--- title: "Electrical and mechanical requirements of insulators" subject: "Power System Analysis I" chapter: 2 key: "semester_4/power_system_analysis_i:insulator-requirements" source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/insulator-requirements site: BCT Engineering --- # Electrical and mechanical requirements of insulators Power System Analysis I · Chapter 2: Overhead and Underground Transmission - Asked in: 2076 Bhadra, 4 marks - Video: [Insulators and Types of Insulators - Mechanical Design of Overhead Line - Power System Engineering 1](https://www.youtube.com/watch?v=IBZmV0Rflyc) — Ekeeda - 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/insulator-requirements - Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-2 ## Question What are the electrical and mechanical characteristics required for a good insulator used on HV transmission lines? ## Short answer Electrically, insulators need high dielectric strength, high insulation resistance, a glazed non-porous surface, sufficient creepage, and puncture strength exceeding flashover voltage. Mechanically, they must withstand tensile/compressive loads, thermal shocks, be lightweight, and resist aging. ## Answer An overhead line insulator must keep the energised conductor isolated from the earthed tower. It must also carry the mechanical load of the conductor in all weather. #### Electrical requirements - High dielectric strength, so it does not puncture at operating and surge voltages - High insulation resistance, with very low leakage current over the surface - A high ratio of puncture strength to flashover voltage (safety factor), so that it flashes over through air rather than breaking down internally - A non-porous, glazed surface free from impurities and cracks, so that moisture and dirt do not form conducting paths - Good performance under contamination, with enough creepage distance #### Mechanical requirements - High mechanical (tensile and compressive) strength to carry the conductor weight plus wind and ice loads - Ability to withstand sudden temperature changes without cracking - Stable properties over time, with no ageing - Light weight and easy handling and installation Porcelain, toughened glass and composite (silicone rubber) insulators are used. Glass shatters visibly when it fails, which makes inspection easy. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the desired relation between puncture strength and flashover voltage? A. Puncture strength should equal flashover voltage B. Puncture strength should be lower than flashover voltage C. Puncture strength should be much higher, so it flashes over in air rather than breaking down inside D. The two are unrelated Answer: C. A high ratio ensures a flashover through air does not damage the insulator. (Key point: Puncture vs flashover) 2. Why should the insulator have a non-porous, glazed surface? A. To increase weight B. So moisture and dirt do not form conducting paths and leakage current stays low C. To raise mechanical strength D. To increase capacitance Answer: B. Glazing prevents surface wetting and leakage. (Key point: Surface) 3. Which mechanical property is needed? A. Low strength to be flexible B. Strength only against compression C. High tensile and compressive strength to carry conductor weight plus wind and ice loads D. High thermal expansion Answer: C. The insulator carries the conductor load in all weather. (Key point: Mechanical) 4. What inspection advantage does toughened glass have? A. It never fails B. It shatters visibly when it fails C. It is porous D. It has no mechanical load Answer: B. A broken glass disc is easy to see during inspection. 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