[sokka_h2otribe]

Some good reasons:

-gotta hang them somehow, and in a very controlled way.

-thermal expansion, very important not to cause axial strain on a cable, which happens on tight bends.

You might think it would be enough to just have slack "somewhere" but I think you get to have many many micro adjustments when you have it across the entire length.

Why don't HV telephone lines do this?

I have no idea. Maybe because they can hang and droop more easily. I hope someone more knowledgeable gives a real answer.

[matt-p]

You are basically spot on. Thermal expansion (primarily) but you need it to be evenly spread for lots of boring reasons like as you mentioned axial force reduction, fatigue life, anchor load limits and so on. These tunnels just use forced air to cool the (XPLE) cables so they're not oil filled or anything. Tunnels are basically all thermally constrained.

[blackhaj7]

Interesting, thanks!

[blackhaj7]

Thanks for the explanation!