The size of the earth wire is determined on the basis of

Question

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Option B: Current carrying capacity of the service line

Explanation & Concepts:
The size of the earth wire is primarily determined by the maximum fault current it may be required to carry during a short-circuit event. It must have sufficient cross-sectional area to safely conduct the fault current for the duration required for the protective device (like a fuse or circuit breaker) to isolate the circuit without overheating or melting the conductor.

Working Principle: When a phase-to-earth fault occurs, the earth wire provides a low-impedance path back to the supply transformer neutral. The thermal rating of the cable is governed by the adiabatic equation $S^2 = \frac{I^2t}{k^2}$, where $I$ is the fault current and $t$ is the clearing time. The size must be sufficient to ensure that the conductor temperature does not exceed its insulation or structural damage limit.

Key Formulas: $S = \frac{I\sqrt{t}}{k}$ — where $S$ is the cross-sectional area of the earthing conductor, $I$ is the fault current, $t$ is the clearing time, and $k$ is a constant depending on the material.

Key Points: The earth wire must handle the maximum possible fault current in the system.; Material conductivity and thermal stability determine the factor 'k'.; Earthing ensures the potential of non-current carrying metal parts remains near ground potential ($0V$).; IS 3043 provides detailed standards for earthing installations in India.

Advantages: Protects personnel from electric shock by providing a safe return path for fault current.; Prevents fire hazards due to insulation failure and persistent leakage currents.

Disadvantages: Undersized earth wires can melt during a severe fault, failing to trip protection.; Corrosion over time can increase impedance, making the earthing ineffective.

Applications: Domestic wiring earthing systems; Industrial machinery grounding; Distribution transformer neutral grounding

Additional Info: The 'current carrying capacity' in this context refers specifically to the short-time fault current rating, not the continuous load current.. The size of the earthing conductor is also influenced by the mechanical strength required to prevent physical breakage.

Answer: B is correct — The size of an earth wire is determined by the fault current it is expected to carry to ensure protective devices operate safely.

Exam Tip: In competitive exams, always distinguish between continuous current rating (for live conductors) and short-time fault current rating (for earth conductors).

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Option A: Voltage of the service line

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Option C: Atmospheric condition

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Option D: None of above

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