Introduction:
An overhead transmission line is a system used for transmitting electrical power over long distances. It is susceptible to various disturbances, including lightning surges, switching surges, and power frequency overvoltages. To protect the transmission line and associated equipment, an earth wire is provided.

Explanation:
The purpose of an earth wire in an overhead transmission line is to provide protection against lightning surges. Lightning surges occur when lightning strikes the transmission line or nearby objects, causing a sudden increase in voltage and current. These surges can damage the transmission line, the equipment connected to it, and even cause power outages.

Role of the Earth Wire:
The earth wire is installed above the conductors of the transmission line and connected to the ground at regular intervals. It serves as a low-impedance path for lightning surges to flow to the ground, effectively protecting the transmission line and associated equipment.

Protection Mechanism:
When a lightning surge occurs, the earth wire provides a path of least resistance for the surge to follow. The surge is diverted away from the conductors of the transmission line and safely discharged into the ground. This prevents the surge from causing damage to the transmission line and equipment.

Advantages of Earth Wire:
1. Protection against Lightning Surges: The earth wire provides a reliable path for lightning surges to be safely discharged into the ground, preventing damage to the transmission line.
2. Enhanced Safety: By diverting lightning surges away from the conductors, the earth wire reduces the risk of electrical hazards and improves overall safety.
3. Minimized Equipment Damage: Lightning surges can cause significant damage to electrical equipment. The earth wire helps minimize such damage by providing a low-impedance path for the surge to follow.

Conclusion:
In conclusion, an earth wire is provided in overhead transmission lines primarily for protection against lightning surges. It plays a crucial role in diverting the surge away from the conductors and safely discharging it into the ground. This protective measure helps safeguard the transmission line and associated equipment from potential damage and ensures the reliability of the power transmission system.