--- title: "Skin effect and proximity effect" subject: "Power System Analysis I" chapter: 4 key: "semester_4/power_system_analysis_i:skin-and-proximity-effect" source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/skin-and-proximity-effect site: BCT Engineering --- # Skin effect and proximity effect Power System Analysis I · Chapter 4: Line Parameter Calculations - Asked in: 2079 Chaitra, 3 marks; 2075 Baisakh, 6 marks; 2073 Magh, 4 marks (asked 3 times) - Video: [Difference between Proximity Effect and Skin Effect | Skin effect और Proximity effect में अंतर ?](https://www.youtube.com/watch?v=-CgQ2nSqW-w) — NO MORE ratta maar - 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-and-proximity-effect - Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-4 ## Question Explain skin effect and proximity effect. ## Short answer Skin effect: AC current crowds to conductor surface due to higher internal inductance at the center, increasing effective resistance R_ac > R_dc. Proximity effect: non-uniform current distribution caused by the magnetic field of nearby conductors, further raising resistance, important in cables and busbars. ## Answer #### Skin effect **Skin effect** is the tendency of alternating current to crowd towards the outer surface of a conductor. The current density is highest at the surface and lowest at the centre. **Cause:** Imagine the conductor as many thin filaments. A filament at the centre is linked by the flux inside the conductor and also by the flux outside, while a filament at the surface is linked only by the outside flux. So the central filaments have more inductance and a higher reactance $\omega L$. The AC current takes the easier, low-reactance path near the surface. ```text DC: uniform AC: crowded at surface .-------. .-------. / . . . . \ / ####### \ | . . . . . | | ## ## | | . . . . . | | # low # | \ . . . . / \ ####### / '-------' '-------' ``` **Effects:** - The effective area is reduced, so $R_{ac} > R_{dc}$. This increases $I^2R$ loss. - Internal inductance is slightly reduced. **Factors:** the effect increases with frequency, conductor diameter, permeability and conductivity. The depth of current flow is $\delta = \sqrt{\frac{2\rho}{\omega\mu}}$; for copper at 50 Hz, $\delta \approx 9.3$ mm. **Remedies:** stranded and ACSR conductors (the steel core carries little current), hollow or tubular conductors, and bundled conductors. There is no skin effect with DC. #### Proximity effect **Proximity effect** is the non-uniform current distribution in a conductor caused by the alternating magnetic field of a **nearby** conductor carrying current. - When two nearby conductors carry current in the **same** direction, current crowds to their far sides. - When they carry current in **opposite** directions (go and return), current crowds to the sides facing each other. ```text Same direction Opposite directions (# ) ( #) ( #) (# ) crowd outward crowd towards each other ``` **Effects:** the effective resistance rises further, and the inductance changes slightly. **Where it matters:** in cables and closely spaced conductors (bus-bars, cable cores, transformer windings). It is negligible in overhead lines, where the conductors are metres apart. | Point | Skin effect | Proximity effect | |---|---|---| | Cause | Conductor's own flux | Flux of a nearby conductor | | Current shift | Towards the surface | Towards or away from the neighbour | | Depends on | f, diameter, μ, ρ | f, spacing, size, current direction | | Important in | All AC conductors | Cables, bus-bars | | Result | $R_{ac} > R_{dc}$ | Further rise in $R_{ac}$ | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does AC crowd towards the surface of a conductor? A. Surface filaments have higher resistivity B. Central filaments are linked by more flux, so they have higher reactance C. The surface is cooler so resistance is lower D. Central filaments carry the DC component only Answer: B. Central filaments link both internal and external flux, giving higher inductance and reactance, so AC takes the low-reactance surface path. (Key point: Cause of skin effect) 2. What is the main consequence of skin effect on conductor resistance? A. R_ac < R_dc B. R_ac = R_dc C. R_ac > R_dc because effective area is reduced D. Resistance becomes independent of frequency Answer: C. Current uses less of the cross-section, so AC resistance is higher and I^2R loss increases. (Key point: Effect on resistance) 3. Two nearby conductors carry current in opposite directions (go and return). Where does current crowd? A. On the far sides B. Uniformly across each conductor C. On the sides facing each other D. At the centre of each conductor Answer: C. For opposite currents the current crowds to the facing sides; for same-direction currents it crowds to the far sides. (Key point: Proximity effect directions) 4. Which statement correctly distinguishes proximity effect from skin effect? A. Proximity effect is caused by the conductor's own flux; skin effect by a neighbour's B. Proximity effect is caused by a nearby conductor's flux and matters in cables and bus-bars, not in widely spaced overhead lines C. Proximity effect is important mainly in overhead lines with metres of spacing D. Proximity effect occurs only with DC Answer: B. Proximity effect arises from the neighbour's alternating field and is negligible when conductors are metres apart. (Key point: Skin vs proximity) ## Related in Power System Analysis I - [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 - [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 - [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 - [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 - [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 ## Related in other subjects - [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×