A practical guide to testing motor and cable insulation resistance using a Megger, selecting the correct test voltage, interpreting insulation resistance values, and determining whether the insulation condition is acceptable.
Motor and Cable Insulation Resistance Testing
Insulation resistance testing is one of the most useful methods for checking the condition of motor windings and power cables.
The test helps detect moisture, contamination, insulation deterioration, and leakage to earth before a complete insulation failure occurs.
However, the Megger reading must be interpreted correctly. A high reading generally indicates good insulation to earth, while a low reading may indicate moisture, contamination, cable damage, or insulation deterioration.
The Megger test does not confirm the complete condition of the motor. It mainly checks insulation resistance between the conductors and earth. It may not detect a short circuit between turns within the same winding.
What Does a Megger Measure?
A Megger applies a controlled DC test voltage and measures the resistance of the insulation.
The result is normally displayed in:
kΩ – kilo-ohms
MΩ – megaohms
GΩ – gigaohms
A higher resistance means less leakage current through the insulation.
The reading must always be evaluated according to:
Motor operating voltage
Applied Megger test voltage
Winding temperature
Humidity and contamination
Test duration
Manufacturer recommendations
Previous test records
Selecting the Test Voltage
The test voltage must not be selected randomly.
For many low-voltage motors operating at 380 V, 400 V, or 415 V, a 500 V DC insulation resistance test is commonly used.
Typical test-voltage guidance includes:
Equipment rated below 100 V: 250 V DC may be used
Low-voltage equipment up to approximately 600 V: 500 V DC is commonly used
Equipment above 600 V: 1,000 V DC may be used when permitted by the manufacturer
Always follow the motor, cable, and test-instrument manufacturer instructions.
Do not apply a Megger test voltage directly to:
VFD output terminals
Soft starter electronic circuits
PTC or PT100 sensors
Encoders
PLC inputs
Electronic monitoring devices
Surge protection devices
Essential Safety Precautions
Before testing:
Disconnect the electrical supply
Apply lockout and tagout procedures
Confirm zero voltage using a suitable meter
Disconnect the motor and cable from the VFD or soft starter
Disconnect temperature sensors, encoders, and electronic devices
Discharge capacitors and stored energy
Keep personnel away from exposed terminals
After testing, allow the Megger to discharge the tested winding or cable before touching the terminals.
How to Test a Motor
Step 1 – Isolate the Motor
Disconnect the motor from the power supply and from any VFD, soft starter, contactor, or electronic equipment.
For accurate diagnosis, disconnect the motor cable from the motor so that the motor and cable can be tested separately.
Step 2 – Inspect the Motor
Before using the Megger, inspect:
The terminal box
Cable glands
Winding terminals
Star or delta links
Moisture
Dust
Oil contamination
Loose or damaged connections
Step 3 – Select the Test Voltage
For a standard low-voltage 380-415 V motor, select 500 V DC unless the motor manufacturer specifies another test voltage.
Step 4 – Test Each Phase to Earth
Connect one Megger lead to the motor frame or protective earth terminal.
Connect the other lead to each motor terminal separately:
U to earth
V to earth
W to earth
Apply the test voltage for 60 seconds and record the final reading.
Step 5 – Compare the Readings
The three phase-to-earth readings should be high and reasonably similar.
A single phase with a much lower reading than the others may indicate:
Local insulation deterioration
Moisture
Contamination
Damage inside the terminal box
A winding insulation problem
Step 6 – Discharge the Windings
After completing the test, allow the instrument to discharge the windings.
Confirm that no voltage remains before touching or reconnecting the terminals.
How to Test a Motor Cable
Disconnect the cable from both the VFD or starter side and the motor side.
Test:
L1 to earth
L2 to earth
L3 to earth
L1 to L2
L1 to L3
L2 to L3
The cable should be tested separately from the motor. Testing them together may produce a low reading without showing which component is responsible.
Interpreting Insulation Resistance Values
For many low-voltage motors tested at 500 V DC, the following ranges can be used as practical guidance.
These values are not universal acceptance limits and must not replace manufacturer recommendations.
Above 100 MΩ
The insulation condition is generally very good.
20 to 100 MΩ
The insulation condition is generally good and normally acceptable.
5 to 20 MΩ
The reading requires investigation. Check moisture, contamination, temperature, and previous test results.
1 to 5 MΩ
The insulation condition is weak. Cleaning, drying, and further inspection are recommended before operation.
Below 1 MΩ
The insulation condition is generally unacceptable for a normal low-voltage motor. The motor and cable should be separated and tested individually.
Close to zero
This may indicate a severe insulation breakdown or a direct connection to earth.
Important Interpretation Rules
Do not judge the motor using one number only.
A useful assessment should consider:
The absolute resistance value
The similarity of the three phase readings
Whether the reading increases or decreases during the test
Winding temperature
Humidity
Previous readings
The motor manufacturer’s limits
The trend over time is often more useful than a single measurement.
A gradual reduction from hundreds of megaohms to much lower values may indicate developing insulation deterioration even if the motor is still operating.
Effect of Temperature
Insulation resistance decreases as winding temperature increases.
A motor tested while hot may produce a lower reading than the same motor tested when cold.
For useful comparison:
Record the winding or ambient temperature
Test under similar conditions whenever possible
Use temperature-corrected values when required
Do not compare hot and cold readings directly without considering the temperature difference
Effect of Moisture and Contamination
Moisture, dust, oil, and conductive contamination may reduce the insulation resistance.
Low readings do not always mean that the winding must be rewound.
Before making a final decision:
Clean the terminal box
Remove dust and contamination
Check the cable glands
Dry the motor using an approved method
Repeat the insulation resistance test
Record the new reading
Polarization Index
The Polarization Index is calculated using:
PI = 10-minute insulation resistance ÷ 1-minute insulation resistance
General guidance:
PI below 1: poor insulation condition
PI between 1 and 2: requires investigation
PI of 2 or more: generally acceptable for many winding insulation systems
PI results must be interpreted carefully.
Some modern insulation systems produce a very high initial resistance and may show a relatively low PI even when the insulation is in good condition.
For many small low-voltage motors, a stable 60-second insulation resistance reading and historical trend may be more practical than a full ten-minute PI test.
What the Megger Cannot Detect
A good Megger reading does not confirm that the motor is completely healthy.
A motor may have high insulation resistance to earth while still having:
A short circuit between winding turns
Unbalanced winding resistance
A poor internal connection
A damaged rotor
Bearing damage
Mechanical misalignment
Cooling problems
For a complete motor assessment, additional tests may include:
Winding resistance measurement
Phase-resistance comparison
Surge testing
No-load current measurement
Current balance measurement
Bearing and vibration inspection
Temperature-sensor testing
Practical Acceptance Checklist
Before returning the motor to service, confirm that:
The motor and cable were tested separately
The correct DC test voltage was used
The phase-to-earth readings are sufficiently high
The three readings are reasonably balanced
The readings are stable or increasing during the test
Temperature and humidity were recorded
The motor manufacturer’s recommendations were reviewed
The windings were discharged after testing
All sensors and electronic equipment were reconnected correctly
Conclusion
Insulation resistance testing is a simple and effective method for evaluating motor and cable insulation.
For many low-voltage motors tested at 500 V DC:
Readings above 20-30 MΩ are generally comfortable
Readings between 5 and 20 MΩ require investigation
Readings between 1 and 5 MΩ indicate weak insulation
Readings below 1 MΩ are generally unacceptable
The final decision must not depend on a single reading.
The resistance value, phase balance, temperature, moisture, test duration, manufacturer instructions, and historical trend must all be considered.
The Megger verifies insulation to earth, but additional tests are required to confirm the complete electrical and mechanical condition of the motor.