AC Mains Transmission Tower question

[phyzguy]

TL;DR Summary

What are the extra two wires on the transmission towers.

Hi all. Many of you on the forum seem to be very knowledgeable on power transmission. I often notice transmission towers like this on. I know the three large wire groups are the three phases of the AC power, but the towers often have two smaller wires (red circle) on the top. Sometimes
they only have one. Does anyone know the function of these wires?

 

[anorlunda]

They are called shield wires. They protect the line from lightning strikes.

 

[hutchphd]

It is not clear to me how they work. Are the towers not grounded? Reference maybe?

 

[russ_watters]

He said they protect the lines, not the towers. The towers are giant lighting rods anyway and are isolated from the lines. The lines of course can't be grounded...

 

[hutchphd]

Oh I missed the red circles. I thought there were lines lower down (they are on the background towers!). OK never mind. (But thanks!)

 

[artis]

Summary: What are the extra two wires on the transmission towers.

Hi all. Many of you on the forum seem to be very knowledgeable on power transmission. I often notice transmission towers like this on. I know the three large wire groups are the three phases of the AC power, but the towers often have two smaller wires (red circle) on the top. Sometimes
they only have one. Does anyone know the function of these wires?
View attachment 314129

As others already said it's a ground wire that electrically connects every metal tower and serves as a "lightning rod" so that the bolt rather hits it first than any of the phases.

But since the 1980's it has also been used as a fiber optic cable. Most of these shield wires now are double purpose. The outer layer is metal strands that perform the shielding while inside there is a optical fiber cable for internet and communications.
It makes sense because fiber doesn't conduct so is immune from induced voltages and currents but it still passes light.

https://en.wikipedia.org/wiki/Optical_ground_wire

 

[artis]

It is not clear to me how they work. Are the towers not grounded? Reference maybe?

Towers are grounded, at least the ones I have seen all have ground bars that connect to one or more of the tower legs and run into ground/soil, but the upper shield wire connects each tower to the next, thereby I think it makes for a better ground and shield because not all towers stand on equally conductive ground.