A polarity test confirms that line, neutral and earth are connected to the correct terminals throughout an installation: single-pole switches and protective devices are in the line conductor only, the centre contact of Edison screw lampholders is line, and socket-outlets have line on the right-hand pin viewed from the front. It is required by Chapter 64 of BS 7671:2018+A4:2026 for initial verification and is repeated during periodic inspection. Polarity is tested dead as part of the continuity sequence and confirmed live once the supply is on. On an EICR, reversed polarity at a socket is normally coded C2, and line on the accessible thread of an ES lampholder is usually recorded as C1.
- Polarity is checked at the origin, at every protective device and switch, at every ES lampholder and at every socket-outlet within the agreed sample.
- The dead test is done with the same line-to-cpc link used for R1+R2, so it costs almost no extra time.
- Live confirmation uses the loop tester or an approved voltage indicator; a plug-in socket tester is a supplement, not a substitute.
- Reversed line and neutral at a socket or accessory is a C2 under Electrical Safety First Best Practice Guide 4; a reversal that exposes a live part is C1.
- Record the result per circuit on the schedule of test results and describe any fault in the observations with the location.
Why does polarity matter?
Polarity is the assignment of line, neutral and protective conductors to the correct terminals at every point of an installation. BS 7671 requires that a single-pole switching or protective device is connected in the line conductor only. The reason is isolation: if a light switch, an MCB or a fuse is in the neutral, turning it off stops current flowing but leaves everything beyond the switch at 230 V to earth, and someone changing a lamp with the switch off is exposed to a live conductor.
The second reason is fault clearance. A line-to-earth fault downstream of a device in the neutral bypasses that device entirely and relies on the upstream one, which may not disconnect in the required time. Correct polarity is therefore part of making earth fault loop impedance and disconnection times mean anything.
What is checked during a polarity test?
Chapter 64 lists the points that must be verified. In practice the inspector is confirming four things.
| Item | What must be true | Why |
|---|---|---|
| Origin of the installation | Incoming line and neutral correct at the cut-out, meter tails and main switch | A reversed supply reverses every circuit at once |
| Single-pole devices | Every fuse, MCB, RCBO pole and single-pole switch is in the line conductor | A device in the neutral does not isolate |
| Edison screw lampholders | Centre contact connected to line, outer thread to neutral (except fully shrouded E14/E27 to BS EN 60238) | The thread is touchable when changing a lamp |
| Socket-outlets and accessories | Line, neutral and earth on the terminals marked L, N and E; on a BS 1363 socket, line is the right-hand pin viewed from the front | Appliances and their internal switches and fuses depend on it |
Bayonet cap lampholders have two identical contacts, but the switch feeding them still has to be in the line. Fused connection units, cooker switches and shaver units are checked so that the fuse is on the line side. A neon indicator that glows with the switch off is a common sign of a reversed feed.
How is polarity tested dead?
The dead polarity test is done during the continuity tests, using the same set-up as the R1+R2 method. With the circuit isolated and proved dead, the line conductor is disconnected from its device and linked to the cpc at the board.
- At each socket-outlet, measure between the line terminal and the earth terminal. A low reading, the same as your R1+R2, confirms the terminal marked L is the line conductor. Measure between neutral and earth: it should be open circuit (or a very high reading on a shared-neutral installation). A low reading on N and open circuit on L means the conductors are reversed.
- At each switch, with the switch open, measure between the incoming terminal and earth. The reading confirms the switch is in the line. Close the switch and confirm the outgoing terminal now reads the same.
- At each ES lampholder, with the switch closed, measure between the centre contact and earth. Low reading: correct. Then between the outer thread and earth: open circuit.
- At the board, before reconnecting, confirm each circuit's line is going back onto the device it came from and not onto a neighbour.
Done dead, this catches reversals at the accessory before anything is energised. It does not confirm the polarity of the supply itself, which is why a live check follows.
How is polarity confirmed live?
Once the installation is energised, polarity is confirmed with an approved voltage indicator to GS38 or with the loop test function of a multifunction tester. Most MFTs display a polarity warning or the L, N and E wiring status when plugged into a socket or connected with probes, and refuse to run the loop test if line and neutral are swapped. Measuring line to earth (about 230 V), line to neutral (about 230 V) and neutral to earth (close to 0 V) at the origin and at the far point of each circuit confirms the supply polarity and the circuit together.
A reversed supply at the cut-out is rare, but it happens on older overhead services and after meter changes, and it reverses every single-pole device in the property. It is a distributor matter, reported to the DNO immediately, and recorded on an EICR as C1 because the main switch and every MCB is now in the neutral.
A plug-in socket tester is fine for a quick check after energising a new socket, but it cannot detect a cpc replaced by another path, does not always distinguish line and earth reversed, and tells you nothing about switches and lampholders. Guidance Note 3 treats it as an indication, not a test.
What faults are found and how are they coded?
Polarity faults are common on DIY work, on extensions run from the wrong terminal of an existing socket and on older installations that have been altered many times. The codes below follow the approach in Electrical Safety First Best Practice Guide 4 and GN3; the inspector's judgement about the actual situation still applies.
| Fault found | Typical code | Reasoning |
|---|---|---|
| Line and neutral reversed at a socket-outlet | C2 | Appliance switches and fuses no longer isolate the line; potentially dangerous |
| Single-pole switch in the neutral of a lighting circuit | C2 | Lampholder remains live with the switch off |
| Line on the outer contact of an unshrouded ES lampholder | C1 | Live part accessible during normal lamp changing; danger present |
| MCB or fuse connected in the neutral | C2 (C1 if a live part is thereby exposed) | Circuit not isolated by its device; fault clearance compromised |
| Reversed polarity at the origin (supply) | C1 | Every single-pole device in the installation is in the neutral; report to DNO |
| Line and earth reversed at a socket | C1 | Exposed-conductive-parts of any connected Class I appliance become live |
| Unable to verify polarity at an inaccessible accessory | Note as a limitation, or FI | Cannot confirm; further investigation if there is reason to suspect a fault |
The C1 for line on the ES thread generates debate because the lamp is usually in place. The argument for C1 is that the lampholder is designed to be handled, the only shield is the lamp cap, and the hazard exists at every lamp change without any further fault. Most inspectors record C1 for a bare ES holder with reversed polarity and C2 where the holder is a shrouded E14 or E27 type. Whichever you choose, write the reasoning in the observation. Reversed polarity is never a C3: it is a non-compliance with a safety consequence, so C2 is the minimum and the report is Unsatisfactory.
How is polarity recorded?
On the schedule of test results there is a polarity column per circuit, completed with a tick or the word "correct" once every point in the sample on that circuit has been verified. Do not tick a circuit where only the socket nearest the board was checked; the extent of the check is governed by the agreed sample and any limitations recorded on the report.
Where a fault is found, the schedule entry for that circuit is left unticked or marked with the observation number, and the fault is written up in the observations 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 a landlord or the next electrician to find it. The guide on writing EICR observations has the pattern.
On an EIC or a Minor Works certificate, the polarity tick confirms that the new work is correct, not the rest of the installation. If you find an existing reversal during minor works, note it in the comments on the existing installation and tell the client in writing.
What are the common mistakes?
- Testing polarity only at the board. A neat board proves nothing about a socket where someone swapped the cores.
- Skipping switches. The test at the lampholder with the switch closed does not prove which conductor the switch is in unless you also test with it open.
- Trusting the socket tester's three green lights and skipping the loop test.
- Coding a reversal C3 because "it works". It works until someone relies on the switch for isolation.
- Not checking the supply. Confirm L-N, L-E and N-E voltages at the origin on every EICR.
How Certio helps
Certio produces the EICR, EIC, Minor Works certificate and PAT records on the BS 7671 model forms and turns them into branded PDFs. Photograph the consumer unit and the circuit schedule drafts itself; point the phone camera at a Megger MFT-X1 and the reading goes straight into the schedule of test results, or dictate it while your hands stay on the probes. Suggested observation wording with the regulation reference is there for you to check and confirm. The judgement stays yours; the typing goes. Starter is free for 7 days, then £5 a month.