Phase sequence: L1, L2, L3
Three Phase Systems · Lesson 3 · Intermediate
Purpose
Explain phase sequence, its measurement, and why swapping any two line conductors reverses the sequence.
Learning objectives
- Define phase sequence
- Distinguish labels from sequence
- Predict reversal
- Plan a safe verification
Phase sequence is the order in which the phase quantities pass the same reference point. Equipment rotation and converter operation can depend on it.
Core theory
A positive sequence may be designated L1-L2-L3 under the adopted convention; labels alone do not prove the actual order at a termination.
Interchanging any two line conductors reverses the sequence. Cyclic relabelling such as L2-L3-L1 describes the same rotational order, not a reversal.
Sequence must be checked with a suitable rated indicator at the required point under a controlled live-testing procedure, then documented. A correct sequence does not prove polarity, earthing, or motor direction after mechanical coupling.
| Term | Meaning | Symbol | Unit |
|---|---|---|---|
| Phase sequence | Order in which phase waveforms reach an equivalent reference point | Not applicable | Not applicable |
| Phase rotation | Common practical description of phase sequence | Not applicable | Not applicable |
| Cyclic order | Equivalent rotations such as L1-L2-L3 and L2-L3-L1 | Not applicable | Not applicable |
Assumptions: No other conductor changes.
- Original: Start with the verified positive order.
- Swap: Interchange one pair: L2 and L3.
- Result: The downstream order reverses.
The phase sequence is reversed.
Reasonableness check: Any single pair swap changes the orientation of the three-phase set.
- Inferring sequence from insulation colour
- Calling a cyclic shift a reversal
- Energising a coupled motor to guess direction
Where this appears in practice
Sequence verification matters for motors, pumps, compressors, converters, synchronising, and parallel supplies.
Knowledge check
Does swapping all labels cyclically from L1-L2-L3 to L2-L3-L1 reverse rotation?
No. It is the same cyclic order with a different starting label.
Answer: No. It is the same cyclic order with a different starting label.
Practical exercise
Classify six written phase orders as the same or reversed sequence without energising equipment.
Summary
- Sequence is an order, not a colour
- One pair swap reverses it
- Verification is controlled live testing
Sources and review
- IEC 60038: IEC standard voltages: IEC; 2009+A1:2021; International
- Broken PEN: IET Wiring Matters; Issue 84, 2021; United Kingdom
- Minimizing unnecessary live testing for initial verification: IET Wiring Matters; Issue 105, 2025; United Kingdom
Editorial review date: 2026-08-22. Professional electrical review is pending.