--- title: "T/F: impulse withstand and neutral earthing" subject: "Transmission and Distribution System Design" chapter: 3 key: "semester_8/transmission_and_distribution_system_design:tf-impulse-withstand-neutral-earthing" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-impulse-withstand-neutral-earthing site: BCT Engineering --- # T/F: impulse withstand and neutral earthing Transmission and Distribution System Design · Chapter 3: Overhead Line Insulator Design - Asked in: 2075 Bhadra, 1+3 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/tf-impulse-withstand-neutral-earthing - Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-3 ## Question State whether the following statement is TRUE or FALSE. Justify your answer with a brief explanation: Impulse withstand voltage of an overhead transmission line insulator depends on system (neutral) earthing condition. ## Short answer True. Effective earthing reduces temporary overvoltages, allowing a lower-rated arrester and reduced BIL (e.g., 900 kV instead of 1050 kV for 220 kV). Isolated or ungrounded neutral requires full insulation level. ## Answer **TRUE.** - The insulation level (impulse withstand voltage, BIL) is chosen as protection ratio × protective level of the surge arrester. - The arrester rating depends on the **earthing coefficient**: with effectively (solidly) earthed neutral the healthy-phase voltage rises to only about 0.8 of line voltage, so an "80% arrester" with a lower protective level is used. With isolated neutral, a "100% arrester" is needed. - So effective earthing allows a **reduced BIL** (e.g. 220 kV: 900 kV instead of 1050 kV), and fewer insulator discs; an ungrounded system needs full insulation. The impulse strength of a given disc does not change, but the required impulse withstand level of the line does depend on neutral earthing. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Does the impulse withstand voltage required for a line depend on neutral earthing? A. False: BIL depends only on disc size B. True: earthed systems need a higher BIL C. False: earthing affects only power-frequency current D. True: earthing changes the arrester rating and hence the required BIL Answer: D. Required BIL = PR x arrester protective level, and the arrester level depends on earthing. (Key point: Verdict) 2. With an effectively earthed neutral, healthy phase voltage rise is about: A. 0.8 of line voltage, allowing an 80% arrester B. 1.0 of line voltage with a 100% arrester only C. 1.73 times line voltage D. 0.5 of line voltage Answer: A. Effective earthing limits the rise so a lower-rated arrester and reduced BIL can be used. (Key point: Effective earthing) 3. For a 220 kV line, what does effective earthing allow? A. Reduced BIL, such as 900 kV instead of 1050 kV, and fewer discs B. More discs than an ungrounded system C. Higher BIL of 1050 kV D. No insulation at all Answer: A. Reduced insulation levels follow from the lower arrester protective level. (Key point: Example) ## Related in Transmission and Distribution System Design - [T/F: earth wire choice depends on circuit and voltage](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-earth-wire-choice-circuit-voltage) — Transmission and Distribution System Design, chapter 3 - [T/F: span choice depends on voltage only](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-span-depends-voltage-not-conductor) — Transmission and Distribution System Design, chapter 4 - [T/F: pin insulator cost vs voltage](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-pin-insulator-cost-linear) — Transmission and Distribution System Design, chapter 3 - [T/F: switching surge ratio vs system voltage](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-ssr-system-voltage) — Transmission and Distribution System Design, chapter 3, asked 4× - [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 - [Neutral earthing and insulation level](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aneutral-earthing-insulation-level) — Transmission and Distribution System Design, chapter 3, asked 3× ## Related in other subjects - [True/false: load rejection raises terminal voltage](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Atf-load-rejection-voltage-rise) — High Voltage Engineering, chapter 3 - [True/false: protective ratio and grounding](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Atf-protective-ratio-grounding) — High Voltage Engineering, chapter 4 - [True/false: derivative controllers always used with others](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Aderivative-always-with-others) — Control Systems, chapter 7, asked 3× - [True/false: air conditioner is closed loop](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Atf-air-conditioning-closed) — Control Systems, chapter 1