---
title: "Electromagnetic induction in neighbouring facilities"
subject: "High Voltage Engineering"
chapter: 5
key: "semester_8/high_voltage_engineering:electromagnetic-induction-neighbouring"
source: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/electromagnetic-induction-neighbouring
site: BCT Engineering
---

# Electromagnetic induction in neighbouring facilities

High Voltage Engineering · Chapter 5: High Stress Electric Fields

- Asked in: 2074 Magh, 6 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/high_voltage_engineering/electromagnetic-induction-neighbouring
- Whole chapter: https://www.bctengineer.com/md/semester_8/high_voltage_engineering/chapter-5

## Question

Explain with example how electromagnetic induction occurs in neighboring facilities.

## Short answer

AC current in a power line creates a magnetic field that links a parallel metallic facility, inducing voltage by mutual inductance. Example: 5 kA earth-fault current, 2 km parallel pipeline at 50 m, soil 100 Ωm induces about 1.84 kV, posing shock and corrosion risks.

## Answer

**Electromagnetic (inductive) coupling** is the voltage induced in a nearby metallic facility, such as a pipeline, fence, rail or telephone line, by the alternating magnetic field of the current in a power line. The facility and the power line are coupled through their **mutual inductance**, like the primary and secondary of an air-core transformer.

#### How it occurs
1. The AC current $I$ in the phase conductors sets up an alternating magnetic flux around the line.
2. Part of this flux links the loop formed by the parallel facility and its earth return.
3. By Faraday's law, a longitudinal emf is induced along the facility:
$$
E = \omega M I L\ \ \text{volts}
$$
where $M$ is the mutual inductance per km, $L$ the length of parallel exposure (km) and $I$ the inducing current.
4. With balanced load currents, the fields of the three phases largely cancel. The induction is therefore small in normal operation (it arises only from the unequal distances to each phase).
5. During a **single line-to-ground fault**, a large zero-sequence (earth-return) current flows and does not cancel. The induced voltage can then reach several kV.

#### Mutual inductance with earth return (Carson)
$$
M = 0.2\ln\frac{D_e}{D}\ \text{mH/km}, \qquad D_e = 658.5\sqrt{\frac{\rho}{f}}\ \text{m}
$$

#### Example
A metal pipeline runs parallel to a 132 kV line for $L = 2$ km at $D = 50$ m. Take soil resistivity $\rho = 100\ \Omega$m, $f = 50$ Hz, and an earth-fault current of 5 kA.
$$
\begin{aligned}
D_e &= 658.5\sqrt{100/50} = 931.3\ \text{m} \\
M &= 0.2\ln\frac{931.3}{50} = 0.585\ \text{mH/km} \\
E &= 2\pi \times 50 \times 0.585 \times 10^{-3} \times 5000 \times 2 \\
&= 1838\ \text{V} \approx 1.84\ \text{kV}
\end{aligned}
$$
This voltage would appear between the pipeline and remote earth, which is dangerous to anyone touching it.

#### Effects on neighbouring facilities
- Shock hazard to people touching pipelines, fences or rails
- Noise and interference in telephone and communication circuits
- AC corrosion of pipelines and damage to their coatings and cathodic protection
- Wrong operation of railway signalling

#### Remedies
- Keep adequate separation and short parallel runs; cross at right angles.
- Transpose the power line; use earth wires of low resistance (they carry part of the return current and reduce the net field).
- Earth the facility at intervals, and use insulating joints and gradient-control mats.
- Clear faults quickly with fast protection.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is a voltage induced in a nearby pipeline or fence?
   A. By direct contact with the line conductor
   B. The alternating magnetic flux of the line current links the facility loop, via mutual inductance (Faraday's law)
   C. By the line's electric field charging it only
   D. By corona discharge
   Answer: B. The line and the facility act like an air-core transformer coupled by mutual inductance. (Key point: Coupling mechanism)

2. What is the expression for the longitudinal induced emf?
   A. E = ωMIL
   B. E = ωM/(IL)
   C. E = MI/L
   D. E = IL/(ωM)
   Answer: A. E increases with frequency, mutual inductance per km, current and parallel length. (Key point: Induced emf)

3. Why is induction much larger in a single line-to-ground fault than in normal operation?
   A. Fault current is at a higher frequency
   B. The three-phase fields add in normal operation
   C. Voltage is higher in the fault
   D. A large zero-sequence earth-return current flows and its fields do not cancel
   Answer: D. Balanced currents largely cancel, but zero-sequence current is in phase in all conductors. (Key point: Fault vs normal)

4. How does a low-resistance earth wire reduce induction?
   A. It increases the earth return current
   B. It increases the parallel exposure length
   C. It carries part of the return current and reduces the net field
   D. It raises the line voltage
   Answer: C. The earth wire current opposes the line's field, reducing mutual coupling to the facility. (Key point: Remedy)

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