Maybe for a short outage, I'd expect it to vaporize without the power line noticing. Even if noticed, automated systems will send a test voltage along the lines periodically.

The thin wire will melt appart before the power lines do.

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.

Arcing powerlines generally doesn't cause irreparable damage. Sometimes a swarm of birds will trigger a swarm. The protection circuit normally trips and it's fine.

Over a substation I could see it causing a lot more damage though.

> The thin wire will melt apart before the power lines do.

It's a bit more subtle than that. As far as I understand, the thin wire might melt, but its vaporized remnants would be enough to keep an arc going between the phases or a phase and the ground. Then the protection devices at both ends of the line would detect the short circuit, turn off the line, wait for a bit of time (enough for the vaporized remnants and ionized air to disperse), and turn the line on again automatically. Since most high-voltage power grids have redundant circuits, it's likely that no consumer would lose power even for a moment.