding a weight, the new resonating frequency is a far below of the induced wind excitation frequency. The wind coming from mountains has never a constant low frequency. – Marko Buršič Aug 23 '16 at 9:33
• Your answer would greatly benefit from back-of-the-envelope calculations of the weight which is needed. – Dmitry Grigoryev Aug 23 '16 at 9:36

I think these weights should disable the 1st harmonic and enable the 3rd harmonic oszillation. The cable has more attenuation of the 3rd than the 1st harmonic or the excitation by the wind is lower for the 3rd harmonic.

The distances between poles and blocks look different, this might prevent stable oscillations steadly enforced by the wind until the steeel cable breaks.

The down-hill location shown in the photographs suggests that "buffeting" may be the issue, caused by vortexes shed on the lee side of the hill. Buffeting has an effect like galloping - long wavelength oscillation.

Galloping, caused by interaction of vortexes with cable rotation, can be controlled by controlling the rotation of the cable-bundle. Buffeting can't be controlled that way, because the vortexes are shed off the hill, not off the cable.

The normal solution to buffeting is to not place cables in lee-side vortexes. It's only the photograph which suggests this may be a problem here: are the same static dampers placed in locations away from hills?

• These dampers were on every span of this cable, which extended for miles, sometimes up and down hills. – Olin Lathrop Sep 7 '16 at 10:33