--- title: "Isolator vs CB and why both used" subject: "Switchgear and Protection" chapter: 4 key: "semester_6/switchgear_and_protection:isolator-vs-cb-simultaneous" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/isolator-vs-cb-simultaneous site: BCT Engineering --- # Isolator vs CB and why both used Switchgear and Protection · Chapter 4: Isolators and Contactors - Asked in: 2072 Asoj, 6 marks; 2074 Bhadra, 6 marks; 2075 Bhadra, 6 marks; 2079 Chaitra, 6 marks (asked 4 times) - Video: [Difference between Circuit Breakers and Isolators | Working of Circuit Breakers | Isolator Working |](https://www.youtube.com/watch?v=UeNj8OthGPA) — ElectroMagnetic World - 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-vs-cb-simultaneous - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-4 ## Question What are the basic differences between isolator and circuit breaker? Although CB can be used to isolate the circuit, why do we need isolator? With a neat circuit diagram, explain why we use isolator and circuit breaker simultaneously in power system. ## Short answer An isolator is an off‑load switch providing a visible break, while a CB makes/breaks load and fault currents. An isolator is needed because CB contacts are not visible and for CB maintenance; the two are used together with the CB interrupting current before the isolator opens, interlocked for safety. ## Answer An **isolator** is an off-load disconnecting switch that gives a visible open gap so that a section of the circuit can be safely isolated for maintenance. A **circuit breaker (CB)** is an on-load switching device that can make, carry and break normal as well as fault currents automatically. #### Differences between isolator and circuit breaker | Point | Isolator | Circuit breaker | |---|---|---| | Operation | Off-load only (no current) | On-load and under fault | | Arc quenching | No arc quenching medium | Has medium (oil, air, SF6, vacuum) | | Fault interruption | Cannot break fault current | Breaks fault currents automatically | | Control | Manual or motorised; not tripped by relay | Tripped by protective relays | | Contact gap | Visible open gap | Contacts inside enclosure, not visible | | Cost and size | Cheap, simple | Costly, complex mechanism | | Purpose | Safety isolation for maintenance | Switching and protection | #### Why an isolator is still needed 1. The contacts of a CB are inside a tank or chamber; the open position is **not visible**, so the maintenance crew cannot be sure the circuit is dead. 2. The CB itself needs maintenance (contact replacement, oil or gas filling). It must be **isolated from both sides**, which the CB cannot do for itself. 3. An open CB still has a small gap; a long, visible air gap from an isolator, plus an **earthing switch**, gives a proven safe condition. 4. Isolators are cheap, so they can be put at many points to section a bus or a feeder. #### Using isolator and CB together ```text Bus ---[ I1 ]---[ CB ]---[ I2 ]--- Feeder/line | [ES] earth switch | === I1, I2 = isolators on both sides of CB ``` **Opening sequence (to isolate):** 1. Open the CB first; it breaks the load or fault current. 2. Then open isolator I2 (line side) and I1 (bus side) at no load. 3. Close the earth switch on the dead section. **Closing sequence (to restore):** 1. Open the earth switch. 2. Close the isolators I1 and I2 (no current flows because CB is open). 3. Finally close the CB to energise the feeder. Isolators are **interlocked** with the CB so that they cannot be operated while the CB is closed. In this way the CB does the switching under load and fault, and the isolator gives safe, visible isolation; each covers what the other cannot do. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which statement correctly contrasts isolator and circuit breaker? A. Isolator breaks fault current; CB is off-load B. Isolator is off-load with no arc quenching; CB can make and break normal and fault currents C. Both operate on load D. CB has a visible gap; isolator hides contacts Answer: B. Isolators carry no arc-quenching medium and must be operated at no load; CBs are tripped by relays under fault. (Key point: Isolator vs CB duty) 2. Why is an isolator still needed even though a CB can open the circuit? A. CB cannot carry current B. CB contacts are enclosed so the open state is not visible, and the CB needs isolation from both sides for maintenance C. Isolators are faster than CBs D. Isolators provide arc quenching Answer: B. A visible gap proves the circuit is dead and allows safe CB maintenance. (Key point: Why isolator needed) 3. What is the correct sequence to isolate a feeder? A. Open isolators, then CB B. Close the earth switch, then open CB C. Open CB, open isolators, then close earth switch D. Open bus isolator, then CB, then line isolator Answer: C. CB breaks current first; isolators then open at no load; the earth switch is closed on the dead section. (Key point: Opening sequence) 4. When restoring supply, the circuit breaker is closed: A. First, before the isolators B. Last, after isolators are closed C. At the same time as the earth switch D. Before the earth switch is opened Answer: B. Isolators close with no current while the CB is open; the CB then energises the feeder. (Key point: Closing sequence) ## Related in Switchgear and Protection - [Isolator operation with CB in HV installation](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aisolator-sld-cb-operation) — Switchgear and Protection, chapter 4 - [Vacuum circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Avacuum-cb-construction) — Switchgear and Protection, chapter 6, asked 2× - [SF6 circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Asf6-cb-construction) — Switchgear and Protection, chapter 6, asked 2× - [Puffer type SF6 circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Apuffer-sf6-cb) — Switchgear and Protection, chapter 6 - [Air blast circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aair-blast-cb) — Switchgear and Protection, chapter 6 - [Minimum oil circuit breaker](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aminimum-oil-cb) — Switchgear and Protection, chapter 6 ## Related in other subjects - [Why do we need DFT](https://www.bctengineer.com/question.md?key=semester_7%2Fdigital_signal_analysis_and_processing%3Awhy-need-dft) — Digital Signal Analysis and Processing, chapter 7, asked 6× - [Short circuit MVA of feeder circuit breaker](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Afeeder-cb-short-circuit-mva) — Power System Analysis II, chapter 3, asked 2× - [Basic structure of power system](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Abasic-structure-power-system) — Power System Analysis I, chapter 1 - [Why do we need FFT](https://www.bctengineer.com/question.md?key=semester_7%2Fdigital_signal_analysis_and_processing%3Awhy-need-fft) — Digital Signal Analysis and Processing, chapter 7, asked 2×