--- title: "Isolator function and application" subject: "Switchgear and Protection" chapter: 4 key: "semester_6/switchgear_and_protection:isolator-function-application" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/isolator-function-application site: BCT Engineering --- # Isolator function and application Switchgear and Protection · Chapter 4: Isolators and Contactors - Asked in: 2071 Bhadra, 6 marks - Video: [Electrical Isolators](https://www.youtube.com/watch?v=eX_GjoYo7Ik) — Leap 2 Learn - 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-function-application - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-4 ## Question What is an isolator? Write its function and application in power system. ## Short answer An isolator is a no-load switch providing a visible break for safe isolation. It disconnects equipment after the circuit breaker opens, enabling maintenance. Applications include both sides of circuit breakers, line bays, bus sectionalizers, and transformer isolation. ## Answer An **isolator** (disconnector) is a mechanical switch that opens or closes a circuit **only when no current, or negligible current, is flowing**. It provides a visible, safe air gap between a live part and the part to be maintained. It has no arc-quenching arrangement, so it must never be opened on load. #### Functions of an isolator 1. **Safety isolation:** disconnects equipment (CB, transformer, line, bus section) from all live parts so maintenance can be done safely. 2. **Visible break:** the open blades give an obvious, visible gap that confirms the circuit is dead. 3. **Sectionalising:** divides a busbar or a feeder into sections so that one part can be taken out while the rest stays in service. 4. **Change-over / bus selection:** in double-bus schemes, isolators select which bus a feeder is connected to. 5. **Earthing:** many isolators are combined with an **earth switch** to earth the dead line and discharge trapped charge. 6. **Breaking small currents:** can safely break the small charging current of busbars or short unloaded lines and the magnetising current of small transformers (where permitted). #### Applications in power system - On **both sides of every circuit breaker** in substations, so the CB can be maintained. - At the **incoming and outgoing line bays** of substations (line isolator with earth switch). - **Bus coupler and bus sectionaliser** isolators in single and double busbar arrangements. - **Transformer isolation** on HV and LV sides of power and distribution transformers. - **Pole-mounted (gang-operated) AB switches** on 11 kV and 33 kV distribution lines in Nepal for sectionalising feeders. - Isolating **potential transformers, lightning arresters and capacitor banks**. - **Bypass isolators** used to bypass a CB during maintenance. #### Types (brief) - By construction: double-break, single-break (centre-break), pantograph, vertical-break. - By location: bus-side, line-side, transfer-bus isolators. - By operation: manual (hook stick), gang-operated, motorised. **Example:** in a 33/11 kV substation, the operator first opens the 33 kV breaker, then opens the isolators on both sides and closes the earth switch before work is done on the breaker. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. An isolator should be operated only when: A. Full load current flows B. No current, or negligible current, flows C. Fault current is present D. Voltage is zero and the CB closed Answer: B. It has no arc-quenching means, so it opens or closes at no load. (Key point: Definition) 2. Which function of an isolator confirms that a circuit is dead? A. Visible open air gap B. Arc quenching C. Automatic relay tripping D. Current limiting Answer: A. The open blades show a visible gap. (Key point: Visible break) 3. What does bus sectionalising by an isolator allow? A. Breaking fault current B. One bus section to be maintained while the rest stays in service C. Faster CB operation D. Reduction of fault level Answer: B. Isolators divide the bus so one part can be taken out of service. (Key point: Sectionalising) 4. Why is an earth switch combined with an isolator? A. To earth the dead line and discharge trapped charge B. To break load current C. To raise line voltage D. To quench the arc Answer: A. Earthing the isolated section removes trapped charge and induced voltage. 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