Select a suitable insulation test and understand what a high resistance reading cannot prove.
Insulation resistance assesses unwanted conductive paths between conductors. Polarity checks whether conductors and single-pole devices are connected in the intended places. A high insulation reading says nothing about polarity.
Decide what is connected first
For an ordinary 230 V circuit, the normal insulation-resistance test is 500 V DC with sensitive equipment and surge protective devices appropriately disconnected. Identify equipment that could be damaged or affect the result before applying the test.
Where disconnecting equipment such as a fixed SPD is not reasonably practicable, the applicable rules can permit a 250 V DC test with a minimum of 1 MΩ. That exception is not a reason to choose the lower voltage for every circuit. SELV and PELV have their own requirements.
Interpret the result in context
Confirm the test voltage, connections, equipment left connected and any limitations. If a result changes after equipment is separated, investigate whether you were measuring the installation or the connected equipment.
Decision check: a circuit has a high insulation-resistance reading but its single-pole switch is in the neutral conductor. The insulation result does not correct the polarity defect; the circuit still requires correction and verification.
Carrying out the insulation test
- Insulation resistance is a dead test on a safely isolated circuit. In the initial-verification sequence it follows continuity and precedes energised verification tests. Safe isolation and proving dead come before applying the insulation tester.
- Test between the live conductors, and from the live conductors to earth. Where appropriate, joining line and neutral avoids applying test voltage across them. The full test voltage still appears to earth, so equipment may still need disconnection or another permitted precaution. Neutral-earth faults can divert current into protective paths or operate an RCD; a line-neutral-only test does not check neutral-earth insulation.
- Test voltages and minimum values: SELV and PELV circuits, 250 V DC and 0.5 MΩ; other circuits up to 500 V such as 230/400 V, 500 V DC and 1.0 MΩ; circuits over 500 V up to 1,000 V, 1,000 V DC and 1.0 MΩ. Record results in megohms.
- Close ordinary switches where it is safe, so the wiring beyond them is included. Remove lamps, and disconnect or account for dimmers, electronic equipment and surge protective devices, which the test voltage can damage and which can lower the reading. Where an SPD cannot reasonably be disconnected, test at 250 V DC, still to a minimum of 1 MΩ, and record the alternative method or limitation.
Reading the result
- Sound new wiring normally reads far above 1 MΩ. Investigate a very low result such as 0.05 MΩ, checking the method and connected equipment before diagnosing deterioration or damage. If an unexpectedly low combined result, for example below 2 MΩ, is obtained, test the circuits individually. Many parallel paths can lower the total without proving a fault in one circuit. A rise after lamps or a dimmer are removed supports equipment affecting the earlier reading.
- Circuits tested together combine like parallel resistances, so the whole board reads lower than any single circuit.
Worked example. Two circuits reading 300 MΩ and 150 MΩ read together about 1 ÷ (1/300 + 1/150) = 100 MΩ.
Polarity
Polarity confirms that single-pole switches and protective devices are in the line conductor only, that the centre contact of an Edison-screw lampholder goes to line where that check is required (E14 and E27 types complying with BS EN 60238 are exempt), and that every socket-outlet has line, neutral and protective conductor on the right terminals. Check it dead during the test sequence, confirm it live where needed, and check it at the origin and at the points of use. If reversed polarity is found, isolate, correct the cause and retest. A single-pole switch in neutral can leave parts connected to line when the load is off. Correct polarity and ordinary switching do not replace safe isolation and proving dead before work.