Polarity Testing (BS 7671)
Polarity testing confirms every switch, fuse and protective device is in the line conductor and nothing is wired the wrong way round.
It is not enough for a circuit to work — the switching and protection have to be in the right conductor. If a switch or fuse ends up in the neutral, the light goes on and off quite happily while the fitting stays live to earth even when "off". Polarity testing is the check that catches exactly this kind of hidden danger.
This guide explains why polarity matters, how to carry out the dead polarity test by continuity and the live polarity check once energised, the specific faults each catches, and where polarity sits in the initial verification test sequence.
Key takeaways
- Polarity testing confirms single-pole switches and protective devices are in the line conductor, not the neutral.
- The dead test uses a continuity method to prove the line conductor connects through to switches, fuses and the centre of screw lampholders.
- The live test verifies polarity at the origin and confirms it hasn’t been disturbed once the installation is energised.
- It catches switches in the neutral, reversed line and neutral, and incorrectly wired Edison screw lampholders.
- Polarity is confirmed as part of the dead tests before energising and re-checked live afterwards.
Why polarity matters
A single-pole switch or protective device must break the line conductor. Put it in the neutral and the circuit still functions, but the fitting downstream remains connected to line potential even when switched off — so anyone changing a lamp or working on the accessory can get a shock from a circuit they believe is dead. That is the core hazard polarity testing exists to prevent.
The same applies to fuses and single-pole MCBs: they have to interrupt the line conductor to disconnect the fault current safely. A protective device in the neutral leaves the fault path live. Polarity is therefore not a cosmetic check — it is a safety-critical part of proving the installation.
The dead polarity test
Polarity is first confirmed dead, with the installation isolated, using a continuity method. In fact it is usually verified as a by-product of the continuity of protective conductors and ring continuity tests: as you prove continuity you also confirm that the line conductor — not the neutral — runs through to each single-pole switch, each fuse or breaker, and the centre contact of every Edison screw lampholder.
The technique is to link the line conductor to the CPC at the board and measure continuity at each point, confirming the switching and protection sit in the line side. Because it is done dead, there is no shock risk, and it lets you correct any reversed connection before the installation is ever energised.
The screw lampholder rule
For an Edison screw lampholder the line conductor must connect to the centre contact and the neutral to the outer screw thread — so the exposed thread is dead when a lamp is removed. Confirming this is a classic part of the dead polarity test.
The live polarity check
Once the dead tests pass and the installation is energised, a live polarity check confirms nothing has been disturbed and that the supply itself is the right way round. At the origin you verify the incoming line and neutral are correctly identified, so the whole installation is referenced correctly from the start.
You then spot-check accessories using an approved voltage indicator or test instrument to confirm line, neutral and earth are where they should be. The live check is a confirmation step — the substantive proof is done dead — but it catches anything introduced or missed once the supply is on.
What polarity testing catches
The headline fault is a single-pole switch or protective device wired into the neutral, which leaves the load live when switched off. Polarity testing also catches reversed line and neutral throughout a circuit or at an accessory, and incorrectly wired centre-contact lampholders where the thread rather than the centre is live.
It will also flag a borrowed or transposed neutral between circuits and, at the origin, a supply that has been connected line-neutral reversed. Each of these looks harmless because the equipment still works — which is exactly why a deliberate test rather than a functional check is needed.
Where polarity fits in the test sequence
In the initial verification sequence, polarity is confirmed during the dead tests, before the installation is energised — it goes hand in hand with continuity testing, since the same measurements prove both. Establishing correct polarity dead means you never energise a circuit with protection or switching in the wrong conductor.
After energising, polarity is confirmed again live at the origin and by spot checks, alongside the other live tests such as earth fault loop impedance and RCD operation. Recording both the dead and live confirmations on the schedule of test results completes the picture.
Capture it on the certificate
TradePlanr’s EICR app has polarity as a line on the schedule of test results, so you tick the dead and live confirmation for each circuit and it flows straight through onto the issued PDF.
Frequently asked questions
Is polarity tested dead or live?
Both. Polarity is first proven dead by a continuity method before energising, which confirms switches and protective devices are in the line conductor. It is then re-checked live at the origin and by spot checks once the installation is energised, to confirm nothing has been disturbed and the supply is correct.
What is the most common polarity fault?
A single-pole switch or protective device wired into the neutral rather than the line conductor. The circuit still works, but the fitting stays live to earth when switched off, exposing anyone changing a lamp or working on the accessory to a shock from a supposedly dead circuit.
How should an Edison screw lampholder be wired?
The line conductor connects to the centre contact and the neutral to the outer screw thread. That way the exposed thread is dead when the lamp is removed. Confirming this connection is a standard part of the dead polarity test.
From guidance to action
Related guides
Initial Verification: The BS 7671 Testing Sequence
The correct dead-then-live testing sequence for initial verification, step by step.
Safe Isolation Procedure
The step-by-step safe isolation procedure, proving dead with a GS38 tester, and locking off.
Insulation Resistance Testing
Test voltages, minimum acceptable values, and how to test insulation resistance safely.
