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the "high tension" is only a reference to the voltage dielectric tension , not the mechanical stress
a copper conductor has very weak mechanical properties , to string it from poll to poll some support is needed
the conductors moving in the wind are a big issue ,
High voltage lines are usually pretty bare and will touch each other if tossed about on a big wind .
It is quite spectacular in a disaster sort of way , the isolators are fixed on the pylons and will not move much at all , their biggest problem is being coated with dust , then it get wet , the isolation is going way down and there is arcing ..yukk
check this out !
https://www.youtube.com/watch?v=oBJyyEAw-6gthe good practice for an overhead conductor is one tie for 1.5 M , it is simpler to have a steel core or a steel sheath
to provide mechanical strength , it also is used as a heath fault detector ,
if you have too much voltage on the steel , there IS something very very wrong with your isolation
it doesn't have anything to do with current carrying capacity and is not even considered in the voltage losses calculations
I know , I did