A string efficiency of 100% implies that

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Option B: Potential across each disc is same

Explanation & Concepts:
String efficiency is defined as the ratio of the total voltage across the string to the product of the number of discs and the voltage across the disc nearest to the conductor. A string efficiency of 100% means that the voltage across every disc in the string is identical.

Working Principle: In a transmission line insulator string, shunt capacitance to the tower causes non-uniform voltage distribution, where the disc nearest to the conductor experiences the highest stress. If the shunt capacitance were zero, the voltage distribution would be perfectly uniform across all discs. Since string efficiency is defined as $\eta = \frac{V_{total}}{n \cdot V_{conductor_disc}}$, reaching 100% implies $V_{conductor_disc} = \frac{V_{total}}{n}$, which signifies an equal potential drop across every disc.

Key Formulas: $\eta = \frac{V_{total}}{n 	imes V_{disc_max}} 	imes 100\%$ — General formula for string efficiency; $V_{disc_i} = \frac{V_{total}}{n}$ — Condition for 100% efficiency

Key Points: Uniform voltage distribution is ideal but practically impossible due to shunt capacitance.; String efficiency is always less than 100% in practical overhead transmission lines.; Higher number of discs results in lower string efficiency for a given voltage level.

Advantages: Equalizes electrical stress on all insulators.; Prevents premature failure of the disc nearest to the conductor.

Disadvantages: Requires complex grading of capacitance or long cross-arms to achieve near-uniform distribution.; 100% is physically unattainable in standard suspension strings.

Applications: Design of HV transmission lines.; Insulator string configuration analysis.

Additional Info: Shunt capacitance is the stray capacitance between the metal link of each insulator disc and the earthed tower structure.. Option C is incorrect because potential across each disc must be equal to V/n, not zero. Option D would short one disc, reducing the total effective number of discs rather than equalizing voltage.

Answer: B is correct — A string efficiency of 100% signifies that the potential is uniformly distributed, meaning each disc supports an identical fraction of the total string voltage.

Exam Tip: Always remember that the disc closest to the line conductor carries the maximum voltage stress in a practical string due to shunt capacitance; therefore, efficiency is never 100% in actual installations.

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Option A: Shunt capacitance is 1 Micro Farad

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Option C: Potential across each disc is zero

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Option D: One of the insulator disc is shorted

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