An insulation tester — also called a megohmmeter — is one of the most important instruments for electrical contractors and maintenance engineers. But with test voltages from 50V to 5000V and prices varying enormously, choosing the right instrument can be confusing.
What Does an Insulation Tester Actually Measure?
An insulation tester applies a known DC voltage across insulation and measures the resulting current, calculating insulation resistance in megohms or gigaohms. Good insulation resists current flow — you want values in the hundreds of megohms to teraohms range. As insulation degrades due to moisture, heat, mechanical stress or age, resistance drops, eventually leading to equipment damage or electrical shock. A standard multimeter cannot do this job — its ohms function uses only a few volts and cannot stress insulation enough to reveal developing faults.
Step 1: Choose the Right Test Voltage
- 250V: Single-phase equipment, household wiring, small motors
- 500V: Standard industrial equipment, three-phase motors up to 415V, distribution panels — the most common range in India
- 1000V: High-voltage motors, generators, transformers, HV cables
- 2500–5000V: Extra-high voltage equipment, solar PV systems, large transformers
The Hioki IR4056 covers 500V and 1000V and is the standard choice for most industrial applications in India.
Step 2: Do You Need PI and DAR Tests?
The Polarisation Index (PI) and Dielectric Absorption Ratio (DAR) are time-based tests that reveal insulation condition more clearly than a single reading. PI compares resistance at 10 minutes to resistance at 1 minute — values above 2.0 indicate good insulation. DAR compares 60-second to 30-second readings. These tests are essential for motor, generator and transformer maintenance. The Hioki IR4057 includes automatic PI and DAR calculation with memory storage.
Step 3: Solar PV Applications
Testing solar PV systems requires a specialised approach — standard insulation testers with earthed measurement circuits cannot be used safely on ungrounded PV systems. The Hioki IR5051 tests at up to 5000V, measures up to 10 TΩ, and is designed specifically for solar string and array testing without an earth connection.
Recommended Models by Application
| Application | Recommended Model |
|---|---|
| Routine panels and wiring up to 415V | Hioki IR4056 |
| Motor and transformer maintenance with PI/DAR | Hioki IR4057 |
| High-voltage cables and generators | Hioki IR4058 |
| Solar PV systems, string testing | Hioki IR5051 |
Minimum Acceptable Insulation Resistance (500V test)
- Above 100 MΩ: Good condition
- 10–100 MΩ: Caution, investigate further
- Below 10 MΩ: Do not put into service until repaired
- Below 1 MΩ: Immediate action required
Browse our full range of Hioki insulation testers or contact our technical team for help choosing the right model.