For large-capacity equipment such as transformers, long cables, generators, etc., a single Insulation Resistance (IR) can only reflect the instantaneous state. Polarization Index (PI) and Dielectric Absorption Ratio (DAR) can determine whether the insulation is affected by moisture or is slowly deteriorating. In addition, temperature can significantly interfere with measurement results.
You don't know how to read the results - the MΩ, GΩ, PI, and DAR displayed on the meter are just a string of numbers to you, and you cannot judge whether the equipment is safe.
This article takes the EM3500 series high-voltage insulation resistance tester as an example to introduce the judgment standards and temperature effects of PI and DAR.
Ⅰ. Which equipment must be tested for PI/DAR?
PI: 110kV and above transformers, long-distance high-voltage cables (several kilometers), large generators
DAR: Transformers and medium-long cables with capacity ≥4000kVA
Generally, only IR can be measured for low-voltage lines or short cables.
Ⅱ. Determination Criteria Cheat Sheet
Polarization Index (PI) = Resistance at 10 min / Resistance at 1 min
PI Value Range | Insulation Condition |
> 4.0 | Excellent |
2.0 ~ 4.0 | Good |
1.0 ~ 2.0 | Abnormal（shorten test cycle） |
< 1.0 | Poor（process promptly） |
Dielectric Absorption Ratio (DAR) = Resistance at 1 min / Resistance at 15 s
DAR Value Range | Insulation Condition |
> 1.4 | Excellent |
1.25 ~ 1.4 | Good |
1.0 ~ 1.25 | Abnormal |
< 1.0 | Poor |
Note: For specific calculation formulas and operation methods, please refer to the official website article "Application of High Voltage Insulation Resistance Tester-2-How to Use the Self-setting Function of Measurement Parameters".
Ⅲ. Effect of temperature on insulation resistance
Insulation resistance decreases as temperature increases. Generally, for every 10°C increase in temperature, the insulation resistance may drop by about half. Therefore, try to measure in an environment close to 20℃~30℃.
If testing at high or low temperatures is necessary, record the temperature at the time of testing and compare with historical data at similar temperatures.
The EM3500 series instrument is equipped with a temperature sensor, which will display the temperature inside the instrument during testing (it does not represent the surface temperature of the object being measured, but it can be used as a reference).
Simple judgment principle:
When the temperature is high, the actual measured value will be lower, but if it is much lower than the standard or historical value, it may be really defective.
The actual measured value is higher when the temperature is low, which does not mean it is absolutely safe. It still needs to be compared with the historical data at the same temperature.
Iridium Electric specializes in the R&D and production of high-voltage insulation resistance testers, including the EM3500, EM3520, 5 kV high-voltage insulation resistance testers, EM3500B, EM3520B, 10 kV high-voltage insulation resistance testers, EM3520C, 15 kV high-voltage insulation resistance testers, and the EM3580 online insulation resistance tester.
ETCRMETER is a high-end brand of ETCR. All products are manufactured by ETCR, which also provides technical support and after-sales service.
For more information, please visit the Yidian Measurement & Control official website: www.etcrmeter.com
10th Floor, Block 5, Liandong Intelligent Manufacture Park, No. 105, Shilong North Road, Nanhai District, Foshan City, Guangdong, China
