--- title: "Air clearances for 220 kV single circuit line" subject: "Transmission and Distribution System Design" chapter: 4 key: "semester_8/transmission_and_distribution_system_design:air-clearances-220kv-single-circuit" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/air-clearances-220kv-single-circuit site: BCT Engineering --- # Air clearances for 220 kV single circuit line Transmission and Distribution System Design · Chapter 4: Conductor and Support Selection - Asked in: 2073 Bhadra, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/air-clearances-220kv-single-circuit - Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-4 ## Question Compute the various air clearances required for a 220 kV single circuit transmission line. ## Short answer For 245 kV max voltage, phase-to-earth clearance is 2.54 m, phase-to-phase 2.9 m. With 11-disc insulator string length 2.0 m and swing angle 45°, cross-arm length is 4.0 m. Vertical span spacing is 4.5 m, horizontal 9.5 m, earth wire above top conductor 8.3 m, and ground clearance 6.91 m. ## Answer The air clearances of a 220 kV single-circuit line are found from the maximum system voltage, the insulator string and its swing. **Assumptions:** $V_m = 245$ kV, swing 45°, 254 × 154 mm discs, tower width at cross-arm level 1.5 m. #### 1. Phase-to-earth (tower) clearance $$ \begin{aligned} \text{Method I:}\quad & V_{ph,peak} = \frac{245\sqrt{2}}{\sqrt{3}} = 200.0\ \text{kV} \Rightarrow a = 200 + 30 = 230\ \text{cm} \\ \text{Method II:}\quad & V_{ph,rms} = \frac{245}{\sqrt{3}} = 141.45\ \text{kV} \Rightarrow a = 6.5 \times 14.145 + 8 = 99.9\ \text{in} = 2.54\ \text{m} \end{aligned} $$ Adopt the larger value, **a = 2.54 m**. #### 2. Phase-to-phase clearance $$ a_{pp} \approx 1.15a = 2.9\ \text{m} $$ The actual spacing chosen below is much larger. #### 3. Insulator string length 10 discs meet the wet (395 kV) and impulse (900 kV) levels. Adding one spare gives 11 discs: $$ l = 11 \times 0.154 + 0.3 \approx 2.0\ \text{m} $$ #### 4. Cross-arm length (clearance with swing) $$ L_{ca} = a + l\sin45° = 2.54 + 1.41 = 3.95 \approx 4.0\ \text{m} $$ #### 5. Vertical spacing between conductors $$ V = l + a = 2.0 + 2.54 = 4.54 \approx 4.5\ \text{m} $$ #### 6. Horizontal spacing between conductors on opposite sides $$ H = 2 \times 4.0 + 1.5 = 9.5\ \text{m} $$ #### 7. Earth wire clearance (shielding angle 30°, one earth wire) $$ y = \frac{4.0 + 0.75}{\tan30°} = 8.23 \approx 8.3\ \text{m above the top conductor} $$ Also check the midspan clearance between the earth wire and the conductor, using the IS 5613 rule of thumb: at least $0.012L + 1$ m, about 4.6 m for a 300 m span. #### 8. Minimum ground clearance $$ H_g = \frac{V - 33}{33} + 17 = \frac{220 - 33}{33} + 17 = 22.67\ \text{ft} = 6.91\ \text{m} $$ This compares with 7.0 m in IS practice. | Clearance | Value | |---|---| | Phase to earth | 2.54 m | | Phase to phase (minimum) | 2.9 m | | Cross-arm length | 4.0 m | | Vertical / horizontal conductor spacing | 4.5 m / 9.5 m | | Earth wire above top conductor | 8.3 m | | Ground clearance | 6.91 m | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which value is adopted for phase-to-earth clearance at 220 kV? A. 2.30 m (Method I), the smaller B. 2.54 m (Method II), the larger C. 2.9 m D. 1.5 m Answer: B. The larger of the two methods is adopted, 2.54 m. (Key point: Phase-to-earth clearance) 2. How is cross-arm length computed? A. a + l sin45° = 2.54 + 1.41 = about 4.0 m B. a + l = 4.5 m C. 2a = 5.1 m D. l sin45° only Answer: A. The cross arm holds clearance a from the tower body when the string swings by 45°. (Key point: Cross-arm length) 3. How is horizontal spacing between conductors on opposite sides found? A. H = 2 L_ca + tower width = 9.5 m B. H = L_ca C. H = 1.15a D. H = l + a Answer: A. Two cross arms plus the tower width at the cross-arm level: 2 x 4.0 + 1.5. (Key point: Horizontal spacing) 4. What fixes the height of the earth wire above the top conductor? A. Ground clearance B. A 30° shielding angle with the horizontal offset of the outer conductor C. Insulator disc count D. Wind pressure Answer: B. y = (4.0 + 0.75)/tan30° = 8.23 m ensures the conductor lies within the shielded zone. (Key point: Earth wire placement) ## Related in Transmission and Distribution System Design - [Air clearances for 220 kV double circuit line](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aair-clearance-220kv-double-circuit) — Transmission and Distribution System Design, chapter 4 - [Air clearances, 220 kV line for 200 MW](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aair-clearances-220kv-200mw-120km) — Transmission and Distribution System Design, chapter 4 - [Air clearances for 132 kV double circuit line](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aair-clearance-132kv-double-circuit) — Transmission and Distribution System Design, chapter 4 - [Tower dimensions for 220 kV single circuit line](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atower-dimensions-220kv) — Transmission and Distribution System Design, chapter 4, asked 2× - [Number of insulator discs for 220 kV line](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Ainsulator-discs-220kv-appendix) — Transmission and Distribution System Design, chapter 3 - [Tower dimensions for 220 kV double circuit line](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atower-dimensions-220kv-double-circuit) — Transmission and Distribution System Design, chapter 4, asked 2× ## Related in other subjects - [Capacitor for zero regulation, 70 km 220 kV line](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3A70km-220kv-zero-regulation-capacitor) — Power System Analysis I, chapter 6 - [RRRV from short circuit test (220 kV)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Arrrv-220kv-test) — Switchgear and Protection, chapter 6, asked 2× - [Surge impedance and velocity of 220 kV line](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Asurge-impedance-220kv-200km) — High Voltage Engineering, chapter 3, asked 2× - [220 kV line, 200 MW: sending end from ABCD](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aabcd-220kv-200mw) — Power System Analysis I, chapter 6