In an extra high voltage overhead transmission line earth wire is provided to protect the line against

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Option B: Lightening surges

Explanation & Concepts:
An earth wire (or ground wire) is a non-current-carrying conductor installed at the highest point of an overhead transmission line. Its primary purpose is to intercept direct lightning strikes, thereby preventing them from hitting the phase conductors and causing flashovers or equipment damage.

Working Principle: The earth wire is connected to the tower body, which is grounded through an earth electrode. When a lightning stroke occurs, the earth wire provides a low-impedance path to the ground, diverting the surge current away from the phase conductors. Furthermore, it creates a shielding angle, ensuring that the lightning stroke terminates on the ground wire rather than the phase lines, effectively reducing the probability of direct strikes.

Key Formulas: $Z_s = \sqrt{\frac{L}{C}}$ — Represents the surge impedance of the transmission line, which influences the magnitude of traveling waves.; $	heta = \arccos(\frac{d}{h})$ — The shielding angle calculation where $d$ is the horizontal distance and $h$ is the vertical height of the earth wire.

Key Points: The earth wire is placed at the top of the tower to provide a protective shielding zone.; It helps in reducing the magnitude of the lightning stroke current reaching the conductors by providing a ground path.; The effectiveness of the shield is governed by the shielding angle, which is typically kept below $30^\circ$ for high voltage lines.; It acts as a ground return path during a line-to-ground fault.

Advantages: Significant reduction in transmission line outages due to lightning strikes.; Provides a low resistance return path for fault currents.

Disadvantages: Increases the structural load on the transmission towers.; Increases the initial capital cost of the transmission project.

Applications: High Voltage (HV) and Extra High Voltage (EHV) overhead transmission lines.; Substation structures to protect equipment like transformers and circuit breakers.

Additional Info: Switching surges are generally controlled by circuit breaker design and pre-insertion resistors, not primarily by the earth wire.. Corona effect is a surface phenomenon controlled by conductor diameter and spacing; earth wire does not mitigate it.. The earth wire is often referred to as the 'Static Wire' or 'Shield Wire'.

Answer: B is correct — An earth wire is installed on transmission towers specifically to intercept lightning strikes, protecting the phase conductors from direct surges.

Exam Tip: Always remember that the earth wire protects against *direct* lightning strikes, while surge arresters protect against *traveling* surges that reach the substation.

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Option A: Switching surges

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Option C: Corona effect

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Option D: Ensure fault voltages

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