Last updated on December 18th, 2025 at 01:37 pm
Use a megohmmeter to measure insulation resistance in the field (non-destructive, trend monitoring). Use a hipot (dielectric withstand) tester to verify insulation breakdown thresholds in manufacturing or safety tests. Choose based on your use case, voltage needs, and test objectives.
Why Use Insulation Testing?
Insulation testing ensures the electrical integrity and safety of cables, wiring, motors, and electrical equipment. Two widely used methods are:
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Megohmmeter testing — measures insulation resistance
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Hipot (high potential) testing — stresses insulation to detect breakdown
Both apply high voltage (AC or DC) to the device under test (DUT) and monitor resulting current leakage.
Megohmmeters: How They Work & When to Use Them
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A modern megohmmeter (aka insulation resistance tester) applies a DC voltage to the DUT and measures the resulting nanoamp to microamp current. Ohm’s Law converts that to a resistance value. -
The device is sometimes called a “megger” (a trademarked brand name).
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For equipment rated ≤ 500V, test voltage is often up to 2× rated voltage. For higher voltage systems, the test max may align nearer to rated voltage.
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Because of capacitance and dielectric absorption within the DUT, readings take time to stabilize—often 60 seconds or more is used for steady insulation resistance (IR) reading.
Techniques & Best Practices
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Stepped voltage tests: increase test voltage in stages. A declining IR trend as voltage rises can signal insulation degradation.
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Trend comparisons: periodic IR measurements at the same voltage and dwell time provide insight into insulation aging.
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DAR & PI:
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DAR (Dielectric Absorption Ratio) = IR at 60s ÷ IR at 30s
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PI (Polarization Index) = IR at 10 min ÷ IR at 1 min
A DAR < 1 or PI < 1.5 often indicates contamination, moisture, or insulation aging.
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Environmental consistency: temperature and humidity affect readings, so perform tests under comparable conditions.
Many megohmmeters today are handheld, support voltages up to 1 kV (or even 15 kV for high-end units), and may combine functions (e.g. DMM + IR).
Hipot Testers: Purpose & Usage
A hipot test (also called dielectric withstand) pushes insulation beyond normal operating voltage to detect weak spots or breakdown points. Common in factory QA, safety qualification, or final inspections.
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The test applies either AC or DC voltage, with leakage current limits (e.g. 0.1–5 mA depending on standard).
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Many standards (e.g. IEC) require test voltage = (2 × operating voltage) + 1,000 V (or equivalent) for a fixed duration (typically 1 minute).
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The test setup often ties one lead to ground and the other to the device’s power leads.
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If the DUT includes EMI filters or Y-capacitors, AC hipot may falsely flag leakage; in such cases, DC testing or adjusted current limits are used.
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Unlike megohmmeters, hipot testers draw more current, may require mains power (not battery), and include “fail” thresholds (i.e. low and high limits) to detect abnormal leakage or open circuit.
Megohmmeter vs. Hipot: Which to Choose?
| Criteria | Megohmmeter | Hipot Tester |
|---|---|---|
| Use Case | Field diagnostics, preventive maintenance | Safety qualification, factory QA, design validation |
| Test Type | Measures insulation resistance | Stresses insulation to detect breakdown |
| Current / Voltage | Low current, safe for insulation | Higher stress current, riskier to insulation |
| Damage Risk | Minimal | Can damage weak insulation if failed |
| Trend Monitoring | Yes | Limited (pass/fail) |
| Standards Role | N/A | Common in regulatory and safety standards |
Broadly, megohmmeters are for ongoing diagnostics, while hipot testers are for pass/fail safety validation.
FAQs
Q: What’s the difference between a megohmmeter and a hipot tester?
A megohmmeter measures insulation resistance by applying DC voltage and measuring leakage current. A hipot tester stresses insulation with higher voltage to verify breakdown thresholds in safety or QA tests.
Q: Can a hipot test damage insulation?
Yes—if insulation is weak, the stress from hipot may cause breakdown. That’s why hipot tests are typically controlled in manufacturing or lab settings, not for routine field use.
Q: What is DAR and PI in insulation testing?
DAR = IR(60s) ÷ IR(30s). PI = IR(10 min) ÷ IR(1 min). Lower values suggest deteriorated insulation (e.g. moisture, contamination, aging).
Q: When should I choose megohmmeter testing over hipot?
Use a megohmmeter for field diagnostics, insulation trends, non-destructive testing. Use a hipot for safety qualification, factory inspection, or troubleshooting design weaknesses.
Q: Do megohmmeter tests follow standards?
Yes—many megohmmeters support standards-based techniques (e.g. DAR, PI) and maintain compliance across industries (industrial, utility, OEM).
Q: What does a megohmmeter measure?
A megohmmeter measures insulation resistance in megohms by applying a DC voltage and detecting leakage current.
Q: What is a hipot test used for?
A hipot test (dielectric withstand) verifies insulation can handle over-voltage stress—used in production and safety testing.
Q: Is hipot safer than megohmmeter testing?
No—hipot testing applies higher stress and can damage weak insulation; it’s not for general field use.
Q: What is polarization index (PI) in insulation testing?
PI = IR at 10 minutes ÷ IR at 1 minute, used to assess insulation cleanliness and moisture.