---
title: "Protection requirements; protective system vs scheme"
subject: "Switchgear and Protection"
chapter: 1
key: "semester_6/switchgear_and_protection:protective-system-vs-scheme"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/protective-system-vs-scheme
site: BCT Engineering
---

# Protection requirements; protective system vs scheme

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

- Asked in: 2075 Bhadra, 6 marks
- Video: [Protective Scheme and Protective System - Introduction to Protective Relaying](https://www.youtube.com/watch?v=cezBPcKy4XI) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/protective-system-vs-scheme
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-1

## Question

What are the basic requirements of protection scheme? Differentiate between a protective system and protective scheme.

## Short answer

Basic requirements: selectivity, speed, sensitivity, reliability, stability, simplicity, economy, adequateness. A protective system performs one function using a single principle (e.g., differential), while a protective scheme is the coordinated arrangement for an element, comprising multiple systems plus breakers.

## Answer

#### Basic requirements of a protection scheme
1. **Selectivity:** only the faulty section is disconnected by the nearest breakers.
2. **Speed:** the fault is cleared quickly to limit damage and preserve stability.
3. **Sensitivity:** operates for the minimum fault current (high-resistance faults, minimum generation).
4. **Reliability:** operates whenever required (dependability) and never wrongly (security).
5. **Stability:** stays inactive for through-faults, load swings and transformer inrush.
6. **Simplicity and economy:** cost proportional to the importance of the equipment; easy to maintain.
7. **Adequateness:** covers all likely faults and abnormal conditions of the equipment.

#### Protective system vs protective scheme
A **protective system** is a complete arrangement of protection equipment and associated devices that performs one specified protective function, based on one protection principle. It includes the relay(s), CTs and VTs, wiring, trip circuit, auxiliary DC supply and, where used, the communication channel (by the IEC definition the circuit breaker itself is excluded). *Example:* the distance protection system of a line, or the differential protection system of a transformer. If any of its components fails, that protection fails.

A **protective scheme** is the coordinated arrangement of protection for one or more elements of the power system. It includes all the equipment needed to make the protection work (relays, CTs, VTs, circuit breakers, batteries) and **may comprise several protective systems**. *Example:* the protection scheme of a power transformer = differential system + restricted earth-fault system + Buchholz and temperature devices + back-up overcurrent/earth-fault system, all tripping the HV and LV breakers.

| Point | Protective system | Protective scheme |
|---|---|---|
| Meaning | Arrangement that performs one protection function | Coordinated arrangement protecting a whole element (or elements) |
| Based on | One protection principle (distance, differential, overcurrent) | Several principles combined (main + back-up) |
| Contents | Relay, CT/VT, wiring, trip circuit, DC supply, communication channel | Several protective systems plus circuit breakers and all supporting equipment |
| Scope | Narrower; a part of a scheme | Wider; may comprise several systems |
| Example | Distance relay + CTs + CVT + DC supply + trip circuit of a line | Complete line protection: main-1 and main-2 distance, back-up overcurrent, breaker-failure, auto-reclose |

```text
 Protective scheme of a transformer
 ┌───────────────────────────────────────┐
 │ Differential system (CTs + 87T relay) │
 │ REF system          (CTs + 64R relay) │
 │ Buchholz / temperature devices        │──► trip HV & LV CBs
 │ Back-up O/C & E/F   (CTs + 51/51N)    │
 │ Common DC supply and trip circuits    │
 └───────────────────────────────────────┘
   each row = one protective system
```

## Check yourself

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

1. Which best describes a protective system?
   A. Only the relay type and its connection
   B. The complete hardware chain: CT/VT, relay, DC supply, trip circuit, CB, wiring
   C. The principle of differential protection
   D. The settings logic of IDMT relays
   Answer: B. A system is the whole set of equipment performing the protective function. (Key point: Protective system)

2. Which is an example of a protective scheme?
   A. Merz-Price differential protection
   B. DC battery
   C. Trip coil
   D. Circuit breaker mechanism
   Answer: A. A scheme is the principle/method of protection, such as differential, distance or overcurrent. (Key point: Protective scheme)

3. A stuck breaker or dead battery is a failure of which?
   A. Protective scheme (wrong principle choice)
   B. Zone overlap
   C. Protective system (component failure)
   D. Setting logic only
   Answer: C. System failures arise from components; scheme failures from wrong principle or setting. (Key point: Failure cause)

4. Which describes the stability requirement?
   A. Stays inactive for through-faults, load swings and transformer inrush
   B. Operates for the minimum fault current
   C. Clears the fault in few cycles
   D. Cost proportional to equipment importance
   Answer: A. That is stability; the others are sensitivity, speed and economy. (Key point: Stability requirement)

## Related in Switchgear and Protection

- [Basic requirements of a protection scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Abasic-requirements-protection-scheme) — Switchgear and Protection, chapter 1, asked 3×
- [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
- [Unit protection scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aunit-protection-scheme) — Switchgear and Protection, chapter 7
- [Converting IDMT scheme to differential protection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-to-differential-transformer) — Switchgear and Protection, chapter 7
- [Knee point voltage; measuring vs protective CT](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aknee-point-measuring-protective-ct) — Switchgear and Protection, chapter 2

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- [Basic requirements of classification](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Abasic-requirements-classification) — Soil Mechanics, chapter 3, asked 2×
- [Basic requirements of linear programming](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Alinear-programming-requirements) — Project Engineering, chapter 3
- [Mutex requirements; token vs non-token](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Amutex-requirements-compare) — Distributed System, chapter 6