The velocity of propagation of electromagnetic waves on overhead line is

Question

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Option A: $3 	imes 10^8$ m/s

Explanation & Concepts:
The velocity of propagation of electromagnetic waves on overhead transmission lines is approximately equal to the speed of light in free space, which is $3 	imes 10^8$ m/s. This occurs because the electromagnetic energy travels through the surrounding air medium rather than through the metallic conductors themselves.

Working Principle: An electromagnetic wave on a transmission line is characterized by the velocity $v = \frac{1}{\sqrt{LC}}$, where $L$ is inductance per unit length and $C$ is capacitance per unit length. For an overhead line, the dielectric constant and magnetic permeability of the air are essentially those of a vacuum, leading to a velocity of propagation $v = \frac{1}{\sqrt{\mu_0 \epsilon_0}} = c = 3 	imes 10^8$ m/s.

Key Formulas: $v = \frac{1}{\sqrt{LC}}$ — Velocity of electromagnetic wave on a transmission line; $v = \frac{1}{\sqrt{\mu_0 \epsilon_0}} \approx 3 	imes 10^8$ m/s — Velocity in free space or air

Key Points: Electromagnetic waves travel in the dielectric space around the conductors.; The velocity is inversely proportional to the square root of the line parameters (inductance and capacitance).; In underground cables, the velocity is lower than $3 	imes 10^8$ m/s due to the higher permittivity of insulation materials.

Advantages: Fast transient signal propagation for protection relaying; Minimal time delay for communication signaling

Disadvantages: High-speed surges can cause insulation breakdown if protection is not fast enough; Reflection phenomenon complicates voltage analysis

Applications: Power system transient analysis; Traveling wave protection schemes; Fault location in transmission lines

Additional Info: The value $3 	imes 10^8$ m/s corresponds to 300,000 km/s.. Option B (3×10^8 km/s) is incorrect as it is $10^6$ times faster than light.. Option C (3×10^10 m/s) is incorrect as it is $10^2$ times faster than light.. Option D (3×10^8 km/hr) is incorrect as it equals approximately 83.33 km/s.

Answer: A is correct — The velocity of propagation of electromagnetic waves on overhead transmission lines is approximately the speed of light, $3 	imes 10^8$ m/s.

Exam Tip: Remember that velocity is always lower in cables than in overhead lines because the relative permittivity $\epsilon_r$ of insulating material is greater than 1, reducing velocity by a factor of $1/\sqrt{\epsilon_r}$.

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Option B: $3 	imes 10^8$ km/s

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Option C: $3 	imes 10^{10}$ m/s

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Option D: $3 	imes 10^8$ km/hr

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