7.2 Polarity Verification

7.2 Polarity Verification

Quick Answer: Polarity verification confirms that every single-pole protective device, switch, and fuse is connected in the phase (line) conductor only, that Edison-screw (ES) lampholders have the centre contact connected to phase, and that socket outlets are correctly wired. Polarity is checked dead (during continuity, test #4) and confirmed live after energisation.

Purpose

Polarity verification is dead test #4 (and is also confirmed live after energisation). It confirms that:

• Every single-pole switch, fuse, and circuit breaker is connected in the phase (line) conductor only — not the neutral
• Edison-screw (ES) lampholders have the centre contact connected to phase and the outer screw thread to neutral
• Socket outlets have the phase conductor on the correct terminal

Why It Matters

A single-pole switch in the neutral conductor leaves the load energised (live) when the switch is "off" — lethal for anyone undertaking maintenance who believes the circuit is isolated. An ES lampholder with the outer screw thread on phase presents a shock hazard when changing a lamp, since the exposed thread is touchable while the lamp is being inserted or removed. Correct polarity is therefore a fundamental safety requirement.

Dead Test Method (Continuity-Based)

With the supply isolated:

1. Link phase and CPC at the distribution board (as for the R1+R2 continuity test).
2. At each accessory, test between the switched phase terminal and the CPC:
   • With the switch ON — a low reading confirms continuity through the phase conductor and switch.
   • With the switch OFF — a high reading confirms the switch breaks the phase conductor. If the reading stays low with the switch off, the switch is in the wrong conductor (neutral) or is bypassed.
3. For socket outlets, test between the phase pin and the CPC terminal at the socket — a low reading confirms the phase conductor is on the correct terminal (consistent with the continuity method).

This method simultaneously verifies continuity of the phase conductor and confirms that single-pole switching devices are in the phase conductor.

Live Test Method (After Energisation)

After the installation is energised, use an approved voltage indicator to confirm polarity at each socket/outlet. The instrument must be proved on a known live source before and after each test — the prove-test-prove sequence required by GS38 and the HSE guidance on electrical test equipment:

1. Prove the indicator on the proving unit or a known live supply — confirm all indicators function.
2. Test at the accessory — confirm L–N, L–E, and N–E readings.
3. Prove again on the known live source — confirm the indicator still functions.

This three-point sequence guards against a failed indicator producing a false "dead" reading. The expected readings at a correctly wired single-phase socket are:

• L–N ≈ 230 V (single-phase nominal)
• L–E ≈ 230 V
• N–E ≈ 0 V (no significant voltage between neutral and earth)

These readings confirm phase is on the correct terminal. If L–E reads ~0 V and N–E reads ~230 V, the phase and neutral have been transposed. A reading of 0 V across all three combinations suggests a lost supply or a faulty indicator — re-prove the instrument before concluding the circuit is dead.

Why Both Dead and Live Tests?

The dead continuity-based test confirms the wiring topology before energisation — that the switch is in the phase conductor and the socket terminals are correctly allocated. The live test confirms the same outcome under real conditions after the supply is connected, and catches any error introduced during final connection at the board or at the accessory. Both are required because either test alone can miss a fault: a dead test cannot reveal a phase-neutral transpose introduced at the final connection, and a live test on its own does not confirm that the switch breaks the phase (it only confirms terminal voltages).

Edison-Screw (ES) Lampholders

For ES lampholders, the centre (eye) contact must be connected to the phase conductor and the outer screw thread to neutral. Polarity testing (dead or live) confirms this arrangement. BC (bayonet cap) lampholders do not have a polarised contact and do not require polarity confirmation at the holder, though the switch must still be in the phase conductor.

Dead vs Live Polarity — Summary Table

Recording

Polarity is recorded on the schedule of inspections (confirmed as satisfactory) and the schedule of test results. Both the dead continuity-based check and the live confirmation contribute to the overall polarity verification.

Worked Example

A radial socket circuit is tested dead with phase and CPC linked at the board. At the first socket, the tester measures between the phase pin and CPC and reads 0.35 Ω — low, confirming the phase conductor is continuous and on the correct terminal. At the switch controlling an outside light, with the switch ON the reading is 0.4 Ω and with the switch OFF the reading is >999 Ω (open circuit), confirming the switch breaks the phase conductor. After energisation, the tester confirms L–N = 230 V, L–E = 230 V, N–E = 0 V at the socket. Polarity is satisfactory and recorded.