--- title: "Isolator construction, principle and application" subject: "Switchgear and Protection" chapter: 4 key: "semester_6/switchgear_and_protection:isolator-construction-principle" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/isolator-construction-principle site: BCT Engineering --- # Isolator construction, principle and application Switchgear and Protection · Chapter 4: Isolators and Contactors - Asked in: 2073 Bhadra, 6 marks - Video: [isolator | what is isolator | difference between isolator and circuit breaker | electric isolator](https://www.youtube.com/watch?v=biy8g0vbI_M) — Mind of Engineer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/isolator-construction-principle - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-4 ## Question Explain the construction, operating principle and application of isolator. ## Short answer An isolator consists of rotating post insulators, a moving blade, and fixed contacts with no arc-quenching. It opens or closes only after the circuit breaker interrupts current, creating a visible air gap. Applications include maintenance isolation on both sides of CBs, bus sectionalizing, and line earthing. ## Answer An **isolator** is an off-load switching device that provides a visible isolation gap. It is operated only after the circuit breaker has interrupted the current. #### Construction A common type is the **double-break, centre-rotating isolator** used in outdoor substations. ```text Fixed contact Moving blade Fixed contact [==]======== ( rotates 90 deg ) ========[==] | | | Insulator Rotating insulator Insulator | | | ==== base channel (steel) + operating mechanism ``` 1. **Base frame:** galvanised steel channel on which the insulator stacks stand. 2. **Post insulators:** porcelain or polymer insulators; the outer two are fixed and the middle one rotates (double-break type). 3. **Moving contact (blade):** copper or aluminium tube mounted on the rotating insulator. 4. **Fixed contacts:** spring-loaded finger contacts, silver plated, giving firm contact pressure. 5. **Operating mechanism:** hand-operated crank or motor drive, connected by rods to all three poles (gang operation). 6. **Earth switch:** attached blade that connects the line side to earth when the main isolator is open. 7. **Interlocks:** mechanical or electrical interlocks with the CB and earth switch. Other types: single-break (centre-break), vertical-break, and **pantograph** (used for connecting to an overhead bus with vertical movement). #### Operating principle 1. The isolator is a simple knife or rotating switch; it has **no arc quenching medium**. 2. **Opening:** after the CB is open (current is zero), the operating handle turns the middle insulator; the blade swings away from both fixed contacts, giving two visible air gaps. 3. **Closing:** with the CB open, the blade is turned back so that it enters the fixed finger contacts. 4. Since no arc control exists, opening on load would draw a heavy arc that can damage the contacts and injure the operator. **Interlocking** makes sure the CB opens first and closes last. 5. The open gap is designed for higher withstand voltage than the open CB, so the isolated part is completely safe. #### Applications - On both sides of circuit breakers in substations for maintenance isolation. - Bus sectionalising, bus coupling and bus selection in double-bus systems. - Transformer, PT, lightning arrester and capacitor bank isolation. - Pole-mounted AB (air break) switches on 11 kV and 33 kV lines for feeder sectionalising. - With earth switch, for earthing lines before work. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In a double-break centre-rotating isolator, what moves? A. Outer insulators rotate B. The middle insulator rotates, swinging the blade away from both fixed contacts C. The base channel slides D. The fixed contacts rotate Answer: B. The blade on the central rotating insulator opens two visible gaps. (Key point: Double-break construction) 2. Why must the CB open before the isolator is operated? A. The isolator has no arc control, so opening on load draws a damaging long arc B. The isolator needs the CB to power its motor C. The CB cools the blade D. The earth switch needs it Answer: A. Opening on load causes a heavy arc that can damage contacts and injure the operator. (Key point: Operating condition) 3. What does interlocking between CB and isolator ensure? A. CB opens first and closes last relative to isolators B. Isolator opens before CB C. Both close together D. Earth switch closes with isolator closed Answer: A. Isolators can move only when the CB is open. (Key point: Interlocking) 4. How is the open gap of an isolator designed compared with an open CB? A. For lower withstand voltage B. For higher withstand voltage, so the isolated part is completely safe C. Equal to the CB gap exactly D. Gap size is irrelevant Answer: B. A larger gap gives a higher withstand than the open CB. (Key point: Gap withstand) ## Related in Switchgear and Protection - [Isolator function and application](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aisolator-function-application) — Switchgear and Protection, chapter 4 - [Isolator in SLD; application of contactors](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aisolator-sld-contactor-application) — Switchgear and Protection, chapter 4 - [Construction and principle of contactor](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acontactor-construction-principle) — Switchgear and Protection, chapter 4 - [Contactor construction and remote control circuit](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acontactor-remote-control) — Switchgear and Protection, chapter 4 - [CT working principle and applications](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Act-definition-principle-applications) — Switchgear and Protection, chapter 2 ## Related in other subjects - [Transformer operating principle and emf equation](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Atransformer-principle-emf-equation) — Electrical Machines I, chapter 2, asked 6× - [Servo motor operating principle](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Aservo-motor-principle) — Electrical Machine, chapter 7, asked 2× - [Capacitor start motor operating principle](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Acapacitor-start-motor-principle) — Electric Machines II, chapter 3, asked 2× - [Operating principle of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-operating-principle) — Electrical Machines I, chapter 5