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SarkariResultOutAdditional Assistant Engineer (Electrical) · 2026

Previous Year Question

In a 3-phase transmission line, if Dm is the geometric mean distance (GMD) between conductors, r is the radius of each conductor, and ε is the permittivity of the surrounding medium, which expression gives the capacitance per phase for a symmetrically spaced line using the GMD method?
  1. C = 2πε / ln(r/Dm)
  2. C = εDm / (2πr)
  3. C = 2πε / ln(Dm/r)
  4. C = πε / ln(Dm/r)

Correct answer: C

Explanation

For a transposed or symmetrically spaced 3-phase line, the phase-to-neutral capacitance per unit length is C = 2πε / ln(Dm/r), where Dm is the effective mutual spacing and r is conductor radius for capacitance calculations.

Formula

C = 2πε / ln(Dm/r) F/m

Step-by-step solution

GMD Method for 3-Phase Capacitance explanatory diagram

Correct Answer

C — C = 2πε / ln(Dm/r)

Explanation

For a transposed or symmetrically spaced 3-phase line, the phase-to-neutral capacitance per unit length is C = 2πε / ln(Dm/r), where Dm is the effective mutual spacing and r is conductor radius for capacitance calculations.

Key Concept

GMD Method for 3-Phase Capacitance

Formula

C = 2πε / ln(Dm/r) F/m

Exam Point

Capacitance has 2πε in numerator and ln(D/r) in denominator.

Common Mistake

Do not reverse the logarithm; ln(r/D) would make the expression physically incorrect.

Common mistake

Do not reverse the logarithm; ln(r/D) would make the expression physically incorrect.

Exam tip

Capacitance has 2πε in numerator and ln(D/r) in denominator.

Quick method

Capacitance has 2πε in numerator and ln(D/r) in denominator.