---
title: "Phase sequence and significance"
subject: "Basic Electrical Engineering"
chapter: 8
key: "semester_1/basic_electrical_engineering:phase-sequence-significance"
source: https://www.bctengineer.com/q/semester_1/basic_electrical_engineering/phase-sequence-significance
site: BCT Engineering
---

# Phase sequence and significance

Basic Electrical Engineering · Chapter 8: Three-Phase Circuit Analysis

- Asked in: 2072 Kartik, 4 marks
- Video: [Phase Sequence | Explained | TheElectricalGuy](https://www.youtube.com/watch?v=1a4kj3Nig0s) — Gaurav J - TheElectricalGuy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/basic_electrical_engineering/phase-sequence-significance
- Whole chapter: https://www.bctengineer.com/md/semester_1/basic_electrical_engineering/chapter-8

## Question

Define phase sequence and explain its significance in three phase system.

## Short answer

Phase sequence is the order (RYB or RBY) in which phase voltages reach their maxima. It determines motor rotation direction, is essential for safe parallel connection of generators, and affects wattmeter readings in unbalanced systems. Protection relays also rely on sequence detection.

## Answer

**Phase sequence** is the order in which the three phase voltages of a three-phase supply reach their positive maximum values. If the order is R, then Y, then B, it is the RYB (positive) sequence. If it is R, B, Y, it is the RBY (negative) sequence.

```text
  Positive RYB:  V_R at 0°, V_Y at −120°, V_B at +120°
  Phasors rotate anticlockwise: R first, then Y, then B.
```

#### Significance
1. **Direction of rotation of motors:** A three-phase induction or synchronous motor rotates in the direction of the rotating magnetic field, which depends on the sequence. Interchanging any two lines reverses the rotation.
2. **Parallel operation:** Alternators and transformers can be connected in parallel only if they have the same phase sequence (along with equal voltage, frequency and phase).
3. **Unbalanced loads and measurement:** Line currents and wattmeter readings in unbalanced loads depend on the sequence.
4. **Protection and safety:** Phase-sequence relays and indicators prevent reverse rotation of pumps, compressors and lifts.
5. **Analysis:** The sequence is needed for correct phasor diagrams and for symmetrical component analysis.

## Check yourself

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

1. What is phase sequence?
   A. The order in which phase voltages reach their positive maximum
   B. The number of phases
   C. The angle between voltage and current
   D. The order of the fuses
   Answer: A. It is the order R, Y, B (or R, B, Y) in which voltages reach their positive peak. (Key point: Definition)

2. What happens to a three-phase motor if two supply lines are interchanged?
   A. It stops permanently
   B. It runs at double speed
   C. Its rotation reverses
   D. Nothing changes
   Answer: C. Interchanging two lines reverses the sequence and the rotating field. (Key point: Motors)

3. Which condition on phase sequence applies when connecting alternators in parallel?
   A. They must have opposite sequence
   B. They must have the same sequence
   C. Sequence does not matter
   D. Sequence must be RBY
   Answer: B. Parallel machines need equal voltage, frequency, phase and the same phase sequence. (Key point: Parallel operation)

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