Every electrical system depends on insulation.

The conductor carries current. The insulation keeps that current where it belongs.

You see this in motors, generators, transformers, switchgear, cables, control circuits, distribution equipment and industrial machinery.

When insulation is healthy, it has high resistance to current. When insulation starts to deteriorate, more leakage current can flow through the insulation or to ground.

That is the basic idea behind electrical insulation testing.

The original JM Test article explains this clearly: good insulation means a relatively high resistance to current, and regular insulation resistance measurements can help show trends toward deterioration. 

For Canadian electrical contractors, maintenance teams, commissioning crews, utilities and industrial plants, insulation testing is not only a troubleshooting step. It is a way to catch problems before they become faults, trips, equipment damage or safety hazards.

What Is Electrical Insulation?

Electrical insulation is the material that surrounds or separates conductors to resist current flow.

It may be found in:

• Cables
• Motors
• Generators
• Transformers
• Switchgear
• Bus systems
• Control wiring
• Windings
• Terminal assemblies
• Electrical panels
• Medium-voltage equipment
• High-voltage equipment

The conductor is designed to carry current.

The insulation is designed to resist current.

The source article compares this to a water pipe: the pipe carries water where it should go, and a leak wastes water and reduces pressure. In an electrical system, deteriorated insulation allows leakage current to flow where it should not. 

What Does “Good” Insulation Mean?

Good insulation has high resistance.

That does not mean infinite resistance.

No insulation is perfect. A very small amount of current may still flow through or across the insulation. The source article notes that this leakage current may be only a microampere, but it is the basis of insulation resistance testing. 

The practical question is not:

“Is there absolutely zero leakage?”

The practical question is:

“Is the insulation resistance high enough, stable enough and trending in the right direction for this equipment and application?”

That is why insulation testing is most useful when results are recorded over time.

A single reading can help.

A trend is better.

How Ohm’s Law Applies to Insulation Testing

The source article uses Ohm’s law to explain 

E = I × R

Where:

• E = voltage
• I = current
• R = resistance

An insulation tester applies a known DC test voltage and measures the small leakage current that flows. From that, it calculates resistance.

If the leakage current is low, the insulation resistance is high.

If the leakage current is high, the insulation resistance is low.

That is the core idea.

The tester is not simply checking whether the circuit is open or closed. It is measuring how strongly the insulation resists current when a test voltage is applied.

Why Insulation Resistance Changes Over Time

Insulation can deteriorate because of operating conditions, environment and age.

Common causes include:

• Moisture
• Dirt
• Dust
• Oil
• Chemical contamination
• Heat
• Thermal cycling
• Vibration
• Mechanical damage
• Cable pulling damage
• Overvoltage
• Partial discharge
• Ageing
• Poor storage
• Poor installation
• UV exposure
• Corrosion
• Conductive tracking
• Loose terminations
• Water ingress

As insulation deteriorates, leakage current can increase.

That usually means insulation resistance decreases.

The drop may happen slowly over months or years, or quickly after a failure event, water ingress, overheating or physical damage.

What an Insulation Resistance Tester Does

An insulation resistance tester, often called a megohmmeter or “megger,” applies a DC test voltage and measures resistance in ohms, megohms, gigohms or teraohms depending on the instrument and test object.

Megger explains that an insulation tester provides a direct reading of insulation resistance and that insulating material can store energy like a dielectric in a capacitor, so the equipment must be discharged after testing for safety. 

A typical insulation tester may be used on:

• Motors
• Generators
• Cables
• Transformers
• Switchgear
• Bus ducts
• Heaters
• Control circuits
• Electrical panels
• Rotating equipment
• Medium-voltage assets

The test helps identify whether insulation is dry, clean and intact enough for the intended service.

Why Insulation Testing Matters

Insulation resistance testing can help find problems before energization or before failure.

It is commonly used during:

• Commissioning
• Preventive maintenance
• Troubleshooting
• Shutdowns
• Motor testing
• Cable testing
• Transformer testing
• Switchgear maintenance
• Post-repair verification
• Post-flood or moisture checks
• Baseline testing on new equipment

It can help identify:

• Moisture in insulation
• Contamination
• Cable damage
• Winding deterioration
• Ground faults
• Low-resistance paths
• Installation damage
• Trending degradation
• Equipment that should not be energized yet

The source article’s main point is simple: insulation should resist current, and measuring that resistance can help show whether the insulation is still doing its job. 

Spot Insulation Resistance Test

A spot test is a single insulation resistance reading taken after a defined test time.

Common examples include:

• 30-second reading
• 60-second reading
• 1-minute reading

A spot test is useful for quick checks, but it has limits.

One reading can be affected by:

• Temperature
• Humidity
• Surface contamination
• Test voltage
• Test duration
• Equipment capacitance
• Previous energization
• Test connection
• Operator setup

A single reading should not be interpreted in isolation unless the procedure, acceptance criteria and equipment condition are clear.

For maintenance, repeated readings taken under similar conditions are more valuable.

Time-Resistance Testing

Time-resistance testing records insulation resistance over time while the test voltage is applied.

Healthy insulation often shows resistance increasing during the test as charging and absorption currents settle.

Poor insulation may show little improvement or may remain low because leakage current dominates.

This is where tests such as PI and DAR become useful.

Polarization Index, or PI

Polarization Index is a timed ratio test.

It commonly compares the 10-minute insulation resistance reading to the 1-minute reading.

Megger describes PI as a well-known time-resistance method and notes that, in general, a low PI ratio indicates little change and poorer insulation, while a higher ratio indicates better insulation behaviour, although some insulation materials show little dielectric absorption by nature. 

PI is often used on motors, generators, transformers and larger equipment where a longer test provides more information than a quick spot reading.

PI should still be interpreted against the equipment type, insulation system, temperature, history and applicable procedure.

Dielectric Absorption Ratio, or DAR

DAR is another timed ratio test.

Megger explains that DAR typically compares insulation resistance after 30 seconds and 60 seconds of applied test voltage. 

DAR is shorter than PI, so it can be useful when a 10-minute test is not practical.

However, DAR is not a universal pass/fail answer for every asset. Like PI, it should be interpreted in context.

Step Voltage and Ramp Testing

Step voltage and ramp testing apply increasing voltage levels to look for insulation weaknesses that may not appear during a simple spot test.

These tests can be useful for identifying defects such as cracks, pinholes or weak insulation behaviour.

The source article highlights Megger MIT525 and MIT1025 testers, which include PI, DAR, DD, step voltage and ramp test capability. 

These diagnostic tests should be used by qualified workers under an approved procedure. The wrong test voltage or method can stress already weakened insulation.

Dielectric Discharge, or DD

Dielectric discharge testing is used on some insulation systems to evaluate discharge current after the test voltage is removed.

It can be helpful for certain multi-layer insulation systems and larger assets.

The important safety point is that insulation can store charge.

After a DC insulation test, the equipment must be discharged properly before it is handled or returned to service. Megger notes that stored energy in insulation must be released at the end of a test and that dielectric absorption energy may require longer discharge time than simple capacitance charging current. 

Choosing the Right Test Voltage

Do not choose the test voltage casually.

The correct voltage depends on:

• Equipment type
• Rated voltage
• Insulation system
• Manufacturer instructions
• Asset condition
• Test purpose
• Site procedure
• Applicable standard or specification
• Whether electronics or surge protection devices are connected
• Whether the asset is low-voltage, medium-voltage or high-voltage

Too low a test voltage may not reveal the issue.

Too high a test voltage may damage weak insulation or connected electronics.

Always isolate sensitive components and follow the manufacturer’s instructions and site procedure before testing.

Safety Before Insulation Testing

Insulation testing can involve hazardous voltage, stored charge and equipment that may not be fully isolated.

Before testing, confirm:

• The work is authorized.
• The worker is qualified.
• The equipment is isolated where required.
• Lockout is applied where required.
• Absence of voltage has been verified.
• The test instrument is suitable and rated for the task.
• Test leads are in good condition.
• The correct test voltage is selected.
• Sensitive electronics are disconnected or protected.
• The area is controlled.
• The equipment will be discharged after testing.
• Results will be documented.

CSA Z462:24 added that absence of voltage must be verified at each point of work when establishing an electrically safe work condition. It also reorganized requirements around electrically safe work conditions and updated several electrical hazard and PPE sections. 

CCOHS also states that only qualified individuals should work on or maintain electrical equipment and that the Canadian Electrical Code serves as the standard for safe electrical installation across Canada, with OHS legislation also covering lockout and PPE requirements. 

Canadian Electrical Safety Context

For Canadian workplaces, insulation testing should be framed around:

• CSA Z462
• CSA C22.1 Canadian Electrical Code where applicable
• Provincial or territorial rules
• Federal OHS requirements where applicable
• Employer electrical safety programme
• Owner procedures
• Manufacturer instructions
• Authority having jurisdiction requirements
• Project specifications
• Qualified-worker requirements

Do not frame OSHA or NFPA 70E as the governing Canadian requirements.

They may be useful technical references for some organizations, but Canadian sites should confirm the applicable Canadian standard, jurisdiction and site procedure.

What Equipment Can Be Tested?

Insulation resistance testing may be used on many assets, including:

Motors

Testing can help identify winding insulation issues, moisture, contamination or ground faults.

Generators

Large rotating machines often use insulation resistance, PI and other diagnostic tests as part of condition assessment.

Cables

Testing can help find damaged insulation, moisture ingress, installation damage or deteriorated cable condition.

Transformers

Insulation testing may be part of transformer maintenance or commissioning, depending on the test plan.

Switchgear

Insulation resistance testing can support commissioning, maintenance and troubleshooting of bus, breakers, compartments and related insulation systems.

Heaters and Industrial Equipment

Heating elements and process equipment may develop leakage paths due to moisture, contamination or insulation breakdown.

What Makes a Reading “Good”?

There is no single insulation resistance number that is good for every asset.

A good reading depends on:

• Equipment voltage
• Equipment type
• Insulation material
• Temperature
• Humidity
• Test voltage
• Test duration
• Equipment age
• Manufacturer recommendation
• Site procedure
• Historical results
• Industry standard or project requirement

A high reading is generally better than a low reading.

But trend matters.

For example:

• A motor that has stayed stable for years is easier to trust.
• A motor that has dropped sharply since the last test needs attention.
• A wet cable may improve after drying.
• A contaminated surface may improve after cleaning.
• A weak insulation system may continue declining even if it still passes a minimum value today.

The source article’s recommendation to take regular measurements and watch the trend is the right practical takeaway. 

Why Temperature Matters

Insulation resistance changes with temperature.

A reading taken on cold equipment may not match a reading taken on hot equipment.

For trending, try to compare results taken under similar conditions or apply the required temperature correction method if the procedure calls for it.

Always record test conditions.

Useful records may include:

• Ambient temperature
• Equipment temperature
• Humidity
• Test voltage
• Test duration
• Asset condition
• Connection points
• Equipment status
• Recent cleaning or drying
• Test instrument used
• Technician notes

Without this context, future teams may misread the trend.

Why Moisture and Contamination Matter

Moisture and contamination can lower insulation resistance.

This is common after:

• Outdoor storage
• Flooding
• Washdown
• Condensation
• Cable damage
• Poor sealing
• Dust buildup
• Oil contamination
• Chemical exposure
• Long shutdowns
• Equipment left open during construction

If the reading is low, the answer is not always “replace the equipment.”

The team may need to inspect, clean, dry, repair or investigate further.

The correct action depends on the asset and risk.

Insulation Testing During Commissioning

Commissioning is one of the best times to perform insulation testing.

It helps confirm that equipment has not been damaged during:

• Shipping
• Storage
• Installation
• Cable pulling
• Termination
• Construction activity
• Weather exposure
• Field modifications

A commissioning insulation test can also create the first baseline record.

That baseline is useful for future maintenance because the team can compare later readings against the original condition.

Insulation Testing During Preventive Maintenance

For maintenance teams, insulation testing is most useful when it is part of a routine programme.

A good preventive maintenance process may include:

• Asset list
• Test interval
• Standard test voltage
• Defined connection points
• Test duration
• Temperature and humidity record
• Pass/fail criteria where applicable
• Trend review
• Follow-up actions
• Calibration status of test equipment
• Test reports stored by asset

A random test result is useful for troubleshooting.

A consistent history is useful for reliability.

What to Record in an Insulation Test Report

A useful report should include:

• Customer or site name
• Asset ID
• Equipment type
• Manufacturer
• Model or serial number
• Rated voltage
• Test voltage
• Test duration
• Test connection points
• Insulation resistance reading
• PI or DAR result if performed
• Temperature
• Humidity where relevant
• Test date
• Technician
• Test instrument make and model
• Test instrument serial number
• Calibration status
• Pass/fail result where applicable
• Notes about cleaning, drying or repair
• Recommended follow-up

Reports should make future comparison easy.

If the next technician cannot tell how the previous test was done, the trend becomes less useful.

Test Instrument Selection

Choose the insulation tester based on the application.

Consider:

• Test voltage range
• Resistance measurement range
• CAT safety rating
• Data logging
• PI and DAR capability
• Step voltage capability
• Ramp test capability
• Guard terminal
• Lead quality
• Battery life
• Display readability
• Environmental rating
• Download/reporting software
• Calibration certificate
• Accessories
• Rental or purchase availability

The source article lists Megger MIT525 and MIT1025 features including PI, DAR, DD, step voltage and ramp tests, CAT IV 600 V safety rating on all terminals, memory with time/date stamp, and resistance measurement up to 10 TΩ for 5 kV models and 20 TΩ for the 10 kV model. 

For Canadian use, confirm the exact model, voltage rating, accessories, calibration certificate and availability before booking.

JM Test Systems Canada Support

JM Test Canada’s rental page lists a Fluke 1587FC Insulation Tester as a featured rental item and states that its rental division includes electrical test equipment, with rentals including applicable accessories and calibration certificates. 

JM Test Canada’s calibration services page lists electrical test equipment calibration services including insulation testers / Megger calibration, hipot testers, low-ohm resistance testers, earth ground resistance testers, circuit breaker test sets and other electrical test equipment. 

For the Canadian page, keep exact product and service claims conditional.

Customers should confirm:

• Insulation tester rental availability
• Test voltage range
• Lead and accessory kit
• Current calibration certificate
• Model number
• Data logging requirements
• Safety rating
• Shipping timeline
• Calibration service availability
• Turnaround time
• Certificate type

Common Insulation Testing Mistakes

Testing Without Verifying Isolation

Insulation testing should not be started until the equipment is correctly isolated and safe to test.

Using the Wrong Test Voltage

The wrong test voltage can produce misleading results or damage sensitive equipment.

Forgetting Stored Charge

Insulation can store energy during a DC test. The equipment must be safely discharged after testing. 

Comparing Readings Taken Under Different Conditions

Temperature, humidity and test duration can affect readings.

Treating One Reading as the Whole Story

A single reading is useful, but trend data is better.

Testing Through Connected Electronics

Drives, surge protection devices, control boards and instrumentation may need to be disconnected or protected before insulation testing.

Ignoring Low but Improving Readings

A drying asset may show improving results. Document the trend and follow the approved procedure.

Ignoring High but Declining Readings

A high reading that drops significantly over time may still deserve attention.

Using Uncalibrated Test Equipment

The test result depends on the tester. Confirm calibration status before relying on the reading.

Practical Takeaway

Good insulation resists current.

Insulation testing helps verify whether cables, motors, transformers, switchgear and other equipment still have enough resistance to leakage current for the intended service.

The original JM Test article explains that no insulation is perfect, but good insulation has relatively high resistance, and regular measurements help identify deterioration trends. 

For Canadian teams, the best approach is to:

• Use the correct insulation tester.
• Select the correct test voltage.
• Follow manufacturer and site procedures.
• Work under CSA Z462-informed electrical safety practices.
• Verify absence of voltage where required.
• Discharge equipment after DC testing.
• Record test conditions.
• Trend results over time.
• Confirm test equipment calibration.
• Use qualified workers.

JM Test Systems Canada can support insulation testing work with electrical test equipment rentals and calibration services where available. Confirm current Canadian availability, accessories, calibration certificates, service scope and shipping timeline before booking.