When there is interference in an overhead communication line running parallel and in close proximity to an ove

Question

When there is interference in an overhead communication line running parallel and in close proximity to an overhead power line in the longitudinal and lateral directions by the power line are due to

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Option C: Both magnetic and electric induction

Explanation & Concepts:
Interference between parallel power and communication lines occurs due to electromagnetic coupling mechanisms. Magnetic induction results from the variation of the magnetic field created by power line currents, while electric induction results from the electrostatic potential differences created by power line voltages.

Working Principle: 1. Magnetic Induction: Alternating currents in power lines produce a time-varying magnetic field ($\Phi$), which links with the communication circuit, inducing an electromotive force (EMF) via Faraday's Law ($e = -N \frac{d\Phi}{dt}$). 2. Electric Induction: The capacitive coupling between the high-voltage power line conductors and the communication line conductors causes charge redistribution on the communication line, inducing voltage due to the potential difference in the surrounding field.

Key Formulas: $e = M \frac{di}{dt}$ — Induced EMF due to magnetic coupling (mutual inductance $M$); $V_c = V_p \frac{C_{12}}{C_{12} + C_{20}}$ — Induced voltage due to capacitive (electric) coupling

Key Points: Magnetic induction depends on the magnitude of the current and the mutual inductance between lines.; Electric induction depends on the voltage magnitude and the capacitance between the power and communication lines.; Interference can be minimized by transposing power lines and increasing the separation distance.; Shielding cables and using twisted pairs in communication lines help reduce inductive interference.

Advantages: Improved signal-to-noise ratio in communication lines.; Prevention of hardware damage due to high induced voltages.

Disadvantages: High cost of installing shielding or underground cabling.; Complex maintenance of transposition schemes.

Applications: Telecommunication network planning near HV transmission corridors.; Railway signaling safety systems.

Additional Info: Magnetic induction is often the dominant factor at power frequencies (50/60 Hz).. Electric induction is significantly reduced if the communication line is shielded with a grounded metallic sheath.

Answer: C is correct — Interference in parallel lines is caused by both magnetic (electromagnetic) and electric (electrostatic) induction from the power line's current and voltage respectively.

Exam Tip: Always remember that 'current' in the power line causes magnetic interference, while 'voltage' causes electric (capacitive) interference.

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Option A: Magnetic induction and electric induction respectively

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Option B: Electric induction and magnetic induction respectively

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Option D: Magnetic induction only

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