---
title: "Overlapping zones and qualities of relaying"
subject: "Switchgear and Protection"
chapter: 1
key: "semester_6/switchgear_and_protection:overlapping-zones-relay-qualities"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/overlapping-zones-relay-qualities
site: BCT Engineering
---

# Overlapping zones and qualities of relaying

Switchgear and Protection · Chapter 1: Principle of Power System Protection

- Asked in: 2079 Chaitra, 6 marks
- Video: [What is zone of protection? || What are internal & external faults?||Why adjacent zones overlapped?](https://www.youtube.com/watch?v=r1fOVOHqakU) — ELECTRICAL PERSPECTIVE (ELECTRICAL PERSPECTIVE)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/overlapping-zones-relay-qualities
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-1

## Question

Why the protective zones are arranged in overlap fashion? Discuss in brief the various essential qualities of Protective Relaying.

## Short answer

Zones overlap to eliminate blind spots; CTs on opposite sides of a breaker create the overlap, covering the breaker itself. Essential protective relaying qualities are selectivity, speed, sensitivity, reliability, stability, simplicity, and economy.

## Answer

#### Why protective zones are overlapped
A zone boundary is fixed by the CT location. If the zones did not overlap, for example if the bus relay took its current from a CT on the bus side of a breaker and the line relay from a CT on the line side, the breaker and the short connection between the two CTs would belong to no zone. A fault there would be a **blind spot** and could be cleared only by slow back-up protection.

Therefore the CT of each zone is placed on the far side of the common breaker, so the zones **overlap** across the breaker:
```text
       Zone 1 (bus)
   ◄──────────────────►
 ──●──CT2──[ CB ]──CT1──●── line
         ◄──────────────────►
             Zone 2 (line)
 CT1 feeds bus relay, CT2 feeds line relay
```
- A fault inside the overlap trips the breakers of both zones; this is acceptable because the overlap is very small.
- Overlap ensures that every point of the system, including the breakers, is protected.
- With dead-tank breakers and bushing CTs, overlap is easily obtained.

#### Essential qualities of protective relaying
1. **Selectivity:** trips only the breakers needed to isolate the faulty element.
2. **Speed:** clears the fault quickly (a few cycles) to reduce damage and maintain system stability.
3. **Sensitivity:** responds to the minimum fault current, e.g. high-resistance earth faults.
4. **Reliability:** dependable (always trips when required) and secure (never trips when not required).
5. **Stability:** remains inoperative for through-faults, power swings and transformer inrush.
6. **Simplicity:** least number of relays and contacts; easy maintenance.
7. **Economy:** cost of protection proportional to the importance of the equipment.

These qualities often conflict, e.g. higher speed and sensitivity may reduce stability; good relaying balances them for each application.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. If CTs of two neighbouring zones were on the same side of a breaker, what would result?
   A. Faster fault clearing
   B. A blind spot between them cleared only by slow back-up
   C. Both zones tripping for every fault
   D. Smaller CT burden
   Answer: B. The region including the breaker would belong to no zone. (Key point: Blind spot)

2. How are zones made to overlap?
   A. CTs of each zone are placed on the far side of the common breaker
   B. CTs are removed
   C. Both CTs are placed on the line side only
   D. One CT serves the whole system
   Answer: A. Placing CTs on opposite sides of the breaker makes the zones overlap across it. (Key point: CT placement)

3. Reliability of a relay includes which two aspects?
   A. Fast and cheap
   B. Sensitive and simple
   C. Dependable (trips when required) and secure (does not trip when not required)
   D. Selective and economical
   Answer: C. These are the two aspects of reliability. (Key point: Reliability)

4. Which statement about relay qualities is true?
   A. All qualities improve together
   B. Economy always increases stability
   C. Sensitivity makes relays more stable
   D. Higher speed and sensitivity may reduce stability
   Answer: D. Qualities conflict, so a balance is needed. (Key point: Conflicting qualities)

## Related in other subjects

- [Importance and types of protective devices](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_and_electronics_engineering%3Aprotective-devices-importance) — Basic Electrical and Electronics Engineering, chapter 4