Electrical insulation is meant to keep current flowing only where it is supposed to flow.

In cables, motors, transformers, switchgear, generators and other electrical equipment, insulation separates energized conductors from ground, other phases and exposed conductive parts. When insulation is healthy, leakage current stays low. When insulation begins to deteriorate, leakage current can increase and the risk of failure becomes higher.

Insulation failure can lead to:

• Ground faults
• Short circuits
• Equipment trips
• Motor burnouts
• Cable failures
• Fire risk
• Shock hazards
• Arc-flash hazards
• Production downtime
• Expensive emergency repairs
• Unsafe touch voltage on equipment

The original JM Test article explains that even modern insulation can degrade because of electrical stress, mechanical damage, vibration, excessive heat or cold, dirt, oil, corrosive vapours, moisture and humidity.

For Canadian industrial plants, utilities, mines, wind farms, solar sites, rail systems, water facilities, data centres and commercial buildings, insulation resistance testing helps identify deterioration before it becomes an in-service failure.

What Is Electrical Insulation?

Electrical insulation is the material that resists current flow between conductive parts.

It may be found around or between:

• Motor windings
• Generator windings
• Transformer windings
• Power cables
• Control cables
• Busbars
• Switchgear components
• Terminations
• Connectors
• Electrical panels
• Rotating machines

A good insulator has very high resistance. A poor or degraded insulator begins to behave partly like a conductor.

The goal of insulation testing is to identify when that resistance is dropping, changing or becoming unstable.

What Causes Electrical Insulation to Degrade?

Insulation degradation is often caused by several stress factors acting together over time.

1. Electrical Stress

Electrical insulation is constantly exposed to voltage stress during operation.

Electrical stress can increase because of:

• Sustained overvoltage
• Voltage surges
• Switching impulses
• Faults
• Harmonics
• Partial discharge
• Poor voltage control
• Repetitive fast voltage pulses from power electronics

The JM Test source article notes that sustained overvoltages or impulses from faults can lead to discharges in voids, which can expand and develop into electrical treeing.

Electrical treeing creates branching degradation paths inside insulation. Once this process begins, insulation strength can decline faster.

2. Heat

Heat accelerates insulation ageing.

Sources of heat include:

• Overloaded conductors
• Loose connections
• Poor ventilation
• High ambient temperature
• Motor overload
• Harmonic heating
• Transformer overloading
• High resistance joints
• Cooling-system problems

Excessive heat can dry, harden, crack or chemically degrade insulation materials.

Cold can also create problems. Repeated expansion and contraction can stress insulation, seals, jackets and terminations, especially in outdoor Canadian environments.

3. Moisture and Humidity

Moisture is one of the most common enemies of insulation.

It can enter through:

• Damaged jackets
• Cracked seals
• Poor terminations
• Condensation
• Flooding
• Washdown areas
• Underground ducts
• Outdoor enclosures
• High-humidity process areas

The JM Test source article explains that once pinholes or cracks develop, moisture and foreign matter can penetrate insulation and create a lower-resistance leakage path. It also notes that flooding can cause a sudden drop in insulation resistance.

For Canadian facilities, moisture-related problems may also appear after freeze-thaw cycles, spring melt, roof leaks, underground duct flooding or condensation inside outdoor equipment.

4. Dirt, Oil and Contamination

Contaminants can settle on insulation surfaces and create leakage paths.

Examples include:

• Dust
• Dirt
• Oil
• Grease
• Salt
• Carbon tracking
• Chemical residue
• Process vapour
• Conductive particles
• Metal filings
• Moisture mixed with dirt

Surface contamination can be especially serious when combined with humidity.

A dry surface may test acceptably, but the same surface may leak current when damp or contaminated.

5. Mechanical Damage

Mechanical damage can weaken insulation immediately or create a defect that worsens over time.

Common causes include:

• Cable pulling damage
• Crushed conduit
• Improper bending radius
• Vibration
• Abrasion
• Poor support
• Impact
• Rodent damage
• Loose hardware
• Cable tray damage
• Termination stress
• Incorrect installation tools

The source article includes mechanical damage and vibration among common insulation-degradation factors.

6. Chemical and Corrosive Vapours

Chemical environments can attack insulation, jackets, seals and terminations.

Examples include:

• Process vapours
• Solvents
• Acids
• Alkalis
• Salt-laden air
• Oil mist
• Industrial chemicals
• Wastewater treatment gases

Chemical degradation may not be obvious during a visual inspection until the insulation is already weakened.

7. Ageing

Even when equipment is operated correctly, insulation ages.

Ageing may be slow at first, but it can accelerate when defects appear. The source JM Test article describes insulation ageing as a gradual spiral of decline where different insulation “enemies” begin to reinforce each other.

For example:

1. Heat dries and hardens insulation.
2. Small cracks develop.
3. Moisture enters the cracks.
4. Leakage current increases.
5. Local heating increases.
6. The defect grows faster.

This is why trend testing matters.

What Is Insulation Resistance Testing?

Insulation resistance testing applies a DC test voltage between conductors, or between a conductor and ground, and measures the resistance of the insulation.

The instrument is commonly called:

• Insulation resistance tester
• Megohmmeter
• Megger tester
• IR tester

The result is usually displayed in:

• MΩ
• GΩ
• TΩ

Fluke explains that insulation resistance testing can detect leakage caused by insulation deterioration or damage and is used on equipment such as switchgear, motors, generators and cables.

A higher reading usually indicates better insulation, while a lower reading may indicate moisture, contamination, damage or deterioration. However, the absolute number should never be interpreted alone. Temperature, humidity, equipment type, test voltage, test duration and historical trend all matter.

Why Periodic Testing Matters

Insulation degradation is often gradual.

A single test may tell you the condition today. Periodic testing tells you whether the insulation is stable, improving, worsening or approaching a failure pattern.

Fluke notes that insulation resistance testing should begin at installation and continue through the equipment life, because historical tracking and trending help identify degradation over time.

Periodic testing can support:

• Preventive maintenance
• Predictive maintenance
• Commissioning
• Post-repair verification
• Troubleshooting
• Post-flood inspection
• Shutdown planning
• Asset condition assessment
• Failure investigation

The strongest value comes from comparing results taken under similar conditions.

Common Equipment Tested

Insulation resistance testing is used on:

Motors

Motor insulation can degrade because of heat, moisture, vibration, contamination, overloads and winding stress.

Testing may help detect:

• Winding-to-ground weakness
• Phase-to-phase insulation issues
• Moisture ingress
• Contamination
• Thermal damage
• Age-related deterioration

Generators

Generator insulation can be affected by heat, vibration, contamination, humidity and mechanical stress.

Timed tests such as PI may be useful for larger rotating machines.

Cables

Cable insulation can be damaged during installation, ageing, flooding, thermal cycling, mechanical stress or termination failure.

Fluke’s insulation testing application note says wires and cables should be isolated from panels and machinery and tested against each other and against ground.

Transformers

Insulation testing may be used as a first check on transformer insulation condition, but transformer insulation systems can be complex.

Megger notes that general-purpose insulation testers can perform spot tests on transformers, but interpreting transformer insulation results can be difficult because of complex oil-cellulose insulation systems; dedicated transformer test equipment may be needed for deeper assessment.

Switchgear and Electrical Installations

Insulation resistance testing can help confirm installation quality and identify defects before energization.

Fluke’s application note states that insulation tests are used to determine the integrity of windings or cables in motors, transformers, switchgear and electrical installations.

Basic Insulation Resistance Test Types

Spot Reading Test

A spot reading test applies a selected DC test voltage and records the insulation resistance after a set time, often one minute.

It is useful for:

• Basic checks
• Installation testing
• Short cable runs
• Low-capacitance equipment
• Quick comparison against previous results

Fluke explains that on low-capacitance equipment such as some cabling or switchgear, time-dependent leakage currents become insignificant quickly, making spot or short-time resistance tests practical.

Time-Resistance Test

A time-resistance test records insulation resistance over time.

It is useful because insulation behaviour changes as charging current, absorption current and leakage current settle.

Large motors, generators and long cable runs may require trend-style tests because readings can change continuously as the insulation charges. Fluke notes that on high-capacitance equipment, time-dependent currents may last for hours, so tests that establish a trend between readings can be more useful than a single reading.

Polarization Index

Polarization Index, or PI, compares the 10-minute insulation resistance reading with the 1-minute reading.

PI = 10-minute insulation resistance / 1-minute insulation resistance

Fluke defines PI as the ratio of the 10-minute resistance value to the 1-minute resistance value.

Megger’s Q&A states that a PI value of 2.0 or higher is generally associated with good insulation and values below 2.0 suggest further investigation, while also noting that interpretation depends on the equipment and insulation system.

PI is especially useful for larger equipment because it gives a relative comparison rather than relying only on one absolute resistance number.

Dielectric Absorption Ratio

Dielectric Absorption Ratio, or DAR, is a shorter timed test.

DAR = 60-second insulation resistance / 30-second insulation resistance

Fluke defines DAR as the ratio of the 60-second resistance value to the 30-second resistance value.

Megger notes that DAR is usually lower than PI because the test duration is shorter, and that DAR values of about 1.4 or higher are typically associated with insulation in good condition.

Step Voltage Test

A step voltage test applies increasing DC test voltages in steps.

The idea is to see whether insulation resistance remains stable as voltage increases.

Megger explains that an ideal insulator should give similar readings regardless of test voltage, while a lower resistance reading at higher voltage may suggest cracks or voids in the insulation.

Ramp Voltage Test

A ramp voltage test gradually increases the applied voltage while monitoring current.

Megger describes ramp testing as a gentler alternative to step voltage testing because the voltage rises slowly, allowing the test to be stopped if current starts increasing rapidly before permanent damage occurs.

The original JM Test article also notes that ramp testing on Megger MIT units can reveal defects that may be difficult to detect otherwise and can reduce the risk of sudden large voltage increments on flawed insulation.

Dielectric Discharge Test

Dielectric discharge, or DD, is used on some larger insulation systems to evaluate discharge behaviour after the test voltage is removed.

It can help assess multilayer insulation systems and absorption characteristics, depending on the equipment and procedure.

Why Temperature Matters

Insulation resistance is strongly affected by temperature.

A colder asset may show a higher resistance reading than the same asset when warm. A warmer asset may show a lower reading, even if the insulation condition has not changed.

Fluke states that for every 10°C above the baseline temperature, insulation resistance roughly halves, and for every 10°C below the baseline, it roughly doubles. Fluke also notes that IEEE 43 recommends correcting rotating-machine insulation resistance measurements to a constant 40°C baseline.

This matters for Canadian field work because outdoor equipment may be tested in very different seasonal conditions.

A motor tested in January and again in July may show different insulation resistance simply because the temperature changed. For useful trend data, record:

• Asset temperature
• Ambient temperature
• Relative humidity where practical
• Test voltage
• Test duration
• Test points
• Recent operating condition
• Whether the asset was wet, cold, recently shut down or recently cleaned

Why Humidity and Moisture Matter

Moisture can lower insulation resistance and create surface leakage paths.

This is especially important after:

• Flooding
• Condensation
• Washdown
• Outdoor exposure
• Underground duct water ingress
• Snow or ice melt
• High-humidity process conditions
• Roof leaks
• Damaged cable jackets

A sudden drop in insulation resistance after a moisture event should be treated seriously.

The original JM Test article specifically notes that insulation resistance can drop suddenly when equipment is flooded.

Selecting the Correct Test Voltage

The insulation tester’s output voltage must match the equipment, insulation system and approved procedure.

Too low a voltage may not reveal a weakness. Too high a voltage may unnecessarily stress insulation or damage sensitive equipment.

Selection should consider:

• Equipment rated voltage
• Manufacturer instructions
• Applicable standard
• Asset owner procedure
• Insulation type
• Age and condition of equipment
• Connected electronics
• Surge suppressors or capacitors
• Whether the test is commissioning, maintenance or troubleshooting

Modern insulation testers may offer selectable test voltages and diagnostic modes. The source JM Test page describes Megger MIT515, MIT525 and MIT1025 insulation testers with 5 kV and 10 kV options, preset voltage ranges, PI, DAR, DD, step voltage and ramp testing depending on model.

Do not assume one test voltage applies to every asset.

Megger MIT515, MIT525 and MIT1025 Context

The original article references Megger MIT525 and MIT1025 instruments.

Megger’s newer product range now includes updated Essential and Advanced models, including MIT515/2, MIT1025/2 and MIT1525/2. Megger says the Advanced 5 kV, 10 kV and 15 kV insulation testers support IR, DAR, PI, DD, step voltage, ramp and PDC testing for transformers, motors, switchgear and cables.

The older JM Test article lists these product capabilities for the MIT515/MIT525/MIT1025 generation:

The Canadian article should mention the specific products as examples, not as the only acceptable instruments. It should also avoid claiming exact Canadian stock, rental or calibration availability unless JM Test Systems Canada confirms it.

How to Get More Reliable Insulation Test Results

1. De-Energize and Isolate the Equipment

Insulation resistance testing is normally performed on de-energized equipment.

Before testing:

• Follow the approved lockout procedure.
• Verify absence of voltage.
• Discharge stored energy.
• Isolate the asset from connected electronics where required.
• Remove or account for surge suppressors, drives, sensors and connected loads.
• Follow the equipment manufacturer’s instructions.

CSA Z462:24 adds that absence of voltage must be verified at each point of work and provides requirements and guidance for safe work procedures, PPE selection and qualified electrical workers.

2. Use the Right Tester

Choose a tester with:

• Correct output voltage
• Correct resistance range
• Suitable category rating
• Guard terminal where needed
• Noise rejection for substations or industrial sites
• PI/DAR/SV/ramp functions where required
• Data storage where trend records matter

3. Record Test Conditions

At minimum, record:

• Asset ID
• Test date
• Test voltage
• Test duration
• Connection points
• Temperature
• Humidity where practical
• Tester model and serial number
• Tester calibration status
• Results by phase or conductor
• Any correction factors used
• Notes about contamination, moisture or repairs

Fluke recommends recording insulation resistance values, timestamps, test voltage, test duration and temperature compensation for useful trending.

4. Compare Like With Like

Do not compare results unless the conditions are comparable.

Try to keep consistent:

• Test voltage
• Test duration
• Connection method
• Asset condition
• Temperature correction
• Test points
• Equipment configuration

5. Trend the Results

A single high reading is not always proof of long-term health.

Trend data is stronger because it shows direction.

Look for:

• Gradual decline
• Sudden drop
• Phase-to-phase differences
• Readings that do not rise during timed testing
• Results that worsen after moisture exposure
• Results that vary strongly with temperature
• Significant changes after maintenance or repair

6. Discharge After Testing

Insulated equipment can retain charge after a DC insulation test.

Always allow the tester or approved grounding method to discharge the asset before disconnecting leads or touching terminals.

This is especially important for long cables, large motors, generators and other high-capacitance equipment.

Common Interpretation Mistakes

Treating One Reading as the Whole Story

One insulation resistance number is useful, but it does not tell the full story.

Trend, temperature, humidity, test voltage and equipment history matter.

Ignoring Temperature

A cold asset may test higher than a warm asset.

Without temperature correction or notes, the trend may be misleading.

Testing Connected Electronics

High DC test voltages can damage drives, sensors, surge protection, electronics or connected loads.

Follow the equipment procedure before testing.

Using the Wrong Test Voltage

A test voltage that is too low may miss a defect. A test voltage that is too high may stress the insulation unnecessarily.

Not Waiting Long Enough

Large or high-capacitance equipment may need a timed test rather than a quick spot reading.

Misusing PI on the Wrong Equipment

PI is useful, but not universal.

Megger notes that some materials, including certain oil-filled equipment, may show PI values close to 1 because their insulation behaviour differs from solid insulation systems.

Assuming Calibration Claims Apply in Canada

The original US article says JM Test provides NIST-traceable and A2LA ISO/IEC 17025-accredited calibration services. For the Canadian page, do not copy this as a blanket Canadian claim unless local capability and scope are confirmed.

Canadian Safety Considerations

Insulation testing can involve high DC test voltage, stored charge and work near electrical equipment.

Canadian teams should consider:

• Employer electrical safety programme
• CSA Z462 workplace electrical safety practices
• Lockout and hazardous-energy control
• Absence-of-voltage verification at each point of work
• Qualified-worker requirements
• Shock and arc-flash risk assessment
• PPE selection
• Test equipment category rating
• Discharge and grounding after test
• Manufacturer instructions
• Site-specific procedures
• Provincial, territorial or federal workplace requirements

CSA Z462:24 specifies requirements and guidance for electrical safety management systems, safe work procedures, PPE selection and qualified electrical worker criteria. It is harmonized with the Canadian Electrical Code, CSA Z460 and CSA M421 for mines.

This article should be treated as educational content, not a complete field procedure.

When to Perform Insulation Resistance Testing

Insulation resistance testing may be appropriate:

• During commissioning
• Before first energization
• During planned maintenance
• After repairs
• After flooding or moisture exposure
• After equipment trips
• After cable faults
• After motor rewind or repair
• After storage
• Before seasonal startup
• Before returning spare motors or cables to service
• As part of a predictive maintenance programme

Fluke’s application note identifies installation testing as a way to help protect against miswired or defective equipment before service and maintenance testing as a way to identify deterioration that can lead to failures.

Choosing Insulation Test Equipment

Before selecting or renting an insulation tester, confirm:

• Asset type
• Equipment voltage rating
• Required test voltage
• Expected resistance range
• Need for PI, DAR, DD, SV or ramp test
• Noise level of the environment
• Need for guard terminal
• Need for data logging
• Need for remote operation
• Category rating
• Battery requirements
• Outdoor or indoor use
• Calibration status
• Included leads and accessories
• Canadian availability
• Rental or purchase requirements

The source JM Test page states that JM Test sells and rents Megger MIT525 and MIT1025 units in the US context. For the Canadian page, confirm exact Canadian product availability, rental inventory, accessories and calibration documents before making firm service claims.

Practical Takeaway

Electrical insulation does not usually fail without warning.

It often degrades because of heat, moisture, contamination, electrical stress, mechanical damage, vibration, chemicals and age. These factors can work together until insulation that once behaved like a strong barrier begins to behave like a partial conductor.

Insulation resistance testing helps identify that change.

The best testing programme uses:

• Correct test voltage
• De-energized and isolated equipment
• Qualified workers
• Proper safety controls
• Consistent test methods
• Temperature and humidity records
• Trend history
• PI, DAR, step voltage or ramp tests where useful
• Post-test discharge
• Clear documentation

JM Test Systems Canada can support electrical teams with insulation resistance testers, megohmmeters, high-voltage test equipment, rentals, calibration support and related electrical safety equipment where available. Confirm exact Canadian inventory, accessories, calibration documentation and service scope before publishing firm claims.