--- title: "Requirements, coordination and protection zone" subject: "Switchgear and Protection" chapter: 1 key: "semester_6/switchgear_and_protection:requirements-coordination-zone" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/requirements-coordination-zone site: BCT Engineering --- # Requirements, coordination and protection zone Switchgear and Protection · Chapter 1: Principle of Power System Protection - Asked in: 2076 Bhadra, 6 marks - Video: [Protection zones](https://www.youtube.com/watch?v=oy-0jSh1YVY) — stephen brooks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/requirements-coordination-zone - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-1 ## Question List down the basic requirements of good protective scheme. What do you understand by coordination of protective device and protection zone? ## Short answer Requirements: selectivity, speed, sensitivity, reliability, stability, simplicity, economy. Coordination means grading pick-up current and time of series devices so the nearest to the fault trips first, with a grading margin of 0.3–0.5 s. A protection zone is the region guarded, bounded by CTs and overlapped to prevent blind spots. ## Answer #### Basic requirements of a good protective scheme 1. **Selectivity** – only the faulty section is isolated. 2. **Speed** – minimum fault-clearing time to limit damage and keep stability. 3. **Sensitivity** – operates on the smallest expected fault current. 4. **Reliability** – operates correctly whenever needed and never falsely. 5. **Stability** – no operation for external faults, load swings or inrush. 6. **Simplicity** – fewer components, easy testing and maintenance. 7. **Economy** – cost matched to the value of the protected equipment. #### Coordination of protective devices **Coordination** is the process of choosing the settings (pick-up current and time) of relays, fuses and breakers in series so that the device nearest to the fault operates first, and each upstream device acts only as time-delayed back-up. - Achieved by time grading, current grading or a combination (IDMT relays). - A **grading margin** (coordination time interval) of about 0.3–0.5 s is kept between successive relays, allowing for breaker opening time, relay overshoot and errors. *Example:* On a radial feeder A→B→C with relays R1, R2, R3, relay R3 at the far end is set at 0.1 s, R2 at 0.5 s and R1 at 0.9 s for a fault at C. R3 clears the fault; R2 and R1 only reset. #### Protection zone A **zone of protection** is the region of the system (generator, transformer, bus, line, motor) guarded by a particular protection; its boundary is fixed by the CTs, and all breakers on that boundary trip for a fault inside it. - Every part of the system is covered by at least one zone. - Neighbouring zones overlap around a circuit breaker so there is no blind spot. ```text ┌─Gen─┐┌─Transf─┐┌─Bus─┐┌──Line──┐┌─Bus─┐ G ──[CB]─────[CB]────[CB]──────[CB]──── load overlaps at each CB ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is coordination of protective devices? A. Making all relays operate at the same instant B. Using one relay for the whole system C. Removing back-up relays D. Choosing settings so the device nearest the fault operates first and upstream ones are delayed back-up Answer: D. Coordination grades pick-up and time so selectivity is obtained. (Key point: Coordination) 2. What is the usual grading margin between successive relays? A. About 0.3-0.5 s B. About 3-5 s C. About 0.01 s D. Exactly zero Answer: A. The margin covers breaker opening time, relay overshoot and errors. (Key point: Grading margin) 3. On radial feeder A-B-C for a fault at C, which relay has the shortest time? A. R1 at the source (0.9 s) B. R2 (0.5 s) C. R3 at the far end (0.1 s) D. All have equal time Answer: C. The relay nearest the fault is set fastest; upstream ones are slower. (Key point: Example grading) 4. Why do neighbouring zones overlap around a breaker? A. To double the fault current B. To leave no blind spot C. To avoid using CTs D. To make relays slower Answer: B. Overlap ensures every part of the system is covered. 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