--- title: "Primary and backup protection" subject: "Switchgear and Protection" chapter: 1 key: "semester_6/switchgear_and_protection:primary-and-backup-protection" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/primary-and-backup-protection site: BCT Engineering --- # Primary and backup protection Switchgear and Protection · Chapter 1: Principle of Power System Protection - Asked in: 2071 Magh, 2 marks; 2077 Chaitra, 6 marks; 2080 Chaitra, 5 marks (asked 3 times) - Video: [Power System Protection Scheme | Primary Protection and Backup protection |](https://www.youtube.com/watch?v=3GLcOW7Bt9c) — Electrical With Abhishek Joshi - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/primary-and-backup-protection - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-1 ## Question What are primary and back up protection? Explain the necessity of back up protection with an example. ## Short answer Primary protection clears faults instantly in its zone; backup protection operates after a time delay if the primary fails due to relay, CT, DC supply, or breaker problems. Backup is essential because no device is 100% reliable and an uncleared fault can cause severe damage, fire, or instability. ## Answer **Primary protection** is the first line of defence: the protection installed for a particular zone (line, transformer, bus, generator) that is expected to clear any fault in that zone quickly and with the smallest possible outage. **Back-up protection** is the second line of defence: a protection that operates, after a deliberate time delay, only when the primary protection or its breaker fails to clear the fault. #### Why primary protection can fail - Failure of the relay itself or its setting. - Open or shorted CT/PT secondary leads. - Failure of the DC tripping supply or trip coil. - Circuit breaker mechanism stuck (breaker failure). - Failure of the communication channel in pilot schemes. #### Necessity of back-up protection - No protective equipment is 100 % reliable, and an uncleared fault can burn equipment, cause fire, collapse system voltage and make generators lose synchronism. - It protects the system while primary protection is out for maintenance. - It covers parts of the system not fully covered by primary protection (e.g. the remote end of a line). - It must be time-graded so that primary protection always has the first chance. #### Example ```text Gen A B C ──●───┼──[CB1]───┼──[CB2]───┼──[CB3]──► load R1 R2 R3 ``` - A fault F occurs on section B–C (beyond CB2). - The primary protection is relay R2 with breaker CB2; it should trip instantly. - If R2 or CB2 fails, relay R1 at the upstream station (set with a higher time, e.g. 0.4 s more) trips CB1 as **remote back-up**. - The cost is that section A–B and its loads are also disconnected, but the fault is removed and the rest of the system survives. In modern substations, **breaker-failure (local back-up) protection** is also used: if CB2 has not opened within about 150–200 ms after its trip signal, all other breakers on the same bus are tripped. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. When does back-up protection operate? A. Instantly together with primary protection B. After a deliberate time delay, only if primary protection or its breaker fails C. Only during normal load conditions D. Only when the primary protection is switched on for testing Answer: B. Back-up is the time-delayed second line of defence. (Key point: Back-up definition) 2. Which of these is a cause of primary protection failure? A. Open CT secondary leads or loss of DC tripping supply B. Correct time grading between relays C. Use of a time delay in back-up relays D. Overlapping of protection zones Answer: A. Failures include relay, CT/PT leads, DC supply, trip coil, stuck breaker and communication channel faults. (Key point: Why primary fails) 3. Why is the back-up relay R1 set with a higher time than R2? A. So R1 trips faster than R2 B. To make R1 less sensitive to load current C. So primary protection always has the first chance to clear the fault D. To reduce the cost of R1 Answer: C. Time grading keeps back-up delayed so only the primary operates when healthy. (Key point: Time grading) 4. In the example, if R2/CB2 fails and R1 trips CB1, what is the drawback? A. The fault is not cleared B. Generator becomes unstable for sure C. CB2 is damaged by the DC supply D. Section A-B and its loads are also disconnected Answer: D. Remote back-up clears the fault but removes a larger part of the system. (Key point: Cost of remote back-up) ## Related in Switchgear and Protection - [Methods of providing backup protection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Amethods-backup-protection) — Switchgear and Protection, chapter 1 - [Zones and backup for breaker 7 failure](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Azones-backup-breaker-7) — Switchgear and Protection, chapter 1 ## Related in other subjects - [Replication objectives; primary-backup](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Areplication-objectives-primary-backup) — Distributed System, chapter 7 - [Fault; primary-backup vs active](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Afault-primary-backup-active) — Distributed System, chapter 7 - [HVDC necessity, configuration and back-to-back](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Ahvdc-necessity-configuration-b2b) — High Voltage Engineering, chapter 1 - [Emergency vs back-up supply, online/offline UPS](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aemergency-vs-backup-ups) — Industrial Power Distribution and Illumination, chapter 9, asked 3×