The top wire in a transmission line is typically a ground wire or earth wire. This wire is primarily used to protect the power-carrying conductors from lightning strikes. It intercepts lightning strikes and safely conducts the high current to the ground, preventing damage to the power lines and potential hazards to the surrounding area.

Here are some key points about the top wire:

• Purpose: Primarily for lightning protection.
• Material: Often made of steel due to its strength and conductivity.
• Location: Situated at the topmost position on the transmission towers.
• Grounding: It is solidly grounded at each tower to ensure effective lightning discharge.

It's important to note that the top wire does not carry the main electrical current. Its sole function is to protect the power-carrying conductors from lightning strikes.