If these Two Factors are Ignored, the Insulation Resistance Value could Vary Significantly
In insulation resistance testing procedures, two factors are critically important; overlooking them can significantly impact test results: temperature and humidity.
Variations in temperature affect the resistance performance of insulation materials, for most insulating materials, resistance decreases as temperature increases，this is because increased temperature enhances the material's conductivity, leading to a drop in insulation resistance, thus, recording temperature during insulation resistance testing helps analyze the test results and determine whether they fall within a normal range. Humidity also has a considerable impact on insulation resistance,when ambient humidity is high, moisture can adhere to the surface of insulation materials, creating a conductive path that reduces resistance. In high-humidity environments, insulation resistance values may drop significantly below normal levels. Recording humidity assists in interpreting the results and in assessing whether high ambient humidity is affecting electrical insulation performance.
Why does IEC 60060 specify that preventive tests on power equipment should be conducted at an ambient relative humidity of less than 80%?
When preventive tests are conducted on power equipment such as transformers, motor windings, or cables under high humidity, the measured data often deviates significantly from actual values, this discrepancy mainly arises from two factors: the impact of water films and the distortion of the electric field,when ambient humidity is high, the surface of insulating materials (such as transformer insulation paper, motor winding enamel, or cable outer insulation) may develop condensation or a thin layer of moisture, substantially reducing surface insulation resistance and significantly increasing surface leakage current. Additionally, condensation and water films may cause distortion of the electric field across conductors and insulation materials, making the field distribution more uneven and potentially creating corona discharge, which directly affects measurement results. Therefore, to ensure accurate measurement, insulation resistance testing is typically conducted in environments with an ambient relative humidity below 65%.
Why is it necessary to record temperature during insulation resistance testing of power equipment?
The insulation materials in power equipment often contain some moisture and soluble impurities (such as salts and acidic substances), which can conduct small currents, as temperature increases, the movement of molecules and ions within the insulating material accelerates, and the moisture and impurities move toward the electrodes under the influence of the electric field, thereby increasing the material's conductivity, consequently, as temperature rises, insulation resistance decreases noticeably, and this decrease is often exponential, for instance, when the temperature increases by 10°C, the resistance of Class B insulation in generators may decrease to about 1/2.8 of its original value, and the resistance of Class A insulation in transformers may decrease to approximately 1/1.7 of its original value.
For equipment with significant moisture ingress, insulation resistance fluctuates more significantly with temperature changes,thus, it is essential to record the ambient temperature during insulation resistance testing. If the equipment has just stopped running and has not fully cooled, the actual internal temperature of the insulation should also be recorded, this approach allows for the conversion of insulation resistance to a common temperature, enabling more accurate comparison and analysis of results.
Why does IEC 60060 allow for “user-defined” values for insulation resistance requirements on certain equipment?
Due to differences in climatic conditions, insulation structure and condition, testing methods, and connections in various countries and regions, IEC 60060 does not specify minimum insulation resistance values for most high-voltage equipment. Exceptions are made only for simpler structures and some low-voltage equipment. Since establishing a uniform “required value” for the insulation resistance of most high-voltage power equipment is difficult, IEC 60060 adopts a “user-defined” approach and emphasizes comprehensive analysis and judgment to accurately assess the insulation status of power equipment.
In summary, temperature and humidity fluctuations can directly impact insulation resistance values. Therefore, recording these two parameters during testing provides more accurate data for fault analysis and equipment condition assessment.
