--- title: "Steps to select number of insulator discs" subject: "Transmission and Distribution System Design" chapter: 3 key: "semester_8/transmission_and_distribution_system_design:steps-selecting-insulator-discs" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/steps-selecting-insulator-discs site: BCT Engineering --- # Steps to select number of insulator discs Transmission and Distribution System Design · Chapter 3: Overhead Line Insulator Design - Asked in: 2072 Asoj, 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/steps-selecting-insulator-discs - Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-3 ## Question Describe the steps to be followed while selecting the number of insulator discs for a transmission line design. ## Short answer Determine required withstand levels (dry, wet, BIL, switching surge) corrected for site. Select the smallest disc count meeting each test, check pollution creepage, and add spare discs. For a 220 kV line, typically 11 discs per suspension string. ## Answer The number of discs is chosen so that the string withstands every voltage stress it will meet: power frequency in dry and wet conditions, lightning impulse and switching surges. Pollution and spare-disc allowances are then added. The steps are: 1. **Fix the system voltage.** Take the nominal voltage $V$ and the maximum system voltage $V_m$ (about $1.1V$, for example 245 kV for a 220 kV line). 2. **Find the required withstand levels** from standards or the given table for $V_m$: - 1-minute power-frequency dry withstand - 1-minute power-frequency wet withstand - lightning impulse withstand (BIL) - switching impulse withstand (BSL), for $V_m > 245$ kV 3. **Correct for site conditions.** Divide the required withstand values by the air-density factor $\delta$ (altitude, temperature) and any humidity factor. This gives the equivalent values at standard conditions. 4. **Power-frequency dry test:** from the disc flashover table, pick the smallest number of discs whose 1-minute dry FOV is at least the required dry withstand. 5. **Power-frequency wet test:** do the same with the wet FOV column. This usually governs over the dry test. 6. **Lightning impulse:** choose the discs whose impulse FOV is at least the BIL. Allow for the uneven voltage distribution (string efficiency) along the string. 7. **Switching surge:** compute the surge $V_{ss} = SSR \times \sqrt{2}V_m/\sqrt{3}$. Use a switching surge ratio of 2–3.5 depending on breaker type and line length. Choose discs whose switching flashover, or equivalent wet FOV, is at least $V_{ss}$. 8. **Pollution (creepage) check:** the number of discs ≥ (specific creepage in mm/kV × $V_m$) / (creepage distance per disc). Use 16–31 mm/kV from light to very heavy pollution. 9. **Take the largest number** from steps 4–8. 10. **Add allowances:** one extra disc for a defective or punctured disc. Use one or two more discs in tension (dead-end) strings than in suspension strings. 11. **Check the string length and air clearance** against the tower window, the swing angle and the cross-arm length. If the string becomes too long, consider a V-string, anti-fog discs or composite insulators. Example: for a 220 kV line ($V_m = 255$ kV row of the table: wet 395 kV, BIL 900 kV), 10 discs meet the wet and impulse levels. Adding one spare disc gives 11 discs per suspension string. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the first step in selecting the number of discs? A. Count the towers B. Add the spare disc C. Fix nominal and maximum system voltage (e.g. 245 kV for 220 kV) D. Choose the conductor colour Answer: C. All withstand levels are obtained from the maximum system voltage. (Key point: Starting point) 2. How is correction for site conditions applied? A. Divide the required withstand by the air-density (and humidity) factor B. Subtract 10% from the withstand C. Multiply the string length by 2 D. Ignore it Answer: A. This gives the equivalent value at standard conditions. (Key point: Site correction) 3. How is the final disc number determined? A. Take the average B. Take the smallest C. Use dry test only D. Take the largest number from dry, wet, impulse, switching and pollution checks, then add allowances Answer: D. The string must satisfy every stress. (Key point: Final number) 4. What allowances are added at the end? A. One extra disc for a defective one, and one or two more in tension strings B. Ten extra discs C. None D. Remove one disc Answer: A. These cover punctured discs and heavier duty of dead-end strings. (Key point: Allowances) ## Related in Transmission and Distribution System Design - [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 - [Insulator discs per string, 180 MW over 200 km](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Ainsulator-discs-180mw-200km) — Transmission and Distribution System Design, chapter 3 - [Initial line design for 200 MW over 120 km](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aline-design-200mw-120km) — Transmission and Distribution System Design, chapter 2 - [Voltage, circuits, discs: 200 MW, 160 km](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Adesign-200mw-160km-voltage-discs) — Transmission and Distribution System Design, chapter 2 - [Voltage, circuits and discs for 260 MW grid link](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Avoltage-circuits-260mw-grid-options) — Transmission and Distribution System Design, chapter 2 - [Effect of parameters on conductor and insulator cost](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aparameters-effect-conductor-insulator-cost) — Transmission and Distribution System Design, chapter 2 ## Related in other subjects - [Transmission line issues in high speed design](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Atransmission-line-issues) — Instrumentation II, chapter 7 - [Selecting main stations; number of offsets](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Afactors-main-station-number-of-offsets) — Surveying I, chapter 3 - [USB protocols followed in USB design](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb-design-protocols) — Instrumentation II, chapter 3 - [Frequency scaling relations and filter design steps](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Afrequency-scaling-design-steps) — Filter Design, chapter 1