---
title: "Proximity effect in cables vs overhead lines"
subject: "Power System Analysis I"
chapter: 4
key: "semester_4/power_system_analysis_i:proximity-effect-cables-vs-lines"
source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/proximity-effect-cables-vs-lines
site: BCT Engineering
---

# Proximity effect in cables vs overhead lines

Power System Analysis I · Chapter 4: Line Parameter Calculations

- Asked in: 2068 Bhadra, 4 marks
- Video: [proximity effect | proximity effect in power system | proximity effect in transmission line | hindi](https://www.youtube.com/watch?v=OzVeBZM2L7o) — Mind of Engineer 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/proximity-effect-cables-vs-lines
- Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-4

## Question

Proximity effect is almost negligible in overhead transmission lines but quite significant in underground power cables. Why?

## Short answer

In overhead lines, conductor spacing is metres, so magnetic field is nearly uniform across each conductor, making current crowding negligible. In underground cables, cores are separated by only mm to cm, causing strong non-uniform field and significant proximity effect, increasing AC resistance.

## Answer

**Proximity effect** is the crowding of current in a conductor caused by the alternating magnetic field of a nearby current-carrying conductor. Its size depends mainly on **how close the conductors are** compared with their size.

#### Overhead lines: negligible

- Phase conductors are spaced several metres apart (e.g. 4–10 m at 132–400 kV) for insulation in air.
- At such distances, the flux from one conductor is almost the same over the whole cross-section of the other (conductor diameter ≈ 2–3 cm, so $d/D$ is tiny).
- So the current distribution is hardly disturbed, and the rise in AC resistance is negligible.

#### Underground cables: significant

- Cable cores are separated only by thin solid insulation, a few mm to a few cm, so cores are almost touching ($D$ close to $d$).
- The field of a neighbouring core changes strongly across the cross-section, so current crowds to one side of each core.
- Cable conductors are also large in cross-section (to carry high current with limited cooling), which strengthens the effect.
- This noticeably increases $R_{ac}$ and $I^2R$ losses, so it must be included in cable ratings.

| Point | Overhead line | Underground cable |
|---|---|---|
| Spacing | Metres | mm to cm |
| Spacing / diameter | Very large | Close to 1 |
| Current crowding | Negligible | Significant |
| Rise in $R_{ac}$ | Very small | Noticeable |

## Check yourself

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

1. Proximity effect is:
   A. Current flowing only at the surface due to its own field
   B. Current crowding caused by the magnetic field of a nearby conductor
   C. Loss in dielectric
   D. Leakage across insulators
   Answer: B. It is distortion of current distribution due to another conductor's alternating flux. (Key point: Definition)

2. Why negligible in overhead lines?
   A. Spacing is metres, far larger than conductor diameter
   B. Conductors are insulated
   C. Current is DC
   D. Conductors are very thin so no flux
   Answer: A. With d/D tiny, the neighbour's flux is nearly uniform across the cross-section. (Key point: Overhead lines)

3. Why significant in cables?
   A. Cables use higher frequency
   B. Cables are laid underground, so earth causes it
   C. Cores are separated only by thin insulation so D is close to d
   D. Cores are far apart
   Answer: C. Nearby cores give a strongly non-uniform field across each core. (Key point: Cables)

## Related in Power System Analysis I

- [Underground cables vs overhead lines](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Acables-vs-overhead-lines) — Power System Analysis I, chapter 2
- [Proximity effect in overhead lines](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aproximity-effect) — Power System Analysis I, chapter 4
- [Conductor materials for lines and cables](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aconductor-materials-selection) — Power System Analysis I, chapter 2
- [Skin effect in transmission lines](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Askin-effect-brief) — Power System Analysis I, chapter 4
- [Skin effect and proximity effect](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Askin-and-proximity-effect) — Power System Analysis I, chapter 4, asked 3×
- [Requirements of conductors for overhead lines](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aconductor-requirements) — Power System Analysis I, chapter 2, asked 2×

## Related in other subjects

- [Underground vs overhead distribution systems](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aunderground-vs-overhead-distribution) — Transmission and Distribution System Design, chapter 5, asked 4×