--- title: "Methods of providing backup protection" subject: "Switchgear and Protection" chapter: 1 key: "semester_6/switchgear_and_protection:methods-backup-protection" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/methods-backup-protection site: BCT Engineering --- # Methods of providing backup protection Switchgear and Protection · Chapter 1: Principle of Power System Protection - Asked in: 2073 Magh, 4 marks - Video: [Primary Backup protection and methods of providing backup protections](https://www.youtube.com/watch?v=4EwmWZwPQZU) — poly ki padhai - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/methods-backup-protection - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-1 ## Question State the various methods used to provide backup protection. ## Short answer Remote back-up uses upstream time-graded relays; relay back-up duplicates independent relays at the same station; breaker back-up (breaker failure) trips all breakers on the bus if a breaker fails; centrally coordinated back-up uses station computers. ## Answer Back-up protection operates, after a time delay, only when the primary protection or its breaker fails to clear a fault. The common methods are: 1. **Remote back-up:** relays at the adjacent (upstream) station act as back-up. They see the fault but are time-graded so that the primary relay has the first chance. - Example: zone-2 and zone-3 of a distance relay, or a time-graded IDMT relay at the source end. - Independent of the primary station's CTs, DC supply and breaker, so very reliable; but it is slow and disconnects a larger part of the system. 2. **Relay back-up (local duplication):** a second, independent relay at the same station (preferably a different principle, fed from separate CT cores and DC supply) trips the same breaker. Example: main-1 and main-2 distance relays on EHV lines. 3. **Breaker back-up (breaker failure protection):** if the breaker does not open within a set time (about 150–200 ms) after the trip signal while current still flows, the breaker-failure relay trips all other breakers on the same bus that feed the fault. 4. **Centrally coordinated back-up:** in modern substations with a central computer, the system state is monitored and, on failure of primary protection, the most suitable breakers are tripped. ```text A B C ─[CB1]────┼──[CB2]────┼── F R1 (remote back-up) R2 primary + local BF relay trips bus B ``` Usually remote back-up is used on distribution systems and local relay + breaker back-up on EHV systems. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a feature of remote back-up? A. Uses the same CTs and DC supply B. Independent of the primary station's CTs, DC supply and breaker, but slow C. Fast and disconnects only the faulty part D. Needs a central computer Answer: B. Remote back-up is very reliable but slow and disconnects a larger area. (Key point: Remote back-up) 2. What does local relay back-up use? A. A second independent relay at the same station, ideally of different principle, on separate CT cores and DC B. The same relay twice C. A relay at the remote station D. A fuse only Answer: A. E.g. main-1 and main-2 distance relays on EHV lines. (Key point: Relay back-up) 3. What does breaker failure protection do after 150-200 ms if current still flows? A. Recloses the failed breaker B. Trips the remote generator only C. Resets the relay D. Trips all other breakers on the same bus feeding the fault Answer: D. It isolates the fault by opening neighbouring breakers. (Key point: Breaker failure) 4. Where are the methods usually applied? A. Remote back-up on EHV; local on distribution B. Only centrally coordinated everywhere C. Remote back-up on distribution; local relay and breaker back-up on EHV D. Breaker back-up on distribution only Answer: C. As stated at the end of the answer. 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