It doesn’t melt, it vaporizes, causing a live plasma channel to form and a giant arc between the lines that is self sustaining. If they have a arc interrupter, it might stop it before it destroys the lines, but not often, and at the power levels we’re talking about here, it is a very difficult problem.

The technique the US Military uses to destroy enemy power grids is to detonate a ball of long stringy aluminum confetti/Mylar above the enemy power substations. The destruction is apparently substantial.

Check out Mylar balloons and power lines (lots of compilations on YouTube if you don’t believe me.