--- title: "Zones of protection and their rules" subject: "Switchgear and Protection" chapter: 1 key: "semester_6/switchgear_and_protection:zones-of-protection" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/zones-of-protection site: BCT Engineering --- # Zones of protection and their rules Switchgear and Protection · Chapter 1: Principle of Power System Protection - Asked in: 2071 Bhadra, 4 marks; 2072 Asoj, 4 marks (asked 2 times) - Video: [Lecture: 7: Zones of protection & Primary backup protection # SKP Lectures](https://www.youtube.com/watch?v=GysYsH69SXk) — SKP Lectures - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/zones-of-protection - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-1 ## Question What do you understand by a zone of protection? Draw the protection zone diagram for a sample (modern) power system network and explain its rules. ## Short answer A zone of protection is the region guarded by a protection scheme, bounded by CTs. Adjacent zones overlap across circuit breakers to avoid blind spots. Rules: every element must lie in at least one zone, the overlap contains a breaker with CTs on opposite sides, and a fault in the overlap trips both zones. ## Answer A **zone of protection** is the part of the power system (generator, transformer, bus, line or motor) protected by a particular protection scheme. A fault anywhere inside the zone makes all the circuit breakers on its boundary trip, so that the zone is fully isolated. The boundary of a zone is defined by the location of the CTs and the circuit breakers. #### Protection zones of a sample power system ```text ┌──────────────┐ │ G Gen │ 1 Generator zone └──┬───────────┘ ┌──┴─[CB]──────┐ │ Gen-transf. │ 2 Transformer zone └──┬───────────┘ ┌──┴─[CB]──────┐ │ HV bus │ 3 Bus zone └──┬───────────┘ ┌──┴─[CB]──────┐ │ Line │ 4 Transmission line zone └──┬───────────┘ ┌──┴─[CB]──────┐ │ Rx bus │ 3 Bus zone └──┬───────────┘ ┌──┴─[CB]──────┐ │ Step-down Tr│ 2 Transformer zone └──┬───────────┘ ┌──┴─[CB]──────┐ │ Feeders/Mtr │ 5 Feeder / motor zone └──────────────┘ Each box is a zone; adjacent boxes overlap around the circuit breaker. ``` #### Rules for drawing zones 1. Each zone is drawn around one main element: generator, transformer, bus, line or motor. 2. Every element of the system must lie in at least one zone; there should be no blind (unprotected) spot. 3. Adjacent zones **overlap**, and the overlap region contains a circuit breaker. This is done by placing the CTs of the two zones on opposite sides of the breaker. 4. A fault inside the overlap region trips the breakers of both zones; the overlap is kept as small as possible so that only a small extra part is disconnected. 5. Zone boundaries are set by CT location; a zone includes the breakers that must trip to isolate it. Using zones makes the scheme **selective**: a fault on the line trips only the two line breakers, while generator, transformers and buses stay in service. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What fixes the boundary of a protection zone? A. The length of the relay setting B. The rating of the DC battery C. Location of the CTs (and the circuit breakers) D. Position of the line insulators Answer: C. CT placement defines the zone boundary and the breakers that trip. (Key point: Zone definition) 2. Why do adjacent zones overlap around a circuit breaker? A. So that there is no blind (unprotected) spot B. To make two relays trip for every fault C. To reduce the number of CTs D. To increase the fault current Answer: A. Overlap by placing CTs on opposite sides of the breaker removes blind spots. (Key point: Overlap) 3. What happens for a fault in the overlap region? A. Only one zone's breakers trip B. Breakers of both zones trip, so the overlap is kept small C. No breaker trips D. Only back-up protection trips Answer: B. Both zones see it; small overlap limits the extra disconnected part. (Key point: Fault in overlap) 4. Which is a rule for drawing zones? A. A zone must contain several different elements B. Zones must never overlap C. Buses need not be covered by any zone D. Every system element must lie in at least one zone Answer: D. No element may be left without protection. (Key point: Zone rules) ## Related in Switchgear and Protection - [Requirements, coordination and protection zone](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Arequirements-coordination-zone) — Switchgear and Protection, chapter 1 - [Differential protection of power transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Atransformer-differential-protection) — Switchgear and Protection, chapter 7 - [Protection requirements; protective system vs scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aprotective-system-vs-scheme) — 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 - [Ring bus rearrangement and zones](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aring-bus-comparison) — Switchgear and Protection, chapter 1 - [Overlapping zones and qualities of relaying](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aoverlapping-zones-relay-qualities) — Switchgear and Protection, chapter 1 ## Related in other subjects - [Typical power system network and components](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Atypical-power-system-network-components) — Power System Analysis I, chapter 1 - [Power electronics in modern power systems](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Apower-electronics-modern-power-systems) — High Voltage Engineering, chapter 1 - [LTI system and closed loop in modern systems](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Alti-define-cl-pronounced) — Control Systems, chapter 1 - [Three phase system and transition zone](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Athree-phase-transition-zone) — Concrete Technology and Masonry Structure, chapter 2, asked 8×