Why Is Motor Insulation Resistance Low? Causes and Testing Guide Malaysia
Meta Description: Discover why motor insulation resistance becomes low and how Malaysian factories can investigate moisture, contamination, overheating and testing errors.
A low motor insulation resistance reading may indicate that unwanted current can pass through or across the insulation system.
Possible causes include moisture, contamination, overheating, ageing, physical damage or an incorrect test procedure.
One low result does not automatically mean the motor must be replaced. Maintenance teams should verify the measurement and investigate the motor’s condition, temperature, environment and electrical connections.
Common Causes of Low Insulation Resistance
The most common causes include:
Moisture or condensation
Dust, oil or chemical contamination
Motor overheating
Insulation ageing
Damaged winding insulation
Damaged motor cable
Long-term storage
Incorrect test voltage
Connected electronic equipment
Testing at a higher motor temperature
1. Moisture and Condensation
Moisture is a common cause of reduced insulation resistance.
It may enter a motor because of:
High factory humidity
Outdoor installation
Water leakage
Washdown activity
Failed seals
Condensation during shutdown
Rapid temperature changes
Long storage
Inspect:
Terminal box
Cable entries
Drain holes
Motor housing
Junction box
Nearby water sources
A temperature-humidity data logger can help determine whether the motor is exposed to prolonged high humidity or condensation conditions.
2. Dust, Oil and Chemical Contamination
Contamination can create a conductive path across insulation surfaces.
Possible contaminants include:
Metal dust
Carbon dust
Oil
Grease
Salt
Chemical residue
Process powder
Cleaning solution
Contamination may be concentrated inside the terminal box or around winding surfaces.
Cleaning must follow the motor manufacturer’s procedure and approved electrical-safety practices.
3. Motor Overheating
Repeated overheating accelerates insulation deterioration.
Possible causes include:
Motor overloading
Blocked ventilation
Bearing friction
Voltage imbalance
Current imbalance
Frequent starting
High ambient temperature
Incorrect VFD settings
Use:
Thermal imaging camera
Infrared thermometer
Clamp meter
Vibration meter
Temperature data logger
A low insulation reading may be the result of a longer-term overheating problem rather than a sudden fault.
4. Insulation Ageing
Motor insulation gradually ages because of:
Thermal cycling
Electrical stress
Vibration
Moisture
Contamination
Operating time
Trend records are useful because a gradual decline may be visible before complete failure.
5. Physical Damage
Insulation may be damaged by:
Cable abrasion
Sharp edges
Loose internal components
Excessive vibration
Incorrect maintenance
Rodents
Mechanical impact
Winding movement
Inspect accessible cables and terminals after proper isolation.
Internal winding damage may require further specialist assessment.
6. Damaged Motor Cable
A low reading may come from the cable rather than the motor.
Where the approved test method permits, test separately:
Motor
Supply cable
Starter circuit
Connected equipment
Separating the sections helps locate the insulation problem.
7. Long-Term Storage
A motor stored for months may absorb moisture, especially in humid environments.
Before energisation, check:
Storage conditions
Motor temperature
Visible condensation
Terminal-box condition
Insulation resistance
Bearing condition
Follow the manufacturer’s procedure before drying or energising the motor.
8. Incorrect Test Voltage
Using the wrong insulation-test voltage can:
Produce an inappropriate result
Stress sensitive equipment
Damage connected electronics
Create a safety hazard
The correct test voltage depends on:
Motor rated voltage
Motor type
Manufacturer instructions
Connected components
Approved test procedure
Do not select the voltage by guesswork.
9. Connected Variable-Frequency Drive
A variable-frequency drive and other electronic components should not normally be exposed to insulation-test voltage unless the manufacturer specifically permits it.
Connected equipment may include:
VFD
Soft starter
PLC input
Surge-protection device
Electronic sensor
Control module
Disconnect or isolate sensitive components according to the approved procedure before testing.
10. Motor Temperature
Insulation resistance changes with temperature.
A warm motor may show a lower resistance than the same motor when cold.
Record:
Motor temperature
Ambient temperature
Test voltage
Test duration
Humidity
Measurement connection
Do not compare a hot-motor reading directly with a cold-motor result without accounting for temperature.
Insulation Tester vs Multimeter
A multimeter uses a low test voltage for general resistance checks.
An insulation resistance tester applies a much higher controlled DC voltage.
InstrumentMain PurposeInsulation resistance testerEvaluate high-resistance insulationDigital multimeterGeneral voltage, resistance and continuityClamp meterMeasure operating currentThermal imaging cameraLocate abnormal temperature patterns
A multimeter continuity test cannot replace an insulation-resistance test.
Basic Motor Insulation Investigation
Step 1: Review the Motor History
Check for overheating, water exposure, long storage or repeated electrical trips.
Step 2: Isolate the Motor
Apply the approved lockout and electrical-isolation procedure.
Step 3: Prove It Is De-Energised
Use the correct voltage-verification method.
Step 4: Disconnect Sensitive Equipment
Isolate the VFD, soft starter and other electronics as required.
Step 5: Inspect the Motor
Check the terminal box, cable, seals, moisture and contamination.
Step 6: Record Motor Temperature
Temperature is important when comparing results.
Step 7: Select the Correct Test Voltage
Follow the motor manufacturer and approved procedure.
Step 8: Perform the Test
Avoid touching the motor, leads or terminals while test voltage is applied.
Step 9: Discharge the Motor
Allow stored electrical charge to discharge safely.
Step 10: Record the Result
Document test voltage, duration, temperature, connection and reading.
Motor vs Cable Insulation Problem
If the motor and cable remain connected during the test, a low reading could come from either part.
Where permitted:
Disconnect the cable from the motor
Test the motor separately
Test the cable separately
Inspect the terminal box
Compare the results
This helps prevent unnecessary motor replacement.
Sudden vs Gradual Reduction
Sudden Reduction
Possible causes include:
Water entry
Cable damage
Electrical fault
Contamination
Mechanical impact
Incorrect test connection
Gradual Reduction
Possible causes include:
Insulation ageing
Repeated overheating
Increasing contamination
Moisture exposure
Progressive cable deterioration
Trend history is often more informative than one isolated result.
Supporting Instruments
What to CheckRecommended InstrumentMotor insulation resistanceInsulation resistance testerMotor temperature patternThermal imaging cameraSurface temperatureInfrared thermometerOperating currentClamp meterSupply voltageDigital multimeterMotor and bearing vibrationVibration meterRoom temperature and humidityTemperature-humidity data logger
Using Thermal Imaging
Thermal inspection during normal operation may identify:
Uneven motor temperature
Hot bearings
Blocked ventilation
Hot terminals
Abnormal cable temperature
Localised electrical heating
Thermal imaging does not directly measure insulation resistance, but it may help identify conditions contributing to insulation deterioration.
Using a Clamp Meter
Measure all phase currents under the same motor load.
Uneven or excessive current may contribute to overheating.
Current readings should be evaluated together with voltage, temperature and mechanical load.
Using a Data Logger
A temperature-humidity logger can determine whether the motor is exposed to:
High night-time humidity
Condensation risk
Hot room conditions
Ventilation shutdown
Seasonal environmental changes
This is particularly useful for outdoor, standby and intermittently operated motors.
Common Insulation Testing Mistakes
Avoid:
Testing an energised motor
Failing to prove the motor is de-energised
Leaving a VFD connected
Using the wrong test voltage
Ignoring motor temperature
Comparing tests with different durations
Touching terminals during the test
Failing to discharge the winding
Assuming the motor is faulty without separating the cable
Recording only the result without test conditions
Using damaged test leads
Energising a motor immediately after an unsafe result
Important Electrical Safety Reminder
An insulation resistance tester intentionally generates a potentially hazardous test voltage.
Testing should only be performed by trained and authorised personnel following:
Lockout and isolation requirements
Approved voltage-verification methods
Appropriate PPE
Manufacturer instructions
Correct test-voltage selection
Safe discharge procedures
Site electrical-safety rules
Motor Insulation Testing Tools Supplier in Malaysia
MTM Precision supplies instruments for investigating low motor insulation resistance, motor overheating, abnormal phase current, vibration and humid operating environments.
Our range includes insulation resistance testers, thermal imaging cameras, Smart Sensor AR600E infrared thermometers, clamp meters, digital multimeters, vibration meters and temperature-humidity data loggers.
Contact MTM Precision
MTM Precision Sdn. Bhd.
Showroom & Service Centre
No. 29-1 & 29-2, Jalan Bandar 18,
Pusat Bandar Puchong,
47160 Puchong, Selangor, Malaysia
🌐 Website: www.mtmpre.com.my
📧 Email: mtmpre@yahoo.com
📱 WhatsApp: +6016-660 7346
31 Aug 2026
