Insulation Resistance Tester

An insulation resistance tester is a specialized instrument used to measure the insulation performance of electrical equipment. It applies a high-voltage DC output to assess whether insulation materials have aged, become damp, or are damaged. Common names include: megohmmeter, hand-crank megohmmeter (referring to the hand-cranked type), insulation resistance meter, megohmmeter, and insulation tester.

Although a multimeter can also measure resistance, its test voltage is very low (typically within 9V), making it unable to simulate the actual insulation conditions of electrical equipment under high voltage. An insulation resistance tester can output DC voltages ranging from 250V to 5000V or even higher, enabling it to detect insulation defects that are only exposed under high voltage.

Insulation materials may appear normal under low voltage, but when high voltage close to their operating voltage is applied, internal micro-cracks, bubbles, or moisture-affected areas will cause breakdown or leakage. Therefore, only high-voltage testing can truly reflect the reliability of the insulation.

The test voltage is generally set at 2 to 3 times the rated voltage of the equipment under test, but must not exceed the equipment’s withstand voltage rating. Common guidelines are as follows:

• Control circuits below 100V: 250V
• 220V/380V low-voltage equipment: 500V or 1000V
• 3kV to 10kV equipment: 2500V or 5000V
• 10kV to 35kV equipment: 5000V or 10000V
• Equipment above 35kV: 10000V to 15000V

For low-voltage equipment, a range of 1 GΩ (1000 MΩ) is generally sufficient. For high-voltage cables, transformers, and similar equipment, a range of 5 TΩ to 10 TΩ or even higher is required. Generally speaking, the higher the test voltage, the greater the required range.

The Damping Ratio (DAR) is the ratio of resistance at 60 seconds to that at 15 seconds, while the Polarization Index (PI) is the ratio of resistance at 10 minutes to that at 1 minute. These two metrics can assess the deep-layer condition of insulation:

• A Damping Ratio greater than 1.3 indicates dry insulation; less than 1.3 may indicate moisture ingress.
• A Polarization Index greater than 2.0 indicates good insulation; less than 1.5 indicates severe aging.
• A standard single measurement only provides the resistance value, whereas the absorption ratio and polarization index can distinguish between two different types of defects: “moisture ingress” and “aging.”

Handheld units are compact, battery-powered, and portable, making them suitable for on-site inspections and routine maintenance. Benchtop units offer more powerful functionality, higher test voltages and ranges, and typically come with data management and analysis software, making them suitable for laboratory or in-depth diagnostic scenarios.

Safety is the top priority. Before operation, you must disconnect the power supply, verify the absence of voltage, and ensure the equipment is fully discharged. Do not touch the equipment under test during the measurement process. After testing, wait for the instrument to complete its automatic discharge, then manually discharge it using a grounding rod. Only disconnect the leads after confirming there is no residual voltage.

Equipment such as cables and transformers possess capacitive properties. They become charged with high voltage during testing and retain a significant amount of charge internally after the test is complete. Removing the connections without discharging may result in electric shock or damage to the instrument. Discharge procedure: First, allow the instrument to discharge automatically (until the voltage on the screen returns to zero), then use a grounding rod to short-circuit the conductor to ground for at least 2 minutes.

This is a normal phenomenon. The capacitive load of the device under test continues to charge during the test, causing the insulation resistance value to rise gradually from a lower value. It typically takes 30 seconds to several minutes for the reading to stabilize. For long cables or high-capacity transformers, the stabilization time is longer. When recording results, use the stabilized reading as the final value.

When the surface of the device under test is damp or dirty, surface leakage current can cause the measured value to be lower than actual. In this case, wrapping a length of bare copper wire around the insulated surface and connecting it to the Guard terminal can eliminate the effect of surface leakage, ensuring that the measurement results reflect the true condition of the internal insulation.

Specialized high-voltage leads with a shielding layer should be used. Ordinary conductors may cause additional leakage during high-resistance measurements due to bending or moisture, which can affect the readings. Test leads should be kept dry and clean, and should not be twisted or knotted.

There is no universally accepted standard; it depends on the type of equipment and voltage rating. Reference values:
Low-voltage motors and power distribution equipment: Greater than 1 MΩ is acceptable; greater than 100 MΩ is good
10 kV power cables: Greater than 10 MΩ is considered acceptable; greater than 1,000 MΩ is considered good
Transformers and generators: Generally require greater than 10 MΩ/kV
A more accurate method of assessment is to compare results with historical data. If a measurement drops by more than 50% compared to previous years, it should be treated with caution even if it remains within the acceptable range.

Insulation resistance decreases as temperature rises, following an exponential relationship. Typically, for every 10°C increase in temperature, insulation resistance decreases by approximately 50%. Therefore, the equipment temperature should be recorded during testing, and temperature compensation should be performed when necessary to avoid misjudgment due to temperature variations.

Possible causes include: different temperatures, different humidity levels, different test voltages, different test times, and different surface cleanliness of the equipment. It is recommended to conduct tests under similar conditions and use relative metrics such as the absorption ratio or polarization index to mitigate some of these influencing factors.

First, check if the battery is low. Replace the battery or recharge it before retrying. If using AC power, check that the power cord is securely plugged in and that the fuse has not blown. If all of the above are normal, there may be an internal circuit failure, and the unit should be returned to the factory for repair.

Possible causes for a reading of 0: short circuit in the device under test, damaged or shorted test leads, or a blown internal fuse. Possible causes for a reading of infinite: disconnected test leads, open circuit in the device under test, or the range setting is too high. First, check the connections, then replace the test leads and retest.

Most insulation testers are equipped with an automatic shutdown feature to conserve power; they typically shut down automatically after 15 minutes of inactivity. If frequent shutdowns interfere with use, you can disable this feature in the menu or check if the battery is low.

Store it in a dry, dust-free environment free of corrosive gases, at temperatures between 0°C and 40°C. During humid seasons, it is recommended to power on the device once a month for approximately one hour to prevent internal moisture damage. Lithium-ion battery models should be charged regularly to avoid damage from over-discharge.

For routine maintenance of conventional power equipment at 110 kV and below, domestic brands such as Guodian Huamei, U-Lead, and Dingsheng Electric offer good value for money, with stable performance and prompt after-sales support. For ultra-high-voltage substations at 220 kV and above, highly interfered environments, or applications requiring in-depth diagnostic capabilities, international brands like Fluke and Megger have advantages in interference resistance and advanced features, though they come at a higher price.

Key parameters include: test voltage ranges, maximum measurement range, short-circuit current, safety rating (CAT III or CAT IV), support for PI/DAR, automatic discharge function, and data storage and upload capabilities. Select the test voltage based on the voltage rating of the equipment being tested, and choose the short-circuit current based on the equipment’s capacity (for long cables and large transformers, models with a short-circuit current of ≥3mA are recommended).

Yes. An insulation resistance tester is a precision measuring instrument; it is recommended to have it calibrated annually to ensure measurement accuracy. The calibration agency should possess CNAS or equivalent accreditation.