Polarity Testing Guide: Electrical Polarity Test Methods

1. Assemble the test circuit. Connect a low-voltage battery (typically 4.5 V or 9 V) in series with a buzzer, bell or indicator lamp. The two free ends are the test probes. Bridge a continuous conductor with them and the indicator sounds or illuminates.
2. Isolate the installation. The test is dead. Switch off, isolate, secure the isolation, and prove dead with an approved voltage indicator.
3. Separate the conductors at the board. Disconnect the outgoing circuit conductors from the protective devices and the neutral bar so each conductor can be identified individually.
4. Test at each accessory. Put one probe on the conductor at the board and the other on the terminal at the socket-outlet, switch or luminaire. Continuity confirms the conductor identity; no continuity where you expected it means a transposition somewhere between the two points.
5. Mark conductors as they are proved. Tag each confirmed conductor at the board so you never test the same one twice.

Why low voltage

The test voltage is far below mains, so it can be applied to a de-energised circuit without shock risk, will not damage connected electronics, and can be used with accessories left in place.

• Null the lead resistance. Short the leads together and null or record the reading before testing. On short runs the leads can be a large share of the total.
• Wander lead technique. Run a long wander lead from the board to the accessory. One instrument terminal stays at the board on the known line conductor; the other is taken to the accessory and tested against each terminal in turn.
• Prove each terminal. At the accessory, continuity back to the line conductor at the board confirms the line connection. Repeat for neutral and CPC so all three are positively identified, not assumed by colour.

• The switched conductor. With the switch on, measure between the switch wire and earth: it should sit at nominal line voltage (230 V to earth on a single-phase supply). With the switch off it should be at or near zero. Voltage still present with the switch off means the switch is in the neutral.
• ES lampholder. With the lamp out and the switch off, measure from the centre contact to earth and from the outer shell to earth. Both should be at or near zero. A live centre contact with the switch off means the switch is in the neutral.
• Socket-outlet. A plug-in socket tester gives a fast indication at every outlet; confirm anything doubtful with a voltage indicator. Line should be the right-hand slot on a BS 1363 outlet, neutral the left-hand, earth the top.

• Transposed at a socket-outlet. Brown into the neutral terminal, blue into the line terminal. The outlet works normally, so it is only found by test. Typical where an outlet has been replaced or added by someone unqualified.
• Switch in the neutral. Common in loop-in ceiling rose wiring, where the permanent line, the switch wire and the neutral all meet in one accessory, and where a switch has been added to an existing circuit without proving which conductor is the line.
• Transposed at the consumer unit. Line conductors landed on the neutral bar and neutrals feeding the protective devices. This is a whole-board reversal affecting every circuit, and it happens when a board is changed and conductors are re-terminated without identification.
• Common and strapper transposed in two-way switching. Swapping the two strappers alone only inverts which switch position is on. The genuine polarity fault is putting the feed or the switch wire on a strapper terminal instead of the common — the lighting may still appear to work from one switch, but the switch is no longer interrupting the line conductor as Regulation 132.14.1 requires.
• Spur or junction added with line and neutral crossed. A joint made without identifying the conductors reverses everything downstream of it while everything upstream tests correct — which is why a reversal at one accessory always means testing the next one along.

1. Confirm the reversal is real. Re-test with a second instrument or a different method. A socket tester plus an independent voltage indicator is a quick cross-check against instrument or lead error.
2. Check the preceding junction. Work back to the last joint box, ceiling rose or accessory on the circuit and test there. Correct at the junction but reversed at the accessory puts the fault in the final connection.
3. Check the board. If polarity is already reversed at the first accessible point on the circuit, the terminations at the consumer unit are the next place to look.
4. Look before you test. A brown conductor in a terminal marked N, or a conductor with no identification sleeve where one is needed, is often visible before an instrument is picked up.
5. Record the location and the remedy. Note where the fault was and what was done. On an EICR, a polarity reversal left in service is an observation with a classification code and a description of its location.

• Per circuit. Confirm polarity for each circuit tested, alongside confirmation of supply polarity at the origin.
• Limitations. Record any circuit or accessory where polarity could not be fully verified — a permanently connected appliance you could not gain access to, for example — and state why.
• Observations. Any polarity fault found and not put right during the visit is recorded as an observation with a classification code, a description and its location, so the person ordering the report can act on it.

Cross-check with a socket tester

A plug-in socket tester speeds up the check at every outlet on a large installation, but it indicates function rather than proving the conductor path, and it cannot distinguish every combination of fault. Use it alongside the continuity method, not instead of it.