---
title: "Skin effect in transmission lines"
subject: "Power System Analysis I"
chapter: 4
key: "semester_4/power_system_analysis_i:skin-effect-brief"
source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/skin-effect-brief
site: BCT Engineering
---

# Skin effect in transmission lines

Power System Analysis I · Chapter 4: Line Parameter Calculations

- Asked in: 2072 Magh, 3 marks
- Video: [What is Skin Effect ? Explained | TheElectricalGuy](https://www.youtube.com/watch?v=GnVPc7K_5qY) — Gaurav J - TheElectricalGuy
- 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/skin-effect-brief
- Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-4

## Question

Briefly describe skin effect in power transmission lines.

## Short answer

Skin effect is the tendency of AC current to crowd near the conductor surface, increasing AC resistance over DC resistance. It results from higher internal flux linkage in central strands causing greater inductive reactance there. It grows with frequency, conductor diameter, conductivity, and permeability.

## Answer

**Skin effect** is the tendency of alternating current to crowd towards the outer surface of a conductor, so the current density is higher near the surface than at the centre.

#### Cause

- The current in the central filaments is linked by more magnetic flux (internal plus external) than the current in the surface filaments.
- So the inner filaments have higher inductive reactance, and the current prefers the outer, lower-reactance path.

```text
   DC: uniform        AC: crowded at surface
    .........           ##########
    .........           #        #
    .........           ##########
```

#### Effects

- The effective cross-section carrying current is reduced, so the **AC resistance is greater than the DC resistance** ($R_{ac} > R_{dc}$).
- Power loss ($I^2R$) increases.
- The centre of a thick conductor is poorly used, which is why ACSR conductors have steel (strength) at the core and aluminium outside.

#### Factors affecting skin effect

It increases with **frequency**, **conductor diameter**, **conductivity** and **permeability**. It is small at 50 Hz for normal line conductors (about 1–3%), but it is more noticeable for large conductors and at high frequency. It is zero for DC.

## Check yourself

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

1. What is skin effect?
   A. AC crowds toward the outer surface of a conductor
   B. AC concentrates at the centre
   C. Current divides equally among strands
   D. Corona on the surface
   Answer: A. Current density is higher near the surface than at the centre for AC; for DC it is uniform. (Key point: Definition)

2. Why does current move to the surface?
   A. Inner filaments are linked by more flux, so they have higher reactance
   B. Surface has lower resistivity
   C. Inner filaments are hotter
   D. Capacitance at the surface
   Answer: A. Central filaments are linked by internal and external flux, giving higher inductive reactance, so current takes the outer path. (Key point: Cause)

3. What is the main effect of skin effect on resistance?
   A. R_ac is less than R_dc
   B. R_ac is greater than R_dc, increasing I^2R loss
   C. No change in resistance
   D. R_dc increases
   Answer: B. The effective cross-section carrying current is reduced, so AC resistance rises. (Key point: Effect)

4. Skin effect increases with:
   A. Lower frequency and smaller diameter
   B. Higher frequency and larger diameter
   C. Lower conductivity only
   D. DC operation
   Answer: B. It increases with frequency, diameter, conductivity and permeability, and is zero for DC. (Key point: Factors)

## Related in Power System Analysis I

- [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×
- [Skin effect with AC and DC current](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Askin-effect-ac-dc) — Power System Analysis I, chapter 4
- [Ferranti effect and skin effect](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aferranti-and-skin-effect) — Power System Analysis I, chapter 6
- [Proximity effect in cables vs overhead lines](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aproximity-effect-cables-vs-lines) — Power System Analysis I, chapter 4
- [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
- [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

- [Skin depth and power in dissipative medium](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Askin-depth-time-average-power-density) — Electromagnetism, chapter 4
- [Band applications and microwave transmission lines](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Aband-applications-transmission-lines) — RF and Microwave Engineering, chapter 1
- [Negative skin friction: causes, effects, remedies](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Anegative-skin-friction-effects) — Foundation Engineering, chapter 8, asked 4×
- [Issues of EHV-AC power transmission](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Aehv-ac-transmission-issues) — High Voltage Engineering, chapter 1