The transmission lines are more to lightning strikes because of the following reasons,
1) Directly exposed to atmosphere,
2) Higher than other structures,
3) Higher charge concentration as they are earthed at every pole,
4) Lower earth resistance
The ground wires at the top of transmission lines act as a shield and protects the live lines from lightning strikes.
The transmission lines are protected from lightning strikes by taking measures like adding overhead ground wires, reducing the tower foot resistance, adding counterpoise wires, increasing insulation, etc.
The overhead ground wires protect the transmission lines from direct lightning strikes. A ground wire is a conductor that runs in parallel to the line conductor and is placed at the top of the tower structure as shown in the picture below.
The ground wire is placed over the tower in such a way that all the lines are placed well within the shielding angle of the ground wires. The height of the shielding earth wire depends on the width of the cross arm.
Since the earth wires are exposed are directly exposed to extreme weather conditions, it should possess sufficient mechanical strength.
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