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Discussion > Questions for electrical transmission engineers

Or, taking JC's 200°C temperature rise under maximum load (I'm guessing ambient of 20°C), that would mean ~0.8m extra sag for 300m spacing between pylons.

I'd have guessed that 0.8m was a fraction of the normal variation of sag due to random variations in cable weight, tensioning, pylon installation. In other words negligible.
May 28, 2013 at 5:40 PM Martin A

I was not paying attention.

The 0.8m is the extra length of the cable, not the extra sag.

How do you work out the sag? There must be a formula for the catenary. Or you could approximate it as a parabola. Or even as a couple of right angle triangles (sag = opposite, ½ distance between pylons = adjacent, ½ length of cable = hypotenuse). I'll try to put some numbers in later.

May 28, 2013 at 8:21 PM | Registered CommenterMartin A

Martin A;
Re 0.8m extra sag.....
The heating due to the current will have been covered in the design calcs. No doubt they would also have covered a much wider range of ambient temps than are ever likely. So the impact of a degree or two rise in the ambient would be a matter of millimetres and well within the design enveloppe.
My guess is that high ice loads combined with strong winds are more of a worry, as happened in Canada a few years ago.

May 28, 2013 at 11:08 PM | Registered Commentermikeh

mikeh - that's what I assumed.

What I am interested in is finding out if the Met Office's claim that the current in overhead cables will need to be reduced in hotter weather resulting from climate change because the hot weather will result in extra sag has any basis at all in reality. It would be good to have it confirmed authoritatively.


May 29, 2013 at 12:02 AM | Registered CommenterMartin A

Martin this may explain it for you.

May 29, 2013 at 10:30 AM | Unregistered Commentergeronimo