Posted: 4/28/2008 11:09:56 AM EDT
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What does "tension" mean? |
and amperage is volume of current flow. Watts are total power (voltage X amperage) |
They're under high physical tension. This is to keep them from drooping dangerously close to the ground. They're also high voltage lines, but "tension" refers to physical tension, not to the voltage carried on the lines.
Tensions of up to 20,000 newtons are fairly routine. (About 4500 pounds.) And, there are two kinds of tension to understand: Natural tension and mechanical tension. Natural tension is the product of gravity on the span, while mechanical tension is the tension placed on the span to keep it away from the ground, and this is often a very significant amount of tension. I've seen power poles that served as anchor points for short runs of very heavy gauge feeder cables, that looked like they were about to launch. They're bent quite considerably from the tension applied. If the feeders were to break, you'd think that the pole tip would launch. CJ |
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I have some old ceramic power line suspension-type insulators that now serve as garden ornaments. Stamped on them is their tensile strength rating - 17,500 pounds. Similar ones are in use on some of the 12 KV lines in my neighborhood. (Not really high for transmission lines but enough to really mess you up.) Mine are somewhat similar to these, but the ceramic part is much wider:  ETA mine are more like this one. The glaze is beautiful dark brown, with areas darkened by electrical discharge.  ETA collecting insulators is fun! And I just ordered one from eBay for $5. Linky |
Assuming frictionless pulleys at the top of telephone poles, does it take more pulling force to tighten a cable stretched over 100 poles than just two, with the distance between each pole being equal? |
Wow! I never knew collecting insulators could be fun. 
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Yes. The tension in that case is due to the weight of the cable. You are applying a lateral load into the cable which pulls out some of the sag in the cable. Lemme do a quick free body diagram and it will make more sense. ETA Diagram  Ok, so the wire has a weight, and that puts tension on the wire at the towers. To pull the wire, you are taking out that slack and counteracting that force. Thus, if you have 2 towers you will have twice the weight, 100 towers, 100 times the weight. So my answer is kinda wrong. If you are pulling to get to a certain sag or height, it will take more force over more towers. But the tension in the wire is from two places. Part is from the tension you put on it by pulling. The rest comes from the weight of the wire. So, more correctly, a longer wire will have a higher initial tension in it compared to a shorter wire. It will take less force to get to a specified load. So, it will take more additional force to get a short wire to, say, 10 kip (kilo-pounds) of tension than a longer one. The longer one will have a higher initial tension. |
I'm no scientist, but I would guess it would be equal power for either. |
We call it potential difference today. Same idea, I see what he means. And unless there is a tension in electrical terms, it refers to the force applied to the line. |
Really? Anyways, if you're going to make them frictionless, might as well make them massless as well and save the time of figuring out the rotational momentum. |
Really? Anyways, if you're going to make them frictionless, might as well make them massless as well and save the time of figuring out the rotational momentum. Okay, assume ideal pulleys. Massless and frictionless. |
Given two telephone poles set distance apart with a cable strung between them. The cable is tied off on one pole, and looped over an "ideal" pulley on the second pole. Your job is to pull on the loose end of the cable until the cable between the poles is either a certain distance off the ground, or at a certain tension. I think you will find that it takes the same amount of force for two poles, than if there was 100 poles. Reason being the middle poles cancel each other out as long as they are ideal pulleys. |
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In an infinite series of spans, applied tension would be the same across all spans. But due to real world considerations including friction, tension can and does vary from span to span and is going to be most different at the ends of the span, where they're anchored. For equal length spans, of equal cable gauge, less tension (applied tension) means more droop in the span. I was told a story once by a fellow phone worker, a guy much senior to me. (I used to work for BellSouth.) There's an aerial phone cable (on poles, not underground) that's about as thick as your wrist and due to age and damage, needs to be replaced. So one guy on the replacement team gaffs his way up the pole, sets his safety strap. has good access to the cable, and proceeds to loosen the cable strand clamp. Well, that's fine, because the idea is to loosen all the clamps and remove the cable by reeling it in from one end of the run. But the ninny doesn't know what he's doing and tries to REMOVE the clamp. The cable weighs about four pounds per running foot. The span is about 160 feet, from the pole on one side of him to the pole on the other side of him. Graphic experiment: Tie a string to one end of a ruler. The string is maybe four inches long. Stand the ruler up next to any convenient vertical surface. Tape the loose end of the string to that surface and let the ruler fall back. It'll fall back a little, until the string tightens, forming an angle. The guy on the pole is at an angle like this, held to the pole by his gaffs on his feet and the strap around the pole, attached to his belt. Now what happens if you drop a weight on the string? Yep...640 pounds of freed cable smack down on his safety strap, driving him face-first into the pole with great enthusiasm and authority, and pushing his gaffs in so deep to the pole that eventually they had to be "abandoned in place". So the guy's stuck up on the pole, half squashed into the pole, screaming for help, and my friend and co-worker is lying on the ground in a pure fit of hysterical laughter. Eventually a bucket truck gets to the scene of the disaster, the cable is lashed up and hoisted away, and the poor guy is removed safely but had to leave his deeply embedded gaffs behind.  CJ |