How to Read and Understand Insulation Tester Results
Quick answer: To read insulation test results, look at the megohm (MΩ) value on your insulation tester. A higher reading means better insulation resistance. For most low-voltage circuits, 1 MΩ is the minimum, while healthy motors and cables usually read well above that. Always compare the result with the test voltage, the temperature, and previous readings before you call it good or bad.
An insulation resistance test (also called a megger test or megohmmeter test) is one of the most important checks in electrical safety testing. The number on the display only helps if you know what it means. This guide explains, step by step, how electricians, maintenance engineers, and factory teams in Bangladesh can read and understand insulation tester results.
In this guide:
- What an insulation tester measures
- Units and what the display shows
- Minimum acceptable values
- How to read results step by step
- Reading PI and DAR
- Temperature and humidity effects
- Why readings are low and what to do
- Common mistakes
- FAQs
What Does an Insulation Tester Measure?
An insulation tester (megohmmeter) applies a high DC voltage (such as 250 V, 500 V, or 1000 V) across the insulation of a cable, motor winding, transformer, or panel. It then measures the tiny leakage current that flows through or along that insulation.
The tester uses Ohm’s law (R = V ÷ I) and shows the answer as insulation resistance in megohms (MΩ) or gigohms (GΩ).
- High resistance means little current is leaking, so the insulation is healthy.
- Low resistance means current is escaping through moisture, dirt, heat damage, or aging.
A good result means the equipment is safe to energize. A poor result warns you about electric shock, short circuits, equipment failure, or fire before they happen.
What Units Will You See on the Display?
| Unit | Meaning | Typical use |
|---|---|---|
| kΩ | 1,000 ohms | Very poor or wet insulation |
| MΩ (megohm) | 1,000,000 ohms | Most field readings |
| GΩ (gigohm) | 1,000 MΩ | New cables, dry motors, clean equipment |
| OL / ∞ / > | Over range | Reading is higher than the tester can display, which usually means very good insulation |
Beginners often panic at “OL” or “>4000 MΩ”. It is not an error, it is normally a good sign. For more on meter markings, see our guide to understanding multimeter symbols and their meanings.
What Is a Good Insulation Resistance Value?
The accepted minimum depends on the circuit voltage and the equipment type.
Low-voltage wiring (based on IEC 60364 practice)
| Circuit nominal voltage | Test voltage (DC) | Minimum insulation resistance |
|---|---|---|
| SELV / PELV (extra-low voltage) | 250 V | 0.5 MΩ |
| Up to 500 V (most building and factory circuits) | 500 V | 1 MΩ |
| Above 500 V, up to 1000 V | 1000 V | 1 MΩ |
Rule-of-thumb condition guide
| Reading (MΩ) | Condition | Action |
|---|---|---|
| Below 1 MΩ | Poor / unsafe | Do not energize; investigate |
| 1 – 5 MΩ | Marginal | Clean, dry, retest, monitor |
| 5 – 100 MΩ | Acceptable | Record and trend |
| Above 100 MΩ | Good | Healthy insulation |
Rotating machines (motors and generators)
A widely used rule is 1 MΩ per kV of rated voltage, plus 1 MΩ. Under IEEE 43 guidance, random-wound motors should typically read at least 5 MΩ, and form-wound windings at least 100 MΩ, when corrected to 40°C.
Important: These are general guides. Always follow the manufacturer’s data sheet, the applicable standard, and your site’s own maintenance procedure.
How to Read Insulation Test Results: Step by Step
Step 1: Confirm the setup was correct
A reading is only meaningful if the test was done properly. Check that the equipment was de-energized, isolated, and discharged, that the correct test voltage was selected, and that the leads were connected properly (line to conductor, earth to frame or ground).
Step 2: Note the test voltage
The same equipment can read differently at 250 V and 1000 V. Always record the test voltage with the result.
Step 3: Wait for the reading to stabilize
When the test starts, the number often climbs for several seconds as the insulation charges up. Read the value at a fixed time, normally 60 seconds, so results stay comparable.
Step 4: Compare against the minimum
Match the stabilized value to the tables above or to your equipment’s rated minimum.
Step 5: Compare against previous results
This is the step most people skip. A single reading is a snapshot, but a trend tells the real story. A motor that reads 500 MΩ one year and 80 MΩ the next is degrading, even though 80 MΩ still looks “good”.
Step 6: Correct for temperature
Insulation resistance changes with temperature (details below). Compare results only after correcting to a common reference temperature, or at least test at similar temperatures each time.
Step 7: Record everything
Write down the date, equipment ID, test voltage, duration, temperature, humidity, and result. A good log makes trend analysis possible.
Understanding the Three Main Test Methods
1. Spot Reading Test (Short-Time Test)
A single reading taken at a fixed time, usually 1 minute. It is fast and simple, but it varies with temperature and surface moisture, so use it alongside historical data.
2. Dielectric Absorption Ratio (DAR)
DAR = resistance at 60 seconds ÷ resistance at 30 seconds
| DAR value | Insulation condition |
|---|---|
| Below 1.0 | Poor |
| 1.0 – 1.25 | Questionable |
| 1.25 – 1.6 | Acceptable to good |
| Above 1.6 | Excellent |
DAR is useful when you don’t have time for a full 10-minute test.
3. Polarization Index (PI)
PI = resistance at 10 minutes ÷ resistance at 1 minute
| PI value | Insulation condition |
|---|---|
| Below 1.0 | Dangerous |
| 1.0 – 2.0 | Questionable |
| 2.0 – 4.0 | Good |
| Above 4.0 | Excellent |
Healthy insulation keeps absorbing charge, so the resistance keeps rising during the test. Wet or contaminated insulation gives a flat curve, because leakage current dominates. A flat or falling curve is a warning sign.
Note: If the 1-minute reading is already very high (several GΩ), the PI may not be meaningful. Trust the absolute value instead.
Bonus: Step Voltage Test
You test at two or more increasing voltages. Healthy insulation gives roughly the same resistance at every step. A significant drop at higher voltage suggests weak spots, cracks, or aging.
How Temperature and Humidity Change Your Results
This is where Bangladesh conditions matter most.
Temperature rule: Insulation resistance roughly halves for every 10°C rise in temperature. A hot motor straight after running will read lower than the same motor when cold, and that does not necessarily mean it is faulty.
Humidity in Bangladesh: During the monsoon, humidity in Dhaka, Chattogram, Narayanganj, and Gazipur often stays very high. Moisture settles on windings, terminal boxes, and cable ends, and it lowers readings sharply. Testing during a humid season and comparing to a dry-season baseline can create false alarms.
Best practice for BD facilities:
- Test at a consistent time of day
- Record ambient temperature and humidity each time
- Warm the equipment slightly or dry it before retesting a low reading
- Avoid testing immediately after washing, rain, or condensation
Why Are My Insulation Readings Low? (And What to Do)
| Possible cause | What to do |
|---|---|
| Moisture or condensation | Dry the equipment (gentle heat, ventilation), then retest |
| Dust, oil, or carbon dirt | Clean terminals and winding surfaces, then retest |
| Aged or overheated insulation | Plan rewinding or replacement |
| Damaged cable (cuts, crushing, rodent bites) | Locate and repair, or replace the cable |
| Connected loads or surge protectors | Disconnect parallel devices that distort the reading |
| Wrong test voltage or poor lead contact | Re-check settings and connections |
If a motor reads low, isolate the cable from the motor and test each separately. This quickly shows whether the fault is in the cable or in the winding.
Common Mistakes When Reading Insulation Tester Results
- Not discharging the equipment after testing, which is both unsafe and distorts later readings
- Using the wrong test voltage for the equipment rating
- Ignoring temperature when comparing readings
- Relying on one reading instead of tracking trends
- Testing live circuits, which can damage the tester and injure the user
- Assuming OL means a faulty meter, when it normally means excellent insulation
- Not logging results, so there is nothing to compare next time
Insulation testing is one part of a larger safety routine. For the full workflow, read our guide on how to perform electrical safety testing in industrial facilities. If you also test grounding, see our article on the best earth resistance tester for electrical safety.
Choosing the Right Insulation Tester for Accurate Readings
Reliable results start with a reliable instrument. When choosing an insulation tester, look for:
- Multiple test voltage ranges (for example 50 V to 1000 V) so one tester covers many jobs
- Wide measuring range into the GΩ region
- Live-circuit warning and auto-discharge for safety
- Clear display and stable readings
- Proper calibration and local service support
At Power Tech Electric, we are the exclusive authorized distributor of HIOKI and Sanwa (Japan) in Bangladesh. We offer HIOKI insulation testers and megohmmeters and a range of Sanwa insulation testers for electricians, factories, and maintenance teams. You can also browse the wider range of HIOKI testers and digital multimeters.
Want to know how one popular model is used in the field? Read what the Hioki IR4057-50 insulation tester is used for. If you are ready to buy, see the IR4057-50 price in Bangladesh and our guide to buying a genuine Hioki IR4057-50 in Bangladesh so you avoid counterfeit units.
Explore our complete product range to find the right tool for your work.
Frequently Asked Questions
What is a good insulation resistance reading?
For most low-voltage circuits, at least 1 MΩ at 500 V DC is the minimum. Healthy equipment commonly reads tens to thousands of MΩ. Compare with the manufacturer’s data and your own past readings.
What does OL mean on an insulation tester?
OL (over limit) means the resistance is higher than the tester’s display range. It usually indicates very good insulation, not a fault.
What is the difference between PI and DAR?
Both compare readings over time. DAR uses 30-second and 60-second readings, while PI uses 1-minute and 10-minute readings. PI gives a deeper assessment of insulation condition.
Why does my insulation resistance change from day to day?
Temperature, humidity, and surface contamination all affect readings. Record these conditions and test under similar conditions each time.
What test voltage should I use?
Use 500 V DC for circuits up to 500 V, 250 V for extra-low-voltage circuits, and 1000 V for higher-rated equipment, following the equipment rating and applicable standard.
How often should insulation resistance be tested?
New installations are tested before energizing. For maintenance, many facilities test once or twice a year, or more often for critical motors and damp environments.
Can I test insulation with a multimeter?
No. A multimeter does not apply enough voltage to reveal insulation weaknesses. Use a dedicated insulation tester.
Final Thoughts
Reading insulation test results comes down to four things: the right test voltage, a stable reading, the correct minimum value, and a comparison with past results after allowing for temperature. Track these consistently and you will catch insulation problems before they cause downtime or accidents.
Need help choosing a tester, or a quote for HIOKI or Sanwa insulation testers in Bangladesh? Contact the Power Tech Electric team for expert guidance and genuine products backed by local service. Visit Power Tech Electric to see more.
Power Tech Electric | Central Electric Market (4th floor), 165 Nawabpur Road, Dhaka-1100 | +88 01819-186441 | sales@powertech.com.bd