How to test circuit polarity on a UK socket circuit

Helpful video reference. GSH Electrical Training's video "Radial Socket Circuit - Polarity and Continuity Tests" is a clear UK training demonstration showing the complete test sequence for a radial socket circuit. It covers the dead continuity of the CPC, polarity using the R1+R2 link method, and recording results on the Schedule of Test Results. The terminology and test sequence follow BS 7671 and the On Site Guide throughout.

1. What is polarity and why does it matter?

In UK wiring, the line (live) conductor is brown, the neutral is blue, and the circuit protective conductor (CPC, or earth) is green/yellow. When a socket is wired correctly:

• The brown conductor connects to the line (L) terminal, which sits behind the right-hand slot on a UK socket when viewed from the front.
• The blue conductor connects to the neutral (N) terminal, behind the left-hand slot.
• The green/yellow conductor connects to the earth (E) terminal, behind the top slot.

If live and neutral are transposed anywhere in the circuit — at the consumer unit, at a junction box, or at the socket face — the result is reversed polarity. The socket works for most appliances because alternating current flows in both directions anyway, but the switch inside a standard lamp or a single-pole switched appliance will then interrupt the neutral conductor rather than the live. That means the element or lamp holder stays live even when switched off. Touch the wrong part and you can receive a shock.

Under BS 7671, reversed polarity at a socket is a C2 (potentially dangerous) defect on an EICR. Reversed polarity at a consumer unit origin, where the meter tails or supply is connected, is a C1 (danger present) defect.

2. The homeowner approach: use a plug-in socket tester

For a quick check after replacing a socket or adding a spur, a plug-in BS 1363 socket tester is the practical tool. They cost a few pounds, require no electrical knowledge to use, and are available from any electrical wholesaler or hardware shop.

Simply plug the tester into the socket and read the LED pattern from the key printed on the tester body. Different models vary slightly, but most will indicate:

• Correct wiring (all three conductors present, polarity correct)
• Live and neutral transposed (reversed polarity)
• Earth missing
• Live and earth transposed
• Neutral missing

Test every socket on the circuit after any wiring work, not just the one you touched. A fault at a junction box further back in the circuit will show up at all sockets downstream of that junction.

3. The electrician approach: dead polarity test using continuity

This is the formal test described in BS 7671 and carried out by an electrician when certifying new or altered work.

With the circuit isolated at the consumer unit and verified dead:

1. At the consumer unit, use a flying lead or shorting link to connect the line terminal of the circuit under test directly to the main earth bar. This creates a return path through the line conductor so the ohmmeter can read through the circuit.
2. At each socket on the circuit, measure resistance between the L terminal and the E terminal. A reading of around 0.2 to 1.5 ohms (depending on cable length) confirms the line conductor reaches the L terminal — polarity is correct at that point.
3. If the resistance is very high (infinite) between L and E but low between N and E, the live and neutral conductors have been transposed somewhere between the consumer unit and that socket.

This test is called R1+R2 because it measures the resistance of the outgoing line conductor plus the return earth conductor in one reading. The On Site Guide tabulates maximum acceptable values by cable size and circuit length.

4. The live check with a voltage indicator

Once the dead tests are complete and the shorting link has been removed, restore power. Using an approved voltage indicator (one that meets HSE Guidance Note GS38), check:

• 230 V between L and N at the consumer unit origin, confirming the incoming supply is the right polarity.
• 230 V between L and N at the consumer unit output terminals for the circuit under test.
• If accessible and safe, 230 V between L and N at the furthest socket on the circuit.

Do not rely solely on the live test. A faulty neutral connection can give misleading voltage readings. The dead continuity test and the live test together confirm polarity correctly.

5. Reversed polarity at the consumer unit origin

If the incoming supply from the Distribution Network Operator (DNO) arrives with reversed polarity — brown meter tail connected to the neutral bar and blue to the line bar — every circuit in the house will be reversed. This is a C1 (immediate danger) defect on an EICR. You cannot fix this yourself. The fault is in the DNO's service head or meter equipment. Your electrician will note it on the certificate and advise you to contact your DNO or energy supplier to have it corrected.

6. What to do if you find reversed polarity

At a socket: isolate the circuit, open the socket, swap the brown and blue conductors to the correct terminals, refit and retest. If the fault persists at multiple sockets, the reversal is upstream at a junction box or at the consumer unit itself.

At the consumer unit (on an installed circuit, not the supply): isolate and check the connections at the MCB and neutral bar. Brown conductors must go to the MCB, blue conductors to the neutral bar.

In old wiring with red (live) and black (neutral) conductors: a reversal is still a reversal. Red to the MCB, black to the neutral bar.

When to call us

A plug-in socket tester is within any homeowner's capability. Opening a consumer unit and making connections at the busbar is not — that job stays with a qualified electrician. If you have had electrical work done and you are not sure the polarity was checked, a basic socket tester check costs nothing. An EICR from Richard will check polarity formally across every circuit and produce a signed certificate.