---
title: "Preventing ground loops; electromagnetic coupling"
subject: "Instrumentation II"
chapter: 6
key: "semester_5/instrumentation_ii:ground-loop-em-coupling"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/ground-loop-em-coupling
site: BCT Engineering
---

# Preventing ground loops; electromagnetic coupling

Instrumentation II · Chapter 6: Grounding And Shielding

- Asked in: 2078 Kartik, 6 marks
- Video: [PCB Ground Loops and How to Prevent Them](https://www.youtube.com/watch?v=3swYweisn5g) — Altium Academy
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/ground-loop-em-coupling
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-6

## Question

How ground loop can be prevented? Explain the Electromagnetic coupling.

## Short answer

Prevent ground loops by single-point grounding, isolation (transformers, optocouplers), differential signaling, grounding shields at one end, and minimizing loop area. Electromagnetic (radiated) coupling occurs when far-field waves induce voltages in conductors acting as antennas. Mitigate with metal shielding, filtering, and minimizing trace lengths.

## Answer

#### Preventing ground loops
A ground loop forms when a circuit is grounded at more than one point, so the ground-potential difference drives noise current. To prevent it:
- Use **single-point (star) grounding** at low frequency.
- Use **isolation**: isolation transformer, opto-coupler or differential amplifier to break the loop.
- Ground the cable shield at only one end (low frequency).
- Use balanced differential signalling and twisted pairs, to cancel the induced voltage.
- Reduce the loop area and keep all equipment on one power circuit.
- Use common-mode chokes or a ferrite on cables.

#### Electromagnetic (radiated) coupling
Electromagnetic coupling occurs when a source emits a radiating wave, and wires, traces or loops of the receiver act as antennas that pick up the energy. It occurs in the far field (distance greater than λ/2π), where electric and magnetic fields travel together with wave impedance of 377 ohm.

- **Sources:** radio/TV transmitters, mobile phones, switching power supplies, digital clocks, motors with sparking brushes, and lightning.
- **Receiver:** any conductor of length near λ/4 or longer; the induced voltage is proportional to the field strength and effective antenna length.
- **Reduction:** enclose the circuit in a grounded metal shield, filter all lines entering the enclosure, reduce lead and trace lengths, use shielded or twisted cables, ferrite beads, increase distance, slow down clock edges.

## Check yourself

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

1. Which technique prevents ground loops at low frequency?
   A. Grounding at multiple points
   B. Single-point (star) grounding or isolation
   C. Using the largest loop area
   D. Grounding shields at both ends
   Answer: B. Removing multiple paths or breaking them with isolation prevents loop currents. (Key point: Ground loop prevention)

2. Which devices break a ground loop?
   A. Isolation transformer, opto-coupler or differential amplifier
   B. Bypass resistor to chassis
   C. Extra ground wires
   D. Unshielded long cables
   Answer: A. These break the conductive path while passing the signal. (Key point: Isolation devices)

3. In which region does electromagnetic (radiated) coupling occur?
   A. Very near field, within millimetres
   B. Inside a closed metal box only
   C. Only on DC lines
   D. Far field, beyond about lambda/2pi, with wave impedance 377 ohm
   Answer: D. Radiated coupling is a far-field effect where E and H travel together. (Key point: Region of EM coupling)

4. What makes a receiver conductor pick up radiated energy well?
   A. Very short length
   B. Being tightly twisted
   C. Length near lambda/4 or longer, acting as an antenna
   D. Being deeply buried
   Answer: C. Induced voltage depends on field strength and effective antenna length. (Key point: Receiver antenna)

## Related in Instrumentation II

- [Ground loop and its elimination](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aground-loop) — Instrumentation II, chapter 6

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