Helpful video reference. The LEARN ELECTRICS video "TESTING LIGHTING CIRCUITS – AMENDMENT 2 – THE 3 PLATE METHOD" walks through the linking lead setup, R1+R2 measurements at each fitting, and the Zs readings you should expect. LEARN ELECTRICS publishes detailed technical tutorials on UK domestic electrical testing and inspection, with a companion website at learnelectrics.com.
1. Understand the 3-plate method
A lighting circuit is wired differently from a ring final circuit. In older installations, junction boxes sit above ceiling roses and the neutral is switched at the rose. In modern wiring, junction boxes are avoided and the switch return runs back to the rose or a central wiring point. Either way, you need to measure R1+R2, the resistance of the line conductor plus the circuit protective conductor (CPC), at each lighting point to confirm both conductors are continuous and correctly connected.
The 3-plate method solves a practical problem: you cannot easily measure R1+R2 at a lighting point without either disconnecting the neutral at every fitting or running a long test lead back to the consumer unit. The solution is a short linking lead (sometimes called a shorting link or bonding lead) that connects the line (L) and earth (E) terminals together at the consumer unit neutral bar and earth bar respectively. With that link in place, your MFT probes on L and E at any fitting in the circuit will read through both conductors, giving you R1+R2 in a single measurement.
The method is described in IET Guidance Note 3 (Inspection and Testing) and is referenced in BS 7671 as the preferred approach for lighting circuit continuity testing.
2. Isolate and verify dead
Switch off the MCB for the circuit you intend to test. If the consumer unit has RCBOs, operate the relevant RCBO. Write a warning notice ("TESTING IN PROGRESS: DO NOT SWITCH ON") and fix it to the MCB handle with tape or a mechanical locking device.
Go to the first and last lighting points on the circuit and verify dead at each using a two-pole voltage tester. Check between L and N, L and E, and N and E. All three should read 0V. If any reading is present on the line or neutral, investigate before opening anything.
Open each ceiling rose or light fitting lid now so it is ready for testing. Do not disconnect anything yet.
3. Test R1+R2 continuity at each point
At the consumer unit, locate the line terminal for the circuit you are testing and the earth bar. Connect your linking lead between these two points. The lead must be rated for the test voltage: a standard short piece of 1mm² or 1.5mm² twin-and-earth with insulated crocodile clips works well, but a proprietary shorting lead from the MFT manufacturer is more convenient.
Set your MFT to the R1+R2 (low-resistance ohms) function. Null the test leads by touching the probes together and pressing the null/zero button. This removes the lead resistance from your readings.
At the first ceiling rose, place one probe on the line terminal and one on the earth terminal. The reading is R1+R2 for that section of cable. Note it down against the circuit reference. Move to each fitting in turn and record the result. The highest reading across all fittings is the R1+R2 figure that goes on the Schedule of Test Results.
Expected values: a 10-metre run of 1mm² twin-and-earth has an R1+R2 of approximately 0.36Ω. Longer circuits or those wired in 0.75mm² flex within the fitting will read higher. A reading of OL (open circuit) means either the linking lead has come off or there is a break in either the line or earth conductor between the consumer unit and that point.
4. Check polarity at the switch
Polarity testing confirms that the switch switches the line conductor, not the neutral. On a one-way plate switch, the incoming cable brings line and neutral to the switch plate. The line goes to the common (COM) terminal; the neutral passes straight through or is not present at the switch if wiring is via the ceiling rose. The switched live (which carries current back to the fitting when the switch is on) leaves on the L1 or SW terminal.
To test polarity with the circuit isolated and the linking lead in place: use the MFT in R1+R2 mode. Place one probe on the switch terminal (L1) and one on earth. If polarity is correct, you measure continuity through the switch conductor loop back to the consumer unit link. If you read open circuit here but correct continuity at the ceiling rose above, the neutral and switch conductor may be transposed.
On circuits wired to the 2016 and later colour code (brown for line, blue for neutral, grey or brown sleeve for switched live), polarity errors are less common but still worth checking, especially after alterations. On pre-2004 red/black wiring, the red should be on the switch terminal at every plate.
5. Energise and measure earth fault loop impedance
Remove the linking lead from the consumer unit. Close all ceiling rose lids. Restore the circuit at the MCB.
Set the MFT to earth fault loop impedance (Zs) measurement. At the furthest lighting point from the consumer unit, test Zs between line and earth. On a modern MFT this is usually a 2-wire test: probes on L and E at the fitting terminals. Some instruments require the neutral to be present: check your instrument's manual.
Compare the measured Zs against the maximum permitted value for the protective device. For a 6A Type B MCB the maximum Zs is 7.67Ω; for a 10A Type B MCB it is 4.61Ω (BS 7671 Table 41.2, 80% of tabulated value applied in practice to allow for temperature). If the measured Zs exceeds the permitted value, the protective device cannot disconnect within the required time under fault conditions, and this is a C2 observation on an EICR.
Record the highest Zs reading across the circuit, which is normally at the furthest point. If you tested multiple points, note them all but report only the maximum.
6. Record results on the electrical certificate
The Schedule of Test Results for a lighting circuit should include: circuit description and reference number, protective device type (MCB Type B, C, D or fuse) and rating, cable size (cross-sectional area), R1+R2 (highest measured value), polarity (tick or cross), insulation resistance (if measured), Zs (highest measured value), and any observations.
For an EICR, observations are graded C1 (danger present, requires immediate action), C2 (potentially dangerous, requires urgent action), C3 (improvement recommended), or FI (further investigation required). A polarity failure is C2. An exceeded Zs is C2. A corroded earth terminal that still passes continuity but is borderline C3 or C2 depending on severity.
For initial verification on new or altered work, a Electrical Installation Certificate (EIC) is required rather than an EICR. The certificate confirms the installation complies with BS 7671 and is signed by the designer, installer and inspector (these can be the same person for straightforward domestic work).
When to call us
EICR inspections and initial verification certificates for new domestic work are the two most common reasons to need a lighting circuit tested. If you have bought a house and want to know whether the lighting circuits are safe, an EICR is the right starting point. If you have just had a new circuit installed and the installer has not provided a certificate, ask for one: it is a legal requirement for notifiable work.
Need an EICR or certificate for a lighting circuit?
Richard carries out EICR inspections and issues electrical installation certificates for domestic work across Sandwich and east Kent. Most single-circuit certificates are issued on the same day as the visit.
Contact Richard