11.1 Check of Phase Sequence

What Phase Sequence Is

In a three-phase supply the three line voltages are identical in magnitude but displaced by 120 electrical degrees. Phase sequence is the order in which they reach their positive peak. The UK convention is L1 → L2 → L3, and BS 7671 requires that this order is maintained throughout the installation.

Regulation 643.9 puts it directly: for polyphase circuits, it shall be verified that the phase sequence is maintained at all relevant points throughout the installation. Note the two things the regulation does not say — it does not name an instrument, and it does not limit the check to the origin. Both omissions are exam material.

Conductor identification and the colour trap

Amendment 2 to BS 7671:2001, published 31 March 2004, introduced the harmonised cable core colours. They could be used from 1 April 2004 and became mandatory for new work on 1 April 2006, with mixing of old and new colours in the same alteration prohibited during the transition.

Read that table across and the hazard is obvious: black is a line conductor in the current scheme and a neutral in the old one, and blue is a neutral now but was L3 before. A mixed-vintage installation — a 1980s distribution board fed by a new submain, or vice versa — is the classic place for both a sequence error and a dangerous misidentification. Regulation 514.3 requires that where both colour schemes exist, the change is marked with durable warning notices at the interface.

Sequence, rotation, transposition and polarity

Four terms get confused, and the exam exploits it:

Polarity confirms that a particular conductor is where it should be. Phase sequence confirms that three correctly identified line conductors are in the right order. An installation can have perfect polarity on every circuit and still have a reversed sequence at a submain, because swapping two lines with each other breaks no polarity rule at all.

How the Check Is Made — and the Dead/Live Trap

Because Regulation 643.9 states a requirement without prescribing a method, candidates routinely assume it means "plug in a phase rotation meter". The IET's own guidance is more precise. IET Wiring Matters issue 105 (May 2025), in its article on minimising unnecessary live testing for initial verification, states that checking phase sequence "is carried out by visual inspection by checking that the line conductors are connected to the appropriate terminals throughout the installation", and that "phase sequence is a dead test and is often confused with phase rotation which is a live test".

So there are two distinct activities, and the 2391-52 examiner expects you to keep them apart:

In the Regulation 643 running order the phase sequence check is listed after the live tests, and long-standing practice is to confirm the result with an indicator once the installation is energised. That practice is not wrong — but the underlying verification is the dead one, and an inspector who only reads an indicator at the origin has not verified that the sequence is maintained at all relevant points.

Instruments

Where an indicator is used it is either a rotating-disc type — a miniature induction motor whose disc turns clockwise for a correct sequence — or an electronic type with L1/L2/L3 indication and a direction arrow. Many multifunction testers include the function. The leads must comply with GS38 and the instrument must carry a CAT rating appropriate to the point of connection; at the origin of a supply that normally means CAT IV.

Procedure:

1. Establish identification dead first. During the dead tests, verify by inspection and continuity that each line conductor is correctly identified and terminated end to end. This is what makes any later live reading meaningful — an indicator will happily report the sequence of three conductors that are labelled wrongly.
2. Work outwards from the origin, confirming at each stage that L1, L2 and L3 remain on the same terminals through every joint, isolator, busbar chamber and distribution board.
3. Where an indicator is used, energise, then connect it to L1, L2 and L3 (and neutral where the instrument requires it) observing the correct connection order.
4. Read the indication. A correct sequence shows L1-L2-L3, or a clockwise rotation on a disc instrument.
5. Repeat at every relevant point — each distribution board, each polyphase submain, and the incoming terminals of any polyphase equipment such as a motor starter or a lift controller.
6. Record the result on the schedule of test results.

Where the sequence is wrong, the cause is a transposition somewhere upstream. Trace it back board by board rather than swapping conductors at the point where you noticed it — otherwise you correct one board and reverse everything else fed from the same source.

Why It Matters

A reversed phase sequence does not usually cause an electrical fault. It causes a mechanical one.

A three-phase induction motor turns in the direction set by the rotating magnetic field, and that direction is set by the sequence. Swap any two lines and the motor runs backwards. The consequences are not trivial:

• Pumps run in reverse and deliver no flow — which can mean no cooling water, no fire-main pressure, or a dry-running seal that destroys itself.
• Compressors driven backwards can lose lubrication and seize.
• Conveyors, hoists and lifts move the wrong way, which is a direct injury risk.
• Machine tools and fans run backwards, damaging cutters or delivering no air.
• Equipment moved between distribution boards on the same site may work in one location and fail in another.

For an inspector this is one of the few checks whose failure is invisible on most instruments and obvious the moment the plant is started. That is exactly why it is verified before the installation is handed over.

Be careful with the reasoning in the other direction. BS 7671 requires the sequence to be verified; a requirement for verification is not in itself a design and erection requirement, so the significance of a departure depends on what is connected. A transposed submain feeding only single-phase final circuits and static loads has a different practical consequence from one feeding a hoist. BS 7671 publishes no table of observation codes, so the classification is the inspector's judgement, informed by GN3 and the Electrical Safety First Best Practice Guides — justified by the presence of polyphase equipment and the consequence of reverse rotation, not applied by rote.

Recording and Periodic Inspection

At periodic inspection the check is repeated at the origin and at boards where alterations may have occurred, because a transposition is a common by-product of board changes, submain replacement and equipment relocation. It is one of the quickest checks on the schedule and one of the easiest to omit, so the examiner likes it as a discriminator: a blank entry against a check that does apply is a defective report, and a tick against a check that does not apply is a false record.