Insulation resistance (IR) testing is one of the most fundamental electrical safety tests performed by Singapore's Licensed Electrical Workers and maintenance engineers. The Workplace Safety and Health Act requires that electrical installations be maintained in a safe condition — and IR testing is the primary method for detecting insulation degradation before it causes earth faults, fires, or electrocution. This guide takes you through the complete test procedure using a megohmmeter (insulation resistance tester).
Equipment Required
- Insulation resistance tester (megohmmeter) — Fluke 1508 (500/1000 V DC) for LV work; Fluke 1555 (to 5000 V DC) for MV work
- Calibration certificate for the tester (current, SINGLAS-traceable recommended)
- Test leads with CAT III-rated probes and insulated clips
- Discharge rod (for discharging capacitive circuits after testing)
- Digital multimeter (Fluke 175 or 87V) for voltage verification before testing
- Personal protective equipment (insulated gloves, safety glasses)
- Warning labels and barriers if working in a shared environment
Safety Precautions — Read Before Testing
- Verify the circuit is de-energised. Use your DMM to confirm zero volts at all terminals before connecting the insulation tester. Never perform an IR test on a live circuit.
- Discharge capacitive equipment. Motors, cables, and capacitors retain charge after de-energisation. Verify residual voltage < 10 V before connecting test leads. Some IR testers (Fluke 1555) include automatic discharge indication.
- Isolate and lock out/tag out. Follow your organisation's Lockout/Tagout procedure. Ensure no other person can re-energise the circuit during testing.
- Disconnect sensitive equipment. Remove or disconnect solid-state devices, electronic starters, and surge protection devices (SPDs/MOVs) that may be damaged by the IR test voltage. VSD input/output terminals must be disconnected.
- Check ambient conditions. High humidity (Singapore's tropical climate) reduces surface insulation resistance. Test in the morning before humidity peaks, or note humidity conditions for record comparison. Dirty, contaminated surfaces significantly reduce insulation resistance readings.
Step-by-Step Procedure: Motor Winding IR Test
This procedure covers insulation resistance testing of a three-phase induction motor winding — the most common IR test performed on Singapore industrial sites.
Step 1: Prepare the Motor
Disconnect all three phases and neutral at the motor terminal box. Open the motor terminal box cover. Verify zero volts on all terminals using your DMM. Check that the motor protection device (overload relay, soft starter) is disconnected or bypassed for testing. If the motor has a VSD, disconnect both the supply-side and motor-side cables from the VSD before testing — IR test voltage will damage VSD components.
Step 2: Select Test Voltage
For 415 V (LV) motors, use 1000 V DC. This is the Singapore industry standard following IECEx 60034-1 and SS 638 guidance. For larger motors (> 1000 V rated), use 2500 V DC.
Step 3: Connect the Tester
Connect the Line (L) terminal of the megohmmeter to the motor terminal (one phase). Short-circuit all three motor phases together using a jumper wire — this ensures you test all windings simultaneously. Connect the Earth (E) terminal of the megohmmeter to the motor frame/earth stud. If your tester has a Guard (G) terminal, connect it to the motor terminal box mounting point to eliminate surface leakage from the result.
Step 4: Perform the Spot Reading Test
Apply the test voltage and record the reading after 60 seconds (1-minute spot reading). Read and note the result in MΩ. After 60 seconds, continue to 10 minutes for the Polarisation Index (PI) test — record the 10-minute reading. Calculate PI = (10-minute reading) / (1-minute reading).
Step 5: Interpret Results
| Measurement | New Motor | In-Service: Acceptable | In-Service: Investigate | In-Service: Critical |
|---|---|---|---|---|
| IR at 1 minute (1000 V) | > 100 MΩ | > 10 MΩ | 1–10 MΩ | < 1 MΩ |
| Polarisation Index (PI) | > 4.0 | > 2.0 | 1.5–2.0 | < 1.5 |
Step 6: Discharge and Reconnect
After testing, remove the test leads. Apply the discharge rod across each terminal to earth for at least 4× the test duration (e.g., 4 minutes for a 1-minute test) or until the residual voltage falls below 10 V. Then reconnect the motor normally.
Step 7: Document Results
Record in the test log: date and time, motor tag number and location, ambient temperature and humidity, test instrument model and calibration certificate number, test voltage applied, 1-minute and 10-minute readings in MΩ, PI value, and the technician's name and signature. Compare with the previous test record — trending is as important as the absolute value.
Testing LV Cables
For LV cable IR testing, disconnect both ends of the cable from equipment. Test at 500 V DC for cables rated up to 500 V, or 1000 V DC for cables rated up to 1000 V. Connect L to the conductor and E to the cable screen/armour/adjacent cables shorted together. For multicore cables, test each core to all other cores shorted together to earth. Minimum acceptance: 1 MΩ (IEC 60364) — but new cables should read much higher (100s of MΩ to GΩ range).
Trending and Predictive Maintenance
A single IR reading tells you whether a motor passes or fails at that moment. Trending — recording IR values over time and comparing the trend — is far more valuable. A motor showing 500 MΩ today is healthy. The same motor at 50 MΩ six months later and 5 MΩ six months after that is telling you that failure is approaching — you have time to plan a rewind or replacement before an unplanned outage. Build a test schedule into your predictive maintenance programme and track trends in your maintenance management system.