Sequence of Tests: Dead then Live
Purpose
This is the order of tests Wilsons follows on a distribution board with its circuits: all the dead tests first, then the live tests. It is a working method. The authority for the order, the method and the acceptable results is the current BS 7671 and IET Guidance Note 3. If they differ from this page, follow them.
Before You Test
- Inspect first. Complete the visual inspection before any test.
- Isolate safely and prove dead. See Safe Isolation.
- Check the tester. In date, undamaged, correct function and range for the test. Record the serial number and calibration date.
- Null the test leads before continuity tests.
- Record results as you go on the Schedule of Test Results.
Dead Tests
The supply is isolated, locked off and proved dead.
1. Earthing conductor continuity
Disconnect the earthing conductor in the distribution board. Test between the main earth at the supply (the earth terminal or clamp at the intake) and the earthing conductor in the board.
2. Main bonding continuity
Disconnect the bonding conductors in the distribution board. Test from the earth bar in the board to the far end of each bonding conductor, at the gas pipe and at the water pipe.
3. R1+R2
Test every circuit.
- Single-phase circuits: link the line conductor to the earth in the board and measure between them at the furthest point.
- Lighting circuits: if you get no reading, a light may be off. Operate all the switches. This also proves polarity.
- Fused spurs and double-pole switches: operate them to prove polarity.
- Three-phase circuits (such as a motor circuit or a three-phase socket): test each phase to earth, one at a time. Operate the isolator to prove polarity.
Check each reading against the expected value for the cable. See Expected Readings.
4. Ring final circuits
5. Insulation resistance
Use an insulation resistance tester at 500 V d.c. on a 230/400 V circuit. The minimum acceptable reading is 1.0 MΩ (confirm against the current BS 7671). Where equipment is sensitive to the test voltage, such as dimmers, LED drivers or electronic controls, follow GN3 on disconnecting it or testing differently.
A method that works on a board with RCBOs:
- Test the ring final circuit first, with its RCBO slider off, between all conductors.
- Reconnect the circuit protective conductors. Leave the RCBO functional earth and neutral lead disconnected.
- Do the global test at the board, from the top of the main switch, with all sliders on except the ring circuit. Test L1-L2, L2-L3, L1-L3, L1-N, L2-N, L3-N, L1-E, L2-E, L3-E and N-E: 10 readings.
- Operate all the light switches and repeat L-N and L-E for the phase the lighting circuit is on.
- Reconnect the RCBO functional earth and neutral lead.
- Then test the cable from a DOL starter to its motor, between all conductors: L1-L2, L2-L3, L1-L3, L1-E, L2-E, L3-E and N-E: 7 readings.
Reconnect Before the Live Tests
Before any live test, check everything disconnected has gone back and is tight.
| Item | Reconnect |
|---|---|
| Earthing conductor | Before the R1+R2 tests |
| Main bonding conductors | Before the R1+R2 tests |
| Circuit protective conductors | Straight after the ring circuit insulation test |
| RCBO functional earth and neutral lead | After the global insulation test and the lighting repeat |
| Ring circuit RCBO slider | After the global insulation test |
| Main earth | After the Ze test |
Live Tests
6. Ze (external earth fault loop impedance)
Disconnect the main earth at the board. The isolator must still be locked off. Test between each phase and the main earth. The highest reading is Ze. Record it on the certificate and at the top of the Schedule of Test Results.
7. Prospective short-circuit current
Reconnect the main earth. The isolator must still be locked off. Test between each phase and neutral, one at a time. Keep the highest value. Record it with Ze.
8. Polarity and phase sequence
Use approved test lamps or a voltage indicator.
| Between | Expect |
|---|---|
| Each phase and phase | 400 V |
| Each phase and neutral | 230 V |
| Each phase and earth | 230 V |
| Neutral and earth | 0 V |
Neutral to earth at 0 V proves polarity. Then connect a phase sequence tester to all three phases and confirm the sequence.
9. Zs (earth fault loop impedance)
Zs does not need to be measured at every circuit. It can be calculated:
Zs = Ze + (R1 + R2)
Compare the result with the maximum permitted for the protective device in BS 7671. You can measure it at a socket with a plug-top lead if you want to, but it is not necessary.
10. RCD tests
- Operate the test button first (the functional test).
- Test at every socket on the ring circuit.
- Do not use the auto setting. Do each test separately.
- Test on both sides of the AC cycle: at 0° and at 180°.
- Test at ½ and ×1 the rated residual current. A 30 mA RCD must not trip at ½ and must trip within 300 ms at ×1 (confirm against the current BS 7671).
11. Functional testing
Confirm that every circuit operates correctly. Include the phase sequence check at the three-phase socket and at the motor, so the motor does not run backwards.
Common Mistakes
- Forgetting to reconnect something after a test.
- Not nulling the test leads.
- Starting live tests without the isolator locked off where this method requires it.
- Measuring Zs when it can be calculated.
- Using the auto setting on the RCD tester, or testing only one side of the cycle.
- Missing the phase sequence check at the three-phase socket and the motor.