--- title: "Classify insulators; string insulator configurations" subject: "Transmission and Distribution System Design" chapter: 3 key: "semester_8/transmission_and_distribution_system_design:classify-insulators-string-configuration" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/classify-insulators-string-configuration site: BCT Engineering --- # Classify insulators; string insulator configurations Transmission and Distribution System Design · Chapter 3: Overhead Line Insulator Design - Asked in: 2072 Magh, 6 marks - Video: [PS38 Types of Insulators](https://www.youtube.com/watch?v=dNzHCVJB6fo) — Lectures in Electrical Engineering - 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/classify-insulators-string-configuration - Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-3 ## Question Classify the overhead line insulators. Write advantages of string insulators for HV lines and state the various configuration of string insulator. ## Short answer Overhead insulators are classified by construction (pin, suspension, strain, shackle, post) and material (porcelain, glass, composite). String insulators allow voltage uprating, cost scalability, and flexible mechanics. Configurations include I‑string, double I‑string, V‑string, tension, and jumper strings. ## Answer Overhead line insulators support the conductors and isolate them electrically from the towers. #### Classification | Basis | Types | |---|---| | Construction / use | Pin type, suspension (disc/string), strain (tension), shackle, post type, stay (egg) insulator | | Material | Porcelain, toughened glass, composite (polymer, silicone rubber) | | Voltage | LV/MV (pin, shackle, post) and HV/EHV (string, long-rod, composite) | | Position on line | Suspension (straight towers) and tension (angle and dead-end towers) | - **Pin type:** fixed on a pin on the cross-arm. Used up to 33 kV. - **Suspension / disc:** discs hung in a string, used above 33 kV. - **Strain:** a disc string in the horizontal plane at dead-ends, sharp angles and river crossings. - **Shackle:** used on LV distribution lines and service drops. - **Post:** used in substations and compact lines. - **Stay insulator:** fitted in stay wires to keep the lower part dead. #### Advantages of string insulators for HV lines 1. Each disc is rated about 11 kV, so any voltage is handled by adding discs. 2. Cheaper than pin insulators above 33 kV. 3. A failed disc is replaced alone. 4. The flexible string reduces mechanical stress. 5. The conductor below the cross-arm is partly shielded from lightning. 6. The voltage can be upgraded later by adding discs. #### Configurations of string insulators 1. **I-string (single suspension):** one vertical string. It is simple and cheap but swings freely, so it needs a longer cross-arm. 2. **Double I-string:** two parallel strings for heavy conductors or important crossings (higher mechanical strength and reliability). 3. **V-string:** two strings in a V shape. The conductor cannot swing, so the tower window and right-of-way are smaller. Used at 400 kV and above. 4. **Tension (strain) string:** horizontal, single or double, at angle and dead-end towers. 5. **Y-string or inverted V, and long-rod / composite strings:** used for compact EHV lines. 6. **Jumper (pilot) string:** holds the jumper loop at tension towers to keep clearance. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Up to about what voltage are pin insulators used, and what is used above? A. Up to 33 kV; suspension disc strings above B. Only above 132 kV C. Only for substations D. Up to 400 kV; strings below Answer: A. Pin type cost rises steeply, so strings take over above 33 kV. (Key point: Pin insulator use) 2. Where is a strain (tension) string used? A. Horizontally at dead-ends, sharp angles and river crossings B. On stay wires only C. Vertically at straight towers D. In LV service drops Answer: A. It takes the full conductor tension. (Key point: Strain insulator) 3. What is the main benefit of a V-string? A. It removes the need for earth wires B. Higher voltage rating per disc C. Conductor cannot swing, so smaller tower window and right-of-way D. Lower cost of discs Answer: C. Fixed transverse position reduces required clearance. (Key point: V-string) 4. When is a double I-string used? A. As a stay insulator B. To reduce discs C. Heavy conductors or important crossings, for higher strength and reliability D. Light LV lines Answer: C. Two parallel strings share load and give redundancy. (Key point: Double I-string) ## Related in Transmission and Distribution System Design - [String insulator configuration and advantages](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Astring-insulator-configuration) — Transmission and Distribution System Design, chapter 3 - [Basic configurations of insulator string](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Ainsulator-string-configurations) — Transmission and Distribution System Design, chapter 3 - [400 kV lines: configuration, V-string, earth wires](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Ajustify-400kv-config-vstring-earthwire) — Transmission and Distribution System Design, chapter 4 - [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 - [T/F: V-type string for 400 kV lines](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-v-string-400kv-mechanical) — Transmission and Distribution System Design, chapter 3 - [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 ## Related in other subjects - [String of 4 insulators at 66 kV, ratio 10](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Astring-4-insulators-66kv-ratio-10) — Power System Analysis I, chapter 2 - [String of 5 insulators on 100 kV, ratio 0.1](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Astring-5-insulators-100kv) — Power System Analysis I, chapter 2 - [Tower top potential and insulator string voltage](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Atower-top-potential-insulator-voltage) — High Voltage Engineering, chapter 3, asked 2× - [Requirements of conductors for overhead lines](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aconductor-requirements) — Power System Analysis I, chapter 2, asked 2×