When an electric motor trips, stops unexpectedly or refuses to start, attempting an immediate restart can turn a manageable fault into a more serious failure.

Insulation-resistance testing helps determine whether the insulation separating motor windings from each other and from the grounded frame has deteriorated. Low resistance may point to moisture, contamination, damaged insulation or another leakage path. However, one acceptable insulation reading does not prove that the entire motor and supply system are safe to energize. The starter, supply circuit, winding resistance, bearings, driven load and control equipment may still contain faults.

For Canadian industrial plants, utilities, mines, commercial facilities, water-treatment operations, manufacturing sites and electrical contractors, the safest approach is to treat insulation testing as one part of a structured troubleshooting process.

This work should only be performed by qualified electrical workers using approved isolation, absence-of-voltage verification, discharge and personal protective equipment procedures.

Why Insulation-Resistance Testing Matters

Electric motors operate pumps, compressors, fans, conveyors, production equipment and mechanical systems throughout industrial facilities. When a motor fails unexpectedly, downtime can affect production, building operations and safety-critical processes.

Insulation-resistance testing applies a controlled DC voltage and measures the small leakage current flowing through or across the motor’s insulation. The tester then calculates and displays resistance, normally in megohms or higher units.

High resistance generally indicates that the insulation is restricting leakage current effectively. Low resistance can indicate:

• Moisture ingress
• Dirt, oil or conductive contamination
• Thermal deterioration
• Damaged winding insulation
• Phase-to-ground leakage
• Deteriorated motor leads
• Problems in the cable, starter or connected circuit

The test can help determine whether the fault is located in the supply circuit, starter, cable or motor, but it should not be used as the only basis for declaring the motor safe to restart.

Safety Before Testing

Insulation testers can produce DC voltages ranging from a few hundred volts to several kilovolts. Those test voltages can damage connected electronic controls and can leave capacitive equipment holding a hazardous charge after the test.

Before connecting an insulation tester:

• De-energize the circuit.
• Apply the approved lockout procedure.
• Verify absence of voltage with a suitably rated tester.
• Isolate variable-frequency drives, soft starters, sensors and electronic controls.
• Review stored mechanical and electrical energy.
• Discharge capacitors and motor circuits as required.
• Inspect the insulation tester and leads.
• Confirm the selected test voltage is appropriate.
• Control access to the work area.

Some modern insulation testers include live-circuit detection and automatic discharge functions, but these features support the work procedure; they do not replace isolation, verification and qualified-worker judgement.

CSA Z462:24 provides requirements and guidance for electrical-safety management systems, safe work procedures, PPE selection and the identification and training of qualified electrical workers exposed to electrical hazards.

Step 1: Perform a Visual and Sensory Inspection

Begin by inspecting the motor and the surrounding equipment before applying test voltage.

Look for:

• Smoke residue or soot
• Burn marks
• Melted insulation
• Discoloured motor leads
• Loose or damaged terminals
• Contamination in the terminal box
• Water or condensation
• Blocked ventilation
• Damaged cooling fans
• Signs of rubbing or mechanical impact
• Oil or chemical contamination

Also note unusual burnt-varnish or overheated-insulation odours. These can indicate winding overheating even when external damage is limited.

Check the driven equipment as well. A seized pump, jammed conveyor, failed bearing or overloaded mechanical system may have caused the motor trip without the insulation itself being the original problem.

The original JM Test and Fluke workflows both recommend beginning with visible damage, unusual odours, loose connections and basic motor information before moving to electrical tests.

Step 2: Gather Nameplate and Baseline Information

Record the motor nameplate information before choosing a test method or voltage.

Important details include:

• Manufacturer
• Model and serial number
• Rated voltage
• Full-load current
• Power rating in kilowatts or horsepower
• Number of phases
• Frequency
• Insulation class
• Service factor
• Duty rating
• Connection configuration
• Temperature-rise information
• Previous repair or rewind history

Also collect relevant operating information:

• What happened immediately before the failure?
• Did a protective device trip?
• Was the motor overloaded?
• Was it operating through a variable-frequency drive?
• Did the motor overheat?
• Was there flooding, condensation or a washdown?
• Has the motor produced vibration or bearing noise?
• Are earlier insulation readings available?

Historical results are valuable because insulation resistance is influenced by temperature, moisture, equipment condition and the test method. Comparable trends are generally more informative than a single isolated reading.

Megger notes that insulation resistance normally decreases as temperature increases; a commonly used approximation is that resistance may halve for each 10°C increase, although the appropriate correction depends on the insulation system.

Perform Basic Circuit Checks

Before insulation testing, a digital multimeter may be used for basic troubleshooting where permitted by the work procedure.

Checks may include:

• Supply voltage
• Fuse continuity
• Control voltage
• Starter or contactor condition
• Grounding and bonding continuity
• Obvious open circuits
• Voltage imbalance

These checks can help identify whether the problem lies upstream of the motor.

Do not apply an insulation-resistance test while electronic controls remain connected. Fluke specifically warns that insulation-test voltages can damage drives, sensors and other electronic devices connected to the circuit under test.

Step 3: Lock Out, Verify and Isolate the Circuit

Apply the site’s lockout procedure at the appropriate disconnect, starter or motor-control equipment.

After isolation:

1. Verify the test instrument on a known source.
2. Verify absence of voltage on the circuit.
3. Reconfirm the voltage tester on a known source.
4. Isolate electronic controls and connected equipment.
5. Identify and discharge stored electrical energy.
6. Separate the motor leads as required by the approved procedure.

Where practical, separating the motor from the supply cable allows the technician to determine whether low resistance originates in:

• The starter
• The supply cable
• The motor leads
• The motor windings
• Another connected component

Testing the entire circuit without isolating sections may reveal that a problem exists but not where it is located.

Fluke’s failed-motor troubleshooting procedure recommends measuring both the line side and load side of the starter to ground, then tracing the problem toward the starter, cable or motor when a low result is found.

Step 4: Perform the Insulation-Resistance Tests

Select the test voltage according to the motor manufacturer’s instructions, the motor rating, the insulation system and the approved maintenance procedure.

The original article gives these general examples:

• 250 V DC for some control circuits
• 500 V DC for many low-voltage motors up to 600 V
• 1,000 V DC or higher for certain higher-voltage motors

These are general examples, not universal settings. Sensitive controls must be isolated, and the tester voltage must be suitable for the specific motor and circuit.

Test the Supply Circuit to Ground

Where the troubleshooting plan includes the starter and cable, measure insulation resistance:

• From the line side of the starter to ground
• From the load side of the starter to ground

A high line-side reading and a low load-side reading may suggest that the fault is downstream of the starter. The motor cable and motor can then be isolated and tested separately.

Test Each Motor Winding to Ground

With the motor isolated and configured according to the manufacturer’s procedure, measure insulation resistance between each winding or phase and the grounded motor frame.

For a three-phase motor, this may involve:

• Phase A to ground
• Phase B to ground
• Phase C to ground

The test connections depend on whether the internal winding junctions are accessible and whether the motor is connected in wye or delta. Fluke recommends disconnecting the stator windings and phases where required and measuring between windings as well as between windings and ground.

Record:

• Test voltage
• Test duration
• Resistance value
• Winding temperature
• Ambient temperature
• Humidity where relevant
• Motor connection
• Tester model and serial number
• Date and technician

Test Between Windings Where Applicable

Where the motor leads and configuration permit, insulation resistance may also be measured between isolated phase windings.

This is an insulation test between phases, not the same measurement as the low-resistance winding comparison used to detect open turns, poor joints or winding imbalance.

That distinction is important:

• Insulation resistance testing measures high resistance from winding to ground or between insulated windings.
• Winding resistance testing measures the very low conductor resistance of each winding and compares phases.

A standard insulation tester is not normally the correct instrument for precise winding-resistance comparison. That work may require a low-resistance ohmmeter or suitable motor-testing instrument.

Allow the Reading to Stabilize

When DC voltage is first applied, charging and absorption currents can make the displayed resistance change over time.

For routine testing, the approved procedure may require a reading after a defined interval, often 60 seconds. Longer time-resistance tests may use dielectric absorption ratio or polarization index measurements.

The polarization-index test compares resistance measurements taken at one minute and ten minutes. Advanced motor and insulation testers may calculate PI and DAR automatically.

Discharge the Motor After Testing

Motor windings, cables and connected circuits can retain charge after the test.

Allow the tester’s discharge process to finish, confirm that the voltage has reduced to a safe level, and follow the tester manufacturer’s discharge instructions before touching terminals or removing test leads.

Automatic discharge is a useful feature, but the operator must still verify that the circuit is safe. Modern Megger instruments may also monitor residual discharge and indicate when insulation is ready to be retested.

Step 5: Interpret the Results and Decide the Next Action

Do not reduce the decision to “high equals pass, low equals fail.”

The result should be assessed against:

• Motor manufacturer criteria
• Applicable engineering standards
• Motor voltage and insulation type
• Winding temperature
• Test voltage and duration
• Previous test history
• Similar motors
• Repair or rewind information
• The circumstances of the failure

Results That Support Further Evaluation

Favourable indicators may include:

• High phase-to-ground insulation resistance
• Similar insulation behaviour across comparable phases
• No sudden decay during the test
• Stable or improving historical readings
• No visible or sensory evidence of winding damage
• Acceptable separate winding-resistance results
• Acceptable supply, starter and cable tests

Even when these results are favourable, they do not prove that the motor is safe to energize. Bearing condition, rotor condition, mechanical loading, phase balance, control operation and other factors may still require evaluation.

Problem Indicators

Further investigation is required when results show:

• Low phase-to-ground resistance
• Low phase-to-phase insulation resistance
• One phase markedly different from comparable phases
• Rapidly falling resistance
• Evidence of moisture or contamination
• Burnt winding odour or visible damage
• Abnormal winding-resistance imbalance
• Low readings isolated to the cable or starter
• Results that remain low after cleaning or drying

Possible responses include:

• Cleaning and drying
• Cable repair
• Terminal repair
• Further winding testing
• Surge or impedance testing
• Motor-shop evaluation
• Rewinding
• Motor replacement

A motor should not be energized merely because one insulation measurement exceeds a generic threshold.

Be Careful With Generic Megohm Limits

The original article and corresponding Fluke guidance mention screening values of more than 2 MΩ to ground for AC equipment and more than 1 MΩ for DC equipment. These can be useful troubleshooting references in the particular workflow described, but they are not universal acceptance limits for every motor.

The acceptable minimum can depend on:

• Rated voltage
• Winding insulation system
• Motor age
• Temperature
• Equipment manufacturer
• Test standard
• Repair history
• Whether the measurement is corrected to a reference temperature

Use the motor manufacturer’s criteria or the project’s approved engineering standard rather than relying solely on a general blog threshold.

Common Causes of Motor Insulation Failure

Moisture and Condensation

Moisture can enter a motor through damaged seals, washdown exposure, flooding, storage conditions or condensation.

Canadian motors in unheated buildings, outdoor installations and facilities with large seasonal temperature changes may be especially vulnerable to condensation during shutdown and restart cycles.

Thermal Ageing

Overloads, poor ventilation, blocked cooling passages, high ambient temperatures and excessive starts can accelerate insulation deterioration.

Repeated overheating can weaken winding varnish and insulation even when the motor continues to operate.

Contamination

Dust, oil, chemicals and conductive deposits can create surface-leakage paths.

Cleaning and drying may improve insulation resistance when contamination is the main cause, but the motor should be evaluated again before returning it to service.

Mechanical Stress

Vibration, loose windings, worn bearings and rotor contact can cause movement and abrasion inside the motor.

The insulation may gradually wear until conductors contact each other or the grounded frame.

Electrical Stress

Voltage transients, harmonics, phase imbalance, switching and variable-frequency drive operation can place additional stress on motor insulation.

Fluke identifies transient voltage, voltage imbalance, harmonics, overload and operating conditions among common contributors to motor winding and bearing failure.

Why Trending Is Better Than One Reading

A failed-motor test is often a point-in-time troubleshooting activity. A preventive-maintenance programme adds more value by comparing readings over months or years.

Trend records should use:

• The same test points
• The same test voltage
• The same test duration
• Similar motor conditions
• Recorded winding temperature
• Appropriate temperature correction
• The same or comparable instrument

Megger offers motor-testing instruments with built-in temperature measurement and compensation because both insulation resistance and low winding resistance are affected by temperature.

A gradual decline may reveal deteriorating insulation before the motor trips. That gives the facility an opportunity to clean, dry, repair or replace the motor during planned maintenance instead of during an unplanned shutdown.

Practical Takeaway

Insulation-resistance testing is an important part of investigating a failed or suspect motor, but it is not a stand-alone restart approval.

A reliable workflow is:

1. Inspect the motor and driven equipment.
2. Record nameplate and failure information.
3. Lock out, verify and isolate the circuit.
4. Test the starter, cable and motor insulation using the correct voltage and connections.
5. Review the results alongside winding resistance, mechanical condition and manufacturer criteria.

One bad insulation reading confirms that further investigation is necessary. One good reading does not prove that nothing else is wrong.

JM Test Systems Canada can support motor-maintenance teams with insulation testers, megohmmeters, low-resistance ohmmeters, motor-testing instruments, equipment rentals and calibration services.