A one-way lighting circuit is easy to test. Add a second switch and three-core and twin-and-earth between the two switches, and the method changes in two places. The first is where you expect the highest R1 + R2 reading. The second is that a single insulation resistance test no longer covers every conductor. This walkthrough uses a two-way lighting circuit on a training rig: one lighting point and two switches.
Where these tests sit in the sequence
Both tests here are dead tests. They are done in the order given in Regulation 643.1, which is continuity of protective conductors (643.2.1), then insulation resistance (643.3), then polarity (643.6). Live tests such as Zs and RCD operation come later. Continuity comes first so that you know the CPC is intact before you rely on it during insulation resistance testing and, later, for fault protection.
Polarity can be proved during the R1 + R2 test using the same link, and that is the approach used here. The test sequence post explains the full order and the reasoning behind it.
Before any covers come off, the circuit must be safely isolated and proved dead. That means a GS38-compliant voltage indicator, proved on a known source before and after the test. The rig in the video was supplied from an isolated board, but on site you cannot skip this step.
Setting up for R1 + R2
The method is the same as for a one-way circuit:
- Isolate the circuit and fit a temporary link between the circuit’s line conductor and its CPC at the board.
- Set the MFT to the low-resistance ohms range.
- Null the test leads so their resistance is not included in the readings.
- At every point on the circuit, measure between the line terminal and the earth terminal.
The reading at each point is the line conductor resistance (R1) plus the CPC resistance (R2) as far as that point.
Why the lighting point reads highest
Here is the part that catches people out. The ceiling rose may be physically closer to the board than either switch, but it is still electrically the furthest point. Follow the line conductor:
- from the board to the loop terminal at the ceiling rose
- down the switch drop to the first switch
- through a strapper in the three-core to the second switch
- back from the common terminal to the switch wire at the rose, and into the lamp
“Even though it looks as if the lighting point is closer to the consumer unit, the amount of cable through the switching… means the lighting point is the furthest point on this radial circuit.”
So you would expect the highest reading at the lighting point. But expectation is not the rule. Record the highest reading wherever you measure it. On the rig the readings were:
| Test point | Reading |
|---|---|
| Lighting point (switch wire to CPC), after settling | 0.21 Ω |
| First switch | 0.19 Ω |
| Second switch | 0.22 Ω |
The second switch gave the highest reading, so 0.22 Ω is the value that goes on the schedule. The differences are small and come from switch contacts that are rarely used and lead placement. That is exactly why you record what you measure rather than what the logic predicts.
Proving polarity through both strappers
At the lighting point, connect to the earth terminal and to the switch wire, not the loop terminal. This makes the switches part of the loop. Then operate them in turn:
- Reading present: the circuit is made through one strapper.
- Operate the first switch: open circuit. The meter shows full-scale deflection, and the line is broken.
- Operate the second switch: a reading returns, now through the other strapper.
- First switch again: open circuit.
- Second switch again: the reading returns.
Working through that sequence proves that both switches open and close the line conductor. Regulation 643.6 requires that single-pole switching devices are connected in the line conductor only. It also proves that each strapper has continuity, because the circuit has been made through each of them in turn.
At the switches themselves you do not need to operate anything. There you are only proving CPC continuity, by probing the earth terminal and one of the switch terminals. A common mistake is to operate the switches at every point. That does no harm, but it shows you have mixed up the continuity test and the polarity test.
Leave every switch in the on position when you finish. The insulation resistance test needs them on.
Recording the result
On the Schedule of Test Results:
- R1 + R2 gets the highest reading, here 0.22 Ω
- Polarity gets a tick
That R1 + R2 value is useful later. Added to Ze, it gives the expected Zs for the circuit, which is a check on your live measurement. The earth fault loop path post explains how it works.
Preparing for insulation resistance
Before the test:
- remove the temporary link
- refit all accessory covers
- remove all loads (on the rig, no lamp was fitted)
- leave the switches on so the switch wires are included in the test
- disconnect or isolate equipment that could be damaged by 500 V DC or could affect the reading
On the rig, the RCCB was switched off because it contains electronic components. Whatever method you use, check that the circuit’s own conductors are still part of the test. Test from the outgoing side of the circuit’s protective device, or with the device closed, so the circuit wiring is included and the vulnerable equipment is not.
The earthing conductor stays connected for insulation resistance testing. You only disconnect it from the main earthing terminal when measuring Ze.
The first insulation resistance test
Set the instrument to the megohm range at 500 V DC. Table 64 gives this test voltage, with a minimum of 1 MΩ, for circuits rated up to and including 500 V. Test:
- line to earth
- neutral to earth
- line to neutral
The order of the three tests does not matter. Every reading on the rig was greater than 999 MΩ, and that is how to record it. Write “>999 MΩ”, not a blank. A blank cell reads as “not tested”.
Why the first test is not enough
“We’ve completed the insulation resistance test exactly the same as we did when we did a one-way lighting circuit; however, it is not complete at this stage.”
Think about which conductors were actually energised by the tester. With the switches in their first position, the line conductor goes down to the first switch, along one strapper (say the brown), and back up through the common to the lamp. The other strapper (grey) is not connected to anything at either end. It is floating, so it was not tested at all.
The fix:
- Go to each two-way pair in the installation and change the switches to the opposite on position. The lamp should still be lit, but now through the other strapper.
- Return to the board.
- Repeat line–earth, neutral–earth and line–neutral.
On the rig, all three readings were again greater than 999 MΩ. Only now has every conductor in the circuit been tested. The same reasoning applies to intermediate switching. Choose switch positions so that every strapper is connected to the line conductor at least once, and test in each of those positions.
Common errors
| Error | Consequence |
|---|---|
| Recording the lighting point reading because “it should be highest” | Understated R1 + R2 when a switch actually read higher |
| Connecting to the loop terminal instead of the switch wire for polarity | The switches are not in the loop, so polarity is not proved |
| Leaving switches off for insulation resistance | Switch wires are left out of the test |
| One insulation resistance test only on two-way lighting | One strapper is never tested |
| Leaving a blank for a >999 MΩ reading | The result reads as “not tested” |
In the exam
The 2391-52 is open book, with 60 questions in 120 minutes and a 75% pass mark. You will not have time to read Part 6 from start to finish during the exam. Know where to find Regulation 643.1 for the order of tests, 643.2.1 for continuity, 643.3 and Table 64 for insulation resistance test voltages and minimum values, and 643.6 for polarity. Tab them before the exam. Questions on two-way strappers and on where to expect the highest R1 + R2 reading test your reasoning, not lookup speed, so practise them until the answer comes quickly.
How 2391 Prep Fits Into This
2391 Inspection & Testing Pro has 556 practice questions across 7 subject areas. The test procedures questions cover this material: the order of tests, linking line to CPC, proving polarity, and Table 64 test voltages and minimum values. The 15 quick reference cards put the insulation resistance values and the dead-test order on one screen. The 4 working calculators help you take a measured R1 + R2 forward into an expected Zs check, and the 6 field tools support the same work on site.
