Insulation Resistance Testing
Minimum IR values, test procedure, temperature correction, and material types
BS 7671 Reg 643.3, Table 64.1 & IET Guidance Note 3
What is Insulation Resistance Testing?
IR testing checks that cable insulation can safely contain the operating voltage. A DC test voltage is applied between conductors and between conductors and earth. The result is measured in megohms (MΩ) — higher is better. DC is used to eliminate capacitive reactance, giving a true resistive measurement.
Minimum IR Values (Table 64.1)
| Circuit Voltage | Test Voltage | Minimum IR | Note |
|---|---|---|---|
| SELV and PELV | 250V DC | ≥ 0.5 MΩ | Extra-low voltage circuits |
| Up to and including 500V (incl. FELV) | 500V DC | ≥ 1.0 MΩ | Most domestic/commercial — 230 V / 400 V |
| Above 500V up to 1000V | 1000V DC | ≥ 1.0 MΩ | Higher voltage systems |
2 MΩ Investigation Threshold
While 1.0 MΩ is the regulatory pass, the HSE and Guidance Note 3 recommend that any reading below 2.0 MΩ on an individual circuit warrants further investigation. For new installations, a reading near the minimum usually indicates cable damage or moisture ingress during construction.
Test Procedure
Isolate and Lock Off
Safely isolate the circuit. Follow safe isolation procedure.
Disconnect Sensitive Equipment
Remove RCDs, dimmers, PIR sensors, LED drivers, SPDs, and any electronics. The 500 V DC test can damage them.
Set All Switches to ON
Turn on all light switches, FCUs, and isolators so the test covers all wiring including switch wires.
Remove Lamps and Accessories
Remove lamps (especially LEDs/CFLs). Disconnect accessories that may give false low readings.
Select Correct Test Voltage
Set the IR tester to the correct DC voltage for the circuit (usually 500 V DC for 230 V circuits).
Test Between Conductors
Test L–N, L–E, and N–E. For three-phase: each phase to every other phase, each phase to N, and each phase to E.
Record & Discharge
Record the reading in MΩ. Keep leads connected after releasing the test button to safely discharge cable capacitance.
SPD & PME Considerations
SPDs activate on voltage transients — a 500 V DC test will trigger them, shunting current to earth and giving a false low reading.
Options:
- - Disconnect SPDs before testing
- - Connect L and N together, test against E (no potential across SPD)
- - Reduce test voltage to 250 V DC (min 1.0 MΩ still applies)
If the neutral is not isolated from the supply before testing, the tester measures the N–E bond at the source — giving a near-zero reading.
Solution: remove the neutral link at the origin or use a 4-pole isolation switch before testing N–E insulation resistance.
Interpreting Results
| Reading | Indication | Notes |
|---|---|---|
| > 200 MΩ | Excellent | New installation or very good condition |
| 50–200 MΩ | Good | Normal for an installation in good condition |
| 2–50 MΩ | Acceptable | Older installation — monitor on future inspections |
| 1–2 MΩ | Marginal | HSE/GN3 recommend investigation below 2 MΩ |
| < 1 MΩ | Fail | Below minimum — investigate and rectify before energising |
Temperature Correction to 20 °C
PVC insulation resistance halves for every 10 °C rise. Multiply the measured value by the factor below to normalise readings to 20 °C for trending.
| Temperature (°C) | Multiplier to Correct to 20 °C |
|---|---|
| 10 °C | 0.50 |
| 15 °C | 0.71 |
| 20 °C | 1.00 |
| 25 °C | 1.41 |
| 30 °C | 2.00 |
| 35 °C | 2.82 |
| 40 °C | 4.00 |
Insulation Material Types
| Material | Type | Max Temp | Failure Symptoms |
|---|---|---|---|
| PVC | Thermoplastic | 70 °C | Embrittlement, plasticiser migration, 'green goo' on copper |
| XLPE | Thermoset | 90 °C | Rarely fails unless mechanical damage — gigaohm-range readings |
| VRI (Rubber) | Thermoset | 60 °C | Cracking, powdering, moisture absorption — pre-1960s |
| EPR | Thermoset | 90 °C | Retains flexibility, ozone-resistant — industrial use |
Factors Affecting Insulation Resistance
| Factor | Effect |
|---|---|
| Moisture / damp | Dramatically lowers IR. Common after flooding or in unheated buildings. |
| Age of installation | Insulation degrades over time. Older wiring will have lower readings. |
| Cable length | IR is inversely proportional to length — runs >250 m naturally read lower. |
| Temperature | PVC IR halves for every 10 °C rise. Use the correction table below. |
| Humidity > 70% | Moisture film on terminations creates surface leakage paths — not bulk insulation failure. |
| Number of outlets | More accessories = more parallel leakage paths. Isolate suspect circuits. |
| Connected equipment | Electronics, heaters, motors give false low readings if not disconnected. |
EICR Classification for IR Observations
| Condition | Code |
|---|---|
| Insulation completely failed, exposed live parts | C1 |
| IR result below minimum on a circuit | C2 |
| IR result between minimum and 2 MΩ | C3 |
| Aged VRI passing tests but showing degradation | C3 |
| Result influenced by loads that cannot be disconnected | FI |
Important Warnings
- - Disconnect electronic equipment before testing — 500 V DC destroys RCDs, dimmers, LED drivers, PIRs
- - Never IR test a live circuit — follow safe isolation procedure first
- - Discharge capacitance — keep leads connected after releasing the test button; long cables store charge
- - Whole-board test: all circuits in parallel = total IR lower than any individual circuit. If below 1 MΩ, test circuits individually
- - Record exact values (not ">200") to allow future inspectors to identify deterioration trends
Key Points
- - Minimum applies to each individual circuit, not the whole installation
- - Test between all live conductors and earth: L–E, N–E, and L–N
- - IR is the second dead test — performed after continuity of protective conductors
- - FELV circuits are tested at 500 V DC (not 250 V) — they lack full separation from LV
- - For occupied premises (EICR), a global test at 250 V DC with L and N linked minimises risk to hidden electronics
- - VRI (pre-1960s rubber) may pass but crumbles when disturbed — treat as C3 minimum