A polarity test confirms that line, neutral and earth land on the right terminals throughout an installation. Regulation 643.6 of BS 7671:2018+A4:2026 asks for three things: every fuse and single-pole control or protective device in the line conductor only; centre-contact bayonet and Edison screw lampholders with the outer or screwed contact on neutral, except E14 and E27 holders to BS EN 60238; and wiring correctly connected throughout, socket-outlets included. It is done dead with the R1+R2 link, then confirmed live once the supply is on. On an EICR, Electrical Safety First Best Practice Guide 4 Issue 7.4 (August 2026) codes incorrect polarity at the origin C1 and at a final circuit, equipment or accessory C2.
- Regulation 643.6 is the test; Regulations 132.14.1 and 530.3.3 are the design rules behind it (single-pole devices in the line only, never a switching device in the neutral alone), and the A4:2026 schedule of inspections checks them at items 4.14 and 5.21.
- The dead test costs almost nothing extra: it uses the same line-to-cpc link as the R1+R2 continuity test and is the last dead test in the Regulation 643.1 sequence before energising.
- Live confirmation uses a GS38 two-pole voltage indicator or the loop function of the instrument; a plug-in socket tester is an indication, not a test.
- A reversed PME supply can read correctly at the main switch because the installation earth follows the line. Check against a true earth.
- BPG4 Issue 7.4: origin C1, final circuit or accessory C2. Separate protective devices in line and neutral (double-pole fusing) is also C2.
- On the A4:2026 model EICR, polarity is column 26 of the schedule of test results, with a "Confirmed: correct polarity" entry in each distribution board header.
Why does polarity matter?
Polarity is the assignment of line, neutral and protective conductor to the correct terminal at every point. BS 7671 fixes the rule at design stage: Regulation 132.14.1 requires single-pole fuses, switches and circuit-breakers to be in the line conductor only, and Regulation 530.3.3 says a switching device shall not be placed in the neutral conductor alone. The reason is isolation. A light switch, MCB or fuse in the neutral stops the current but leaves everything beyond it at 230 V to earth, so someone changing a lamp with the switch off is holding a live conductor.
The second reason is fault clearance. A line-to-earth fault downstream of a device sitting in the neutral bypasses that device and relies on whatever is upstream, which may not disconnect within the Table 41.1 time. Correct polarity is the precondition for earth fault loop impedance and disconnection times to mean anything, which is why the 643.1 sequence puts polarity before any live loop test.
What does Regulation 643.6 require you to verify?
Chapter 64 is specific. Where relevant, the polarity of the supply at the origin is verified before the installation is energised, and then the test confirms the points in the table.
| Point | What must be true | Regulation behind it | Why |
|---|---|---|---|
| Origin | Incoming line and neutral correct at the cut-out, tails and main switch | 643.6 | A reversed supply reverses every circuit at once |
| Fuses, single-pole MCBs and RCBO poles, single-pole switches, dimmers, thermostats, pull cords | In the line conductor only | 643.6(i); 132.14.1; 530.3.3 | A device in the neutral does not isolate |
| Centre-contact bayonet and Edison screw lampholders on circuits with an earthed neutral | Centre contact line, outer or screwed contact neutral | 643.6(ii); 559.5.1.206 | The thread or outer contact is touchable at a lamp change |
| E14 and E27 lampholders to BS EN 60238 | Excepted from the lampholder check | 643.6(ii); 559.5.1.206 | Compliant holders have both contacts recessed and an insulated thread |
| Socket-outlets, FCUs, cooker and shaver units, isolators | Line, neutral and earth on the terminals marked L, N and E; on BS 1363 line is the right-hand pin viewed from the front, neutral left, earth top | 643.6(iii) | Appliance switches and fuses depend on it; Class I appliances depend on the cpc |
Ordinary bayonet cap lampholders have two identical contacts, so the holder itself has no polarity, but the switch feeding it still has to be in the line. Fused connection units and cooker switches are checked so the fuse and the switch are on the line side. A neon that glows with the switch off is a classic sign of a reversed feed.
The exception for E14 and E27 holders is worth getting right, because it changes what you report. Regulation 559.5.1.206 requires the outer contact of Edison screw and single centre bayonet holders on TN and TT circuits to be on neutral, except holders to BS EN 60238 in those two sizes. A modern compliant E27 batten holder has two contacts at the base and a plastic thread, so reversing it exposes nothing. An older brass-thread holder, or any other ES size, is still in scope. Check the holder before you code it.
Where does polarity sit in the test sequence?
Regulation 643.1 sets the order of the dead tests for initial verification: continuity of protective conductors and ring final circuits (643.2), insulation resistance (643.3), SELV, PELV or electrical separation (643.4), insulation of floors and walls (643.5), then polarity (643.6), with the earth electrode test of a TT system (643.7.2) also done before energising. Where a test fails, that test and any earlier one it may have affected are repeated after the fault is fixed. After energising come the live tests: earth fault loop impedance (643.7), RCD operation (643.8), phase sequence on polyphase circuits (643.9), functional testing (643.10) and voltage drop where required (643.11).
On periodic inspection there is no fixed sequence in the same sense, but Regulation 651.2 lists polarity among the Chapter 64 tests that supplement the inspection, and the practical rule is the same: establish polarity before you rely on any live reading whose meaning depends on it.
How is polarity tested dead?
The dead test is done during continuity, using the R1+R2 set-up. With the circuit isolated, locked off and proved dead, disconnect the line conductor from its device and link it to the cpc at the board. Null the leads first.
At each socket-outlet, measure between the L terminal and the E terminal. A low reading, the same as your R1+R2, confirms that the terminal marked L is the line conductor. Measure between N and E: it should be open circuit, or a very high reading if a shared neutral is involved. A low reading on N and open circuit on L means line and neutral are reversed. A low reading on N and on E together, with L open, means line and earth are reversed, which is the dangerous one for Class I appliances.
At each switch, with the switch open, measure between the incoming terminal and earth. A low reading confirms the switch is in the line. Close the switch and confirm the outgoing terminal now reads the same. Testing only with the switch closed proves nothing about which conductor the switch is in.
At each in-scope ES lampholder, with the switch closed, measure centre contact to earth: low reading. Then thread to earth: open circuit. A wander lead from the board does the same job on long runs.
At the board, before reconnecting, confirm each line conductor goes back on the device it came from and not on a neighbour. Two lines swapped at the board is a polarity fault on two circuits at once, and it also makes the R1+R2 you just recorded belong to the wrong circuit number.
Done dead, this catches reversals at the accessory before anything is energised. It does not confirm the polarity of the supply, which is why a live check follows.
How is polarity confirmed live?
Once the installation is energised, confirm with a two-pole voltage indicator to HSE GS38, proved before and after, or with the loop function of the instrument. Three readings at the origin and at the far point of each circuit do the job: line to neutral about 230 V, line to earth about 230 V, neutral to earth close to 0 V. Most loop testers display the socket wiring when connected through the socket adaptor and will not run the test with line and neutral swapped, so a loop reading at the far socket of a radial is a polarity confirmation as well as a Zs. On a three-phase board the Regulation 643.9 phase sequence check is separate: every circuit can have correct polarity and the sequence can still be reversed, because transposing two lines breaks no polarity rule. The three-phase testing basics guide covers it.
A reversed supply is rare, but it happens on older overhead services and after meter or cut-out work, and it puts the main switch and every single-pole device in the property in the neutral. Do not energise the installation; report it to the DNO and record it. On an EICR it is a C1.
A plug-in socket tester is fine for a quick look after energising a new socket. It cannot see a cpc that has been replaced by another path, does not always distinguish line and earth reversed, and tells you nothing about switches, lampholders or the board. IET Guidance Note 3 treats it as an indication, not a test.
Why can a reversed PME supply look correct at the main switch?
This one catches people out. On a TN-C-S supply the earthing conductor is taken from the combined neutral-earth (PEN) at the cut-out. If line and PEN are transposed upstream of the service head, the installation's earth follows the line: L-N reads 230 V, L-E reads near 0 V and N-E reads 230 V, or, if you have already mentally labelled the conductors by colour, everything can look normal because what you are calling earth is at neutral potential and what you are calling neutral is at line. Inside the equipotential zone all the bonded metalwork is at the same potential, so nothing complains.
The reversal shows against a true earth. Measure neutral to an independent reference: a temporary rod, a bonded water pipe coming in from the ground, or the edge of the zone, where an outdoor tap, lamp post or EV charger earth gives you the real thing. A reading of about 230 V from neutral to true earth, with line to true earth near zero, is the giveaway. This is the scenario discussed in the IET EngX thread "Reverse polarity", and the suggested description is a line-PEN transposition rather than simply "L and N reversed". Note that on a PME supply the socket-outlet cpc is at neutral potential anyway, so a socket tester will not help. The earthing arrangements guide explains why TN-C-S behaves like this.
What faults are found and how are they coded?
Polarity faults are common on DIY work, on spurs taken from the wrong terminal of an existing socket, on older installations altered many times, and at the board after a careless swap of two lines. The codes below follow Electrical Safety First Best Practice Guide 4 Issue 7.4 (August 2026) and IET GN3 10th Edition; the inspector's judgement about the actual situation still applies, and the reasoning goes in the observation.
| Fault found | Code | Basis |
|---|---|---|
| Incorrect polarity at the origin (supply) | C1 | BPG4 7.4 lists it as C1: every single-pole device is now in the neutral; report to the DNO |
| Line and neutral reversed at a socket-outlet or accessory | C2 | BPG4 7.4: incorrect polarity at final circuit, equipment or accessory |
| Single-pole switch in the neutral of a lighting circuit | C2 | Same BPG4 entry; lampholder stays live with the switch off |
| MCB, RCBO or fuse connected in the neutral | C2 | Same entry; circuit not isolated by its device, fault clearance compromised. Raise to C1 if a live part is thereby exposed |
| Separate protective devices in line and neutral (double-pole fusing) | C2 | BPG4 7.4 entry; a blown neutral fuse leaves the circuit live and dead at once |
| Line on the metal thread of an unshrouded ES lampholder | C2, C1 where the thread is touchable in normal use | BPG4 lists the accessory case as C2; many inspectors raise a bare brass-thread holder to C1 because the hazard exists at every lamp change without any further fault. Write the reasoning |
| Reversed polarity on an E14 or E27 holder to BS EN 60238 | Not a polarity non-compliance | Excepted by 643.6(ii) and 559.5.1.206; check the holder rather than assume |
| Line and earth reversed at a socket-outlet | C1 | Exposed-conductive-parts of any connected Class I appliance become live; danger present |
| Unable to verify polarity at an inaccessible accessory | Limitation, or FI where there is reason to suspect a fault | Record the extent agreed; FI is advisory on the A4 model report |
Reversed polarity at a final circuit is never a C3. It is a non-compliance with a direct safety consequence, so C2 is the floor and the report is Unsatisfactory. "It works" is not a defence: it works until someone relies on the switch for isolation. The observation codes guide has the general approach to C1, C2, C3 and FI.
How is polarity recorded on the A4:2026 forms?
The IET model forms for BS 7671:2018+A4:2026, dated 15 April 2026, record polarity in three places. On the schedule of test results, column 26 is "Polarity", filled per circuit once every point in the sample on that circuit has been verified. Each distribution board header carries a "Confirmed: correct polarity" entry for the board itself. On the schedule of inspections, item 4.14 in the distribution board section and item 5.21 in the final circuits section both read "single-pole switching or protective devices in line conductor only", referenced to 132.14.1 and 530.3.3.
The form's footnote to column 26 says that an X denoting incorrect polarity cannot be entered when the schedule is issued with an Electrical Installation Certificate: new work has to be right before the certificate is signed. On an EICR the circuit is marked and the fault written up in Section K with the circuit reference, the location, what is reversed and the code. "Circuit 7, double socket-outlet in bedroom 2 rear wall: line and neutral reversed. C2." is enough for the landlord and the next electrician to find it. The writing observations guide has the pattern, and the schedule of test results guide covers the rest of the columns.
Do not tick a circuit where only the socket nearest the board was checked. The extent is governed by the agreed sample and the limitations recorded on the report; see the sampling and limitations guide. On a Minor Works Certificate the polarity entry in Section D confirms the new work only. An existing reversal found during minor works goes in the comments on the existing installation and in writing to the client.
What are the common mistakes?
Testing polarity only at the board. A neat board proves nothing about a socket where someone swapped the cores, and a tidy label does not prove the line went back on the right breaker.
Skipping switches. A reading at the lampholder with the switch closed does not prove which conductor the switch is in. Test open, then closed.
Coding every reversed ES holder without looking at it. An E14 or E27 to BS EN 60238 is excepted; an old brass-thread holder is not.
Trusting three green lights on a socket tester and skipping the loop test. The tester cannot tell you the cpc is real.
Checking supply polarity only at the main switch on a PME supply. Take a reading against a true earth if anything about the service head or the meter work looks recent or unusual.
Confusing a borrowed neutral with reversed polarity. A low N-E reading on a lighting circuit that is otherwise correct often means a shared neutral; the borrowed neutrals guide covers how to tell them apart.
How Certio helps
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