Insulation resistance testing helps determine whether the electrical insulation surrounding conductors, windings, cables and equipment is still effectively preventing unwanted current leakage.

Good insulation generally has high resistance. Contaminated, damaged, damp or deteriorated insulation usually has lower resistance. However, the actual measured value can vary significantly with temperature, moisture, test duration, equipment size and insulation type.

For that reason, insulation resistance should rarely be judged from one number alone. The most useful information usually comes from performing the same test consistently over time and watching for a downward trend.

Canadian utilities, industrial facilities, electrical contractors, renewable energy operators, manufacturers and maintenance teams commonly use insulation resistance testing on equipment such as:

• Motors
• Generators
• Transformers
• Switchgear
• Power and control cables
• Bus systems
• Industrial machinery
• Electrical panels
• Wind and solar equipment

The testing should be completed by qualified personnel using the correct test voltage, manufacturer instructions, lockout procedures and applicable Canadian electrical safety requirements.

Resistance Readings Should Be Considered Relative

Good insulation has relatively high resistance, while poor insulation has relatively low resistance.

However, the same equipment can produce different readings under different environmental and test conditions without necessarily having suffered insulation damage.

Insulation resistance can be affected by:

• Insulation temperature
• Ambient temperature
• Moisture and humidity
• Surface contamination
• Equipment size
• Cable length
• Test voltage
• Test duration
• Previous electrical or thermal stress

As insulation temperature or moisture increases, the measured insulation resistance will generally decrease. Megger provides a common rule of thumb that resistance may approximately halve for every 10°C increase in temperature, or double for every 10°C decrease, although the exact correction depends on the insulation system.

This is particularly relevant in Canada, where equipment may be tested under very different seasonal conditions. A motor tested in a warm production area in July may produce a different result from the same motor tested in an unheated facility in January.

That does not automatically mean the insulation condition has changed. Results should either be obtained under similar temperature conditions or corrected to a common reference temperature before they are compared.

How an Insulation Resistance Tester Works

An insulation resistance tester, frequently called a megohmmeter or Megger tester, is a high-range resistance meter with a built-in DC voltage source.

The instrument applies a controlled direct-current voltage across the insulation under test. This voltage causes a very small amount of current to flow through and across the surface of the insulation.

The tester measures that current and calculates the resistance.

Insulation resistance testers usually display results in:

• Ohms
• Kilohms
• Megohms
• Gigohms
• Teraohms on higher-range instruments

Healthy insulation commonly produces readings in the megohm or higher range, although the acceptable value depends on the equipment type, rating, insulation system, manufacturer and applicable testing criteria.

Test Voltage

Insulation resistance testers are available with different DC test-voltage ranges.

Common test voltages may include:

• 50 V
• 100 V
• 250 V
• 500 V
• 1,000 V
• 2.5 kV
• 5 kV
• 10 kV

The correct voltage depends on the equipment being tested.

A low-voltage electronic circuit should not be tested using the same voltage as a medium-voltage motor or power cable. Applying an unsuitable voltage can damage sensitive equipment or produce an invalid result.

Always follow:

• Equipment manufacturer instructions
• Insulation tester instructions
• Approved maintenance procedures
• Applicable engineering criteria
• Site-specific safety procedures

The original JM Test article describes traditional testing at 500 V DC or more, but that should not be interpreted as a universal minimum for every type of equipment.

What Current Does the Tester Measure?

When DC voltage is first applied to insulation, the measured current is normally made up of several components.

Capacitive Charging Current

The equipment initially behaves partly like a capacitor.

When the test begins, charging current flows and then usually decreases quickly as the insulation charges.

Long cables, large motors and transformers may have greater capacitance and can require more time to charge and discharge safely.

Absorption Current

Absorption current is associated with polarization within the insulation material.

This current usually decreases more slowly than capacitive charging current. Time-based diagnostic tests such as dielectric absorption ratio and polarization index use this behaviour to provide additional information about insulation condition.

Leakage Current

Leakage current is the more persistent current that flows through or across the insulation.

Contamination, moisture, damaged insulation and surface leakage can increase this current and reduce the measured resistance.

The resistance reading generally rises as charging and absorption currents decrease. This is why the duration of the test matters and why results taken at different times should not be compared casually.

A Basic Insulation Resistance Test

The exact procedure depends on the asset and tester, but the basic process normally includes the following stages.

1. De-Energize and Isolate the Equipment

The equipment should be removed from service and isolated from all energy sources according to the approved lockout procedure.

Insulation resistance testing should not be performed on an energized circuit.

Connected electronic equipment, surge protection devices, control components or other voltage-sensitive equipment may also need to be isolated before testing.

2. Verify the Absence of Voltage

Use an appropriately rated voltage detector or test instrument to verify that the circuit is de-energized.

The voltage tester should normally be confirmed on a known source before and after the absence-of-voltage check according to the site’s safe-work procedure.

3. Discharge Stored Energy

Capacitors, long cables, transformer windings and other equipment may retain electrical energy after disconnection.

Stored energy must be discharged using the approved method before the tester is connected.

4. Connect the Test Leads

Connect the insulation tester according to the equipment and test configuration.

Typical measurements may include:

• Conductor to ground
• Phase to phase
• Winding to ground
• Winding to winding
• Conductor to conductor

Connections should be clean, secure and consistent from one test to the next.

5. Select the Test Voltage

Choose the appropriate DC test voltage based on the equipment rating, insulation system and test procedure.

Do not select the test voltage based solely on what the tester is capable of producing.

6. Apply the Test for the Required Time

Apply the test voltage for the specified duration.

A basic spot-reading test may record resistance after a defined period such as one minute. More advanced diagnostic tests may evaluate readings at multiple times or voltages.

7. Record the Result and Conditions

Document more than the final resistance value.

Useful records include:

• Equipment identification
• Test connection
• Test voltage
• Test duration
• Insulation resistance
• Ambient temperature
• Equipment temperature
• Relative humidity
• Date and time
• Instrument model and serial number
• Calibration status
• Technician
• Relevant observations

8. Discharge the Equipment After Testing

The tested insulation may retain a potentially hazardous charge after the test voltage is removed.

Keep the equipment discharged according to the tester manufacturer’s procedure before touching the terminals, removing leads or returning the equipment to service.

This is especially important for long cables, transformers, generators and large motors.

How to Interpret Insulation Resistance Readings

Insulation resistance readings should generally be treated as relative values.

A single resistance value does not always tell you whether the insulation is healthy or failing. The reading must be reviewed in context.

The JM Test source notes that the same motor or machine may produce different readings on consecutive days without necessarily having defective insulation. The more important signal is whether repeated results show a sustained decline over time.

Trend Testing

Periodic trend testing is one of the strongest uses of insulation resistance measurement.

For example, a motor may produce the following temperature-corrected readings:

• Year 1: 500 MΩ
• Year 2: 470 MΩ
• Year 3: 390 MΩ
• Year 4: 230 MΩ

Every result may still appear high when viewed alone. The downward trend, however, may indicate gradual contamination, moisture ingress, thermal ageing or insulation deterioration.

That gives the maintenance team time to investigate before the equipment fails.

Test Consistency

Tests should be performed as consistently as possible.

Use:

• The same connections
• The same test voltage
• The same test duration
• Similar equipment conditions
• Similar temperature conditions or temperature correction
• Comparable humidity information

Megger similarly recommends performing periodic tests using the same connections, test voltage and duration, and either testing at similar temperatures or correcting readings to a common reference temperature. Recording relative humidity also helps with result interpretation.

Without this consistency, changes in the test method may look like changes in insulation condition.

Factors That Can Lower Insulation Resistance

Moisture

Moisture is one of the most common causes of reduced insulation resistance.

It may enter through:

• Damaged seals
• Condensation
• Outdoor exposure
• Flooding
• Humid environments
• Improper storage
• Cooling-system leaks

Canadian equipment operating outdoors, in unheated spaces or through repeated freeze-thaw cycles may be particularly exposed to condensation and moisture-related issues.

Surface Contamination

Dust, oil, salt, chemicals and conductive residue can create leakage paths across insulation surfaces.

A low reading caused by surface contamination may improve after proper cleaning and drying.

Excessive Temperature

High operating temperatures accelerate insulation ageing.

Repeated overheating from overload, ventilation problems, loose connections or high ambient temperature can gradually reduce insulation life. Electrical hot spots can also damage nearby insulation and increase failure risk.

Mechanical Damage

Vibration, abrasion, impact, cable movement and improper installation can damage insulation.

Small defects may allow moisture and contamination to enter, worsening deterioration over time.

Electrical Stress

Surges, partial discharge, overvoltage and repeated switching stress can degrade insulation.

Insulation resistance testing may help identify deterioration, although it may not detect every localized defect. Other diagnostic methods may be required depending on the asset and failure mode.

Common Insulation Resistance Test Methods

Spot-Reading Test

A spot-reading test records insulation resistance after a specified period at a fixed voltage.

It is simple and useful for routine testing, but environmental conditions must be considered when comparing results.

Time-Resistance Test

A time-resistance test records how resistance changes while the test voltage remains applied.

Healthy insulation often shows increasing resistance as charging and absorption currents decline. Flat or poor improvement may suggest moisture or contamination.

Dielectric Absorption Ratio

The dielectric absorption ratio, or DAR, compares resistance readings taken at two different times during the test.

It provides more information than one spot reading because it evaluates how insulation resistance develops during voltage application.

Polarization Index

Polarization index, or PI, commonly compares a later resistance reading with an earlier reading during an extended test.

PI testing is often used on motors, generators and other rotating machinery. Interpretation depends on the insulation type, equipment design, temperature and manufacturer guidance.

Step-Voltage Test

A step-voltage test applies progressively higher DC voltages while monitoring insulation resistance.

If resistance drops disproportionately as voltage increases, the result may indicate insulation weakness or localized defects.

Ramp Test

A ramp test gradually increases test voltage while monitoring current or resistance.

The MIT525 and MIT1025 instruments described by JM Test include PI, DAR, dielectric discharge, step-voltage and ramp-test functions. Their ramp function allows the operator to monitor current against increasing voltage and terminate the test before breakdown if the developing result indicates a defect.

Megger 5 kV and 10 kV Insulation Resistance Testers

The original JM Test article highlights the Megger MIT525 and MIT1025 insulation resistance testers.

The range includes 5 kV and 10 kV instruments for high-resistance measurement and diagnostic insulation testing.

Listed features include:

• PI, DAR, DD, step-voltage and ramp testing
• Advanced memory with time and date stamps
• Data logging
• Rechargeable lithium-ion battery
• Operation from line power when the battery is discharged
• Rugged dual-case construction
• IP65 protection with the case closed
• CAT IV 600 V terminal safety rating
• USB data transfer
• User-selectable test voltages

JM Test lists resistance measurement capability up to 10 TΩ for its cited 5 kV models and up to 20 TΩ for the 10 kV MIT1025.

These higher-voltage testers are used by organizations such as:

• Electrical contractors
• Testing and service companies
• Utilities
• Industrial facilities
• Rail operators
• Wind farm operators
• Solar generation operators
• Power generation and distribution companies

The correct tester should be chosen according to the voltage class, equipment type, required diagnostic tests and approved maintenance procedure.

Canadian Electrical Safety Considerations

Insulation resistance testing can involve high DC test voltages and equipment capable of storing electrical energy.

Testing should only be performed by qualified workers using appropriate:

• Lockout and isolation procedures
• Absence-of-voltage verification
• PPE
• Rated test leads
• Barriers and controlled work areas
• Discharge procedures
• Manufacturer instructions
• Site-specific work practices

CSA Z462 provides Canadian workplace electrical safety requirements and guidance for electrical safety programs, qualified workers, risk assessment, safe-work procedures and PPE selection. It is intended to be used alongside relevant Canadian electrical and occupational safety requirements.

This article should be treated as educational guidance, not as a complete field-testing procedure.

Practical Takeaway

Insulation resistance is measured by applying a known DC voltage to the insulation, measuring the small current that flows and calculating the resulting resistance.

The instrument provides a direct reading, usually in megohms or higher. But the number should not be interpreted in isolation.

The most reliable insulation maintenance programme uses:

• Consistent test connections
• Consistent test voltage
• Consistent test duration
• Temperature correction or comparable test temperatures
• Humidity records
• Periodic trend analysis
• Proper discharge and safety procedures

One reading tells you what the insulation looked like during one test. A consistent record tells you where the insulation is heading.

JM Test Systems Canada can support electrical teams with insulation resistance testers, Megger MIT525 and MIT1025 equipment, megohmmeter rentals, electrical test equipment and calibration services.