--- title: "T/F: SSR not below 1.8" subject: "Transmission and Distribution System Design" chapter: 3 key: "semester_8/transmission_and_distribution_system_design:tf-ssr-below-1-8" source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-ssr-below-1-8 site: BCT Engineering --- # T/F: SSR not below 1.8 Transmission and Distribution System Design · Chapter 3: Overhead Line Insulator Design - Asked in: 2078 Kartik, 4 marks; 2074 Bhadra, 1+3 marks; 2071 Magh, 1+3 marks (asked 3 times) - 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-ssr-below-1-8 - 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: The value of SSR should not be chosen below 1.8. ## Short answer SSR is the ratio of peak switching surge voltage to peak normal phase voltage. With closing resistors and synchronized switching, the practical minimum is about 1.8–2.0; reducing it further is uneconomical and gives little insulation saving, so values below 1.8 are not chosen. ## Answer **TRUE** (as a practical design rule). - SSR is the ratio of peak switching surge voltage to peak normal phase voltage. Switching a line always produces some transient over-voltage due to trapped charge, travelling-wave reflection and the Ferranti rise. - With closing resistors, synchronised (point-on-wave) switching and metal-oxide arresters the SSR can be reduced to about 1.8–2.0 pu; reducing it further needs very costly equipment and gives little saving in insulation. - Temporary over-voltages and arrester protective level also set a lower bound: the insulation must anyway withstand arrester residual voltage plus margin. Hence in design an SSR below about 1.8 is not chosen; values of 1.8–2.0 are used for UHV and higher values for lower voltages. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is SSR? A. Ratio of peak switching surge voltage to peak normal phase voltage B. Ratio of rms line voltage to peak voltage C. Ratio of flashover to puncture voltage D. Ratio of lightning to switching voltage Answer: A. SSR is the peak switching over-voltage divided by the peak phase voltage, in pu. (Key point: Definition) 2. Why is a design SSR below about 1.8 not chosen? A. Switching never causes over-voltage B. Reducing it further needs very costly equipment for little insulation saving C. Arresters stop working below 1.8 D. The standard forbids lines above 1.8 kV Answer: B. Closing resistors, controlled switching and arresters reach about 1.8-2.0 pu; going lower costs more than it saves. (Key point: Why a lower bound) 3. Which is a cause of over-voltage when switching a line? A. Absence of capacitance B. Trapped charge, travelling-wave reflection and Ferranti rise C. Lower frequency D. Reduced load Answer: B. These effects mean switching always produces some transient over-voltage. (Key point: Causes of switching surge) ## Related in Transmission and Distribution System Design - [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× - [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: flashover voltage vs puncture voltage](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-puncture-before-flashover) — Transmission and Distribution System Design, chapter 3, asked 3× - [T/F: ampacity vs ambient temperature](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-ampacity-ambient-temperature) — Transmission and Distribution System Design, chapter 4, asked 2× - [T/F: tower height independent of span](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-tower-height-independent-span) — Transmission and Distribution System Design, chapter 4, asked 2× ## Related in other subjects - [CT secondary should not be kept open](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Act-secondary-open-true-false) — Electrical Machines, chapter 2 - [True/false: CT secondary kept open](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Atf-false-ct-secondary-open) — Electrical Machine, chapter 2 - [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: series motor started at no load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Atf-dc-series-motor-start-noload) — Electrical Machine, chapter 4