Power Lines

[richb14]

Can anyone tell me why powerlines (electrical ones) sang in the summer and are very straight in the winter. Thanx Rich.

[CT]

Metals generally expand by about 0.1-0.2% for every one degree celsius change. So, if the difference between a cold day and a hot one is about 30C then the difference will be about 3-6% in length.

[Mattk]

but surely these lines are carrying electricity so are far hotter than a 'hot day' ?

[woodys_cool]

yes your right, but this is taken into account when tightening the wires, so they sag slightly when first turned on, then this 'base' temperature (even though it is higher than ordinary metal) will be affected by the little extra heat of summer and the cold of winter, this may not seem like a huge change but think of the ammount of metal wire involved (mostly quite a few miles) an this slight change in temperature has a huge affect on the length of the wire.

[Tim Baxter]

Generally power lines are made from aluminium 'rope'.  It's not such a good conductor as copper but it's so much cheaper that it can be made thicker to be as good a conductor as copper and it's stronger in its own right.

All power lines do heat up when carrying their load, there is huge difference throughout the day between when they're carrying peak load (mealtimes, commercial breaks etc) and when they are only carrying normal consumption current.  This results in changes in length (sagging) because the more current (power) the lines are conducting the hotter they get (just like an electric fire gets hot when you turn it on).  As previously explained the hotter they get, the more they expand.

Additonally, again as said before, the variation in ambient temperature causes them to heat up even more and then cool down during the day causing them to expand and contract.

[Space]

apparently it's mostly the ambient temperature that matters, not heat generated by electricity,
otherwise the wires would be looser in winter ! because in winter we consume more power.

I don't think the wires generate much heat, because they carry a very high voltage and low current. That is how they avoid to waste energy.

[Tim Baxter]

Maybe the ambient temperature has most effect on the lines but I stand by what I say.  There are current losses in the lines and they can only be lost through heat.