An insulation resistance tester (often called a “Megger”) is an essential instrument for checking the insulation condition of electrical equipment and wiring, helping you detect leakage current and insulation deterioration early.
It is widely used for inspection of motors, switchboards, and cables—making it a core tool for preventive maintenance and electrical safety management.
This article explains the measurement principle, step-by-step test procedure, and key characteristics of major Japanese manufacturers.
What Is an Insulation Resistance Tester (Megger)?
An insulation resistance tester (insulation tester) is an electrical measuring instrument used to evaluate the insulation condition of conductors, cables, and electrical equipment.
It temporarily applies a high DC test voltage (e.g., 250 V / 500 V / 1000 V) and measures the very small leakage current. The result is displayed as an insulation resistance value (MΩ).
If the insulation resistance is low, it indicates a higher risk of insulation degradation and potential leakage (ground fault).
Main Applications
- Insulation diagnosis for motors, transformers, and cables
- Periodic inspection of distribution boards, control panels, and breaker panels
- Commissioning / acceptance tests for buildings and equipment
- Electrical maintenance and safety verification work
Basic Measurement Principle
An insulation resistance tester contains an internal high-voltage generation circuit. It applies a test voltage to the object under test and detects the leakage current through the insulation.
Using Ohm’s law (R = V / I), the instrument calculates the insulation resistance (MΩ) from the applied voltage and measured current.
In general, a high resistance value (e.g., 100 MΩ or higher) indicates good insulation, while a low value (around 1 MΩ or less) suggests deterioration or leakage risk.
How to Choose the Test Voltage
| Typical Application | Test Voltage | General Guideline |
|---|---|---|
| Low-voltage circuits (100–200 V) | 250 V range | 0.1 MΩ or higher |
| Power circuits / motors | 500 V range | 0.5 MΩ or higher |
| High-voltage equipment / distribution systems | 1000 V range | 1 MΩ or higher |
Note: The appropriate voltage and acceptance criteria may vary depending on equipment type, standards, and site rules. Always follow your facility’s maintenance standards and the manufacturer’s instructions.
Measurement Procedure
- Switch off the power and confirm the equipment is in a completely de-energized state.
- Remove contamination and moisture from terminals and conductor surfaces as much as possible.
- Connect the test leads and select the appropriate range (250 V / 500 V / 1000 V).
- Press the test button and read the displayed insulation resistance value.
- Record the result. If it is below your threshold, perform re-checks and corrective actions (cleaning, drying, repair, etc.).
Important Safety Notes
- Always isolate the power supply before testing to prevent electric shock.
- After testing, the equipment may hold a residual charge. Discharge it before touching terminals or wiring.
- High humidity can reduce measured values. Keep test conditions consistent when comparing results.
- Avoid accidental contact with metal enclosures or protective earth points during setup.
How to Interpret Results (Typical Guidelines)
For many motors and low-voltage devices, the following rough criteria are often used:
- 10 MΩ or higher … Insulation condition is good
- 1–10 MΩ … Insulation is declining; monitor closely
- Below 1 MΩ … Possible deterioration or leakage risk
A best practice is trend management—performing periodic tests and comparing changes over time. This helps you detect abnormal degradation early, before a sudden failure occurs.
Common Abnormalities: Causes and Countermeasures
| Symptom | Likely Causes | Recommended Actions |
|---|---|---|
| Low resistance value | Insulation aging, contamination, moisture | Clean, dry, then re-test |
| Unstable readings | Poor contact, damaged leads | Check terminals, replace leads |
| Display fluctuates during test | Noise, external voltage present | Reconfirm power isolation, check grounding and test environment |
Major Manufacturers and Representative Models
HIOKI (Hioki E.E. Corporation)
A Japanese manufacturer known for high accuracy and reliability. Strong in digital display clarity and safety features, with many models suitable for both field and lab use.
- IR4057-50: Bluetooth enabled—supports smartphone-based result management.
- IR4053: Basic model with LED pass/fail indication.
- Search HIOKI insulation testers on Amazon
KYORITSU (Kyoritsu Electrical Instruments Works, Ltd.)
Highly regarded for rugged housing and durability—well suited for harsh field conditions and maintenance work.
- KEW3126A: Supports 500 V / 1000 V—suitable for insulation testing of higher-voltage equipment.
- KEW6016: Multi-function unit with additional tester capabilities.
- Search KYORITSU insulation testers on Rakuten
SANWA (Sanwa Electric Instrument Co., Ltd.)
Offers many compact, cost-effective models. Popular not only for maintenance work but also for DIY and educational use.
- PM3: Lightweight handheld type—easy to carry on site.
- MG1000: Full-featured model supporting up to 1000 V.
- Search SANWA insulation testers on Yahoo! Shopping
YOKOGAWA (Yokogawa Electric Corporation)
A globally recognized industrial instrumentation manufacturer. Offers many high-reliability, high-accuracy models, including options suitable for industrial and research applications.
- MY40: Simple operation with a durable design.
- MY60: High-accuracy model suitable for more demanding measurement work.
- Search YOKOGAWA insulation testers on Amazon
Useful Accessories for Inspection and Diagnostics
- Search insulation testers on Amazon
- Search insulation testers (Megger) on Rakuten
- Search insulation testers on Yahoo! Shopping
Q&A
Q. If the insulation resistance is low, should I replace the equipment immediately?
A. Not necessarily. Insulation resistance can drop temporarily due to moisture or surface contamination. Re-testing after cleaning and drying is often effective before deciding on repair or replacement.
Q. Which is better: analog or digital?
A. Digital models are the standard for field work today because they provide clear readings and often include safety and judgment functions. Analog models can be useful because trends and needle movement make changes easier to “see” at a glance.
Q. Can I manage test results as data?
A. Yes. If you choose a model with Bluetooth or USB output (available from brands such as HIOKI and KYORITSU), you can store results on a smartphone or PC for reporting and trend analysis.
