Definition of insulation resistance

Insulation resistance (IR) refers to the resistance value between two conductors of dielectric material or equipment at a specific voltage, which reflects the ability of the material to prevent the current from passing through, and is one of the core indicators to evaluate the insulation performance of electrical equipment. High insulation resistance means good insulation performance, can effectively prevent leakage, short circuit, electric shock and other safety hazards, to ensure the stable operation of the power system and personnel and equipment safety. Therefore, understanding the factors that affect insulation resistance is of great significance for optimizing equipment design, selecting materials reasonably and formulating effective maintenance strategies.

Application method and standard of insulation resistance tester

How to use:

1. Preparation work:

   • Device connection: Correctly connect the insulation resistance tester to the device under test to ensure that the wiring is firm and meets the test requirements. Typically, the red test wire (or terminals labeled "+", "L", or "line") is connected to the live part of the device (such as the winding, housing, etc.), and the black test wire (or terminals labeled "-", "E", or "ground") is connected to the ground end of the device or the reference point of the part under test.

   • Power Supply: If it is a battery powered tester, make sure the battery is fully charged. For testers that require an external power supply, connect the power supply and check that the connection is stable.

2. Parameter setting:

   • Test voltage: Select the appropriate test voltage according to the rated voltage and insulation resistance test standard of the device under test. For example, high-voltage equipment may need to use a tester with a voltage class of 500V, 1000V, 2500V, or even higher.

   • Test time: Some test standards may specify a specific test duration, ensuring that the tester's timing function (if any) is set according to the standard.

   • Other Settings: such as test mode (absorption ratio, polarization index, etc.), automatic discharge function, etc., are set according to specific test requirements and equipment functions.

3. Safety measures:

   • Isolating the power supply: Before testing the insulation resistance of the electrical equipment in operation, the power supply must be disconnected to ensure that the equipment is completely unenergized.

   • Personal protection: Wear appropriate personal protective equipment (PPE), such as insulated gloves and shoes, to avoid the risk of electric shock.

   • Equipment discharge: for the equipment that has been powered off, especially high-voltage equipment, manually discharge or use the discharge function of the tester to ensure that the equipment has no residual charge.

4. Measurement process:

   • Startup test: Start the insulation resistance test according to the tester's operating manual. After the measurement begins, the tester applies the selected voltage and displays the insulation resistance value.

   • Observe the reading: Wait for the tester to show a stable reading and record the test result. Pay attention to observe whether there are abnormal conditions, such as excessive fluctuation in reading, equipment heating, etc.

   • Record and compare: Record the measurement data and compare it with the factory value, historical data or standard requirements to determine whether the insulation performance is qualified.

5. Post-test processing:

   • Turn off the test: After completing the measurement, turn off the test function according to the tester operating manual and disconnect the device under test.

   • Discharge: For large capacitance devices, even if the tester has an automatic discharge function, additional discharge should be performed manually after the test to ensure that the device is completely depowered.

Standard:

Standards for insulation resistance testing are usually developed by relevant national or international electrical standards bodies, such as IEC, IEEE, GB, ANSI, etc. The following may be covered in the standard:

• Minimum insulation resistance value: According to different types of electrical equipment (such as high-voltage distribution devices, motors, cables, etc.), the minimum insulation resistance value should be achieved under specific test conditions.

• Test voltage levels: Recommended or specified test voltages for devices with different voltage levels.

• Test conditions: such as ambient temperature, humidity, equipment status (new installation, operation, repair, etc.) and pre-treatment requirements before testing.

• Test method: including wiring method, test procedure, data recording and analysis method, etc.

• Periodic test requirements: Specifies the frequency at which the equipment should be tested for insulation resistance during operation.

The main factors affecting insulation resistance

1. Material property

   a. Material type and purity: Different types of insulating materials (such as plastics, ceramics, rubber, glass, etc.) due to their molecular structure, polarization characteristics, impurity content and other differences, their insulation resistance values are significantly different. In general, materials with high purity, low polarization and low impurities have high insulation resistance. In addition, material aging, degradation, moisture and other factors will cause its insulation performance to decline.

   b. Surface state and thickness: the surface roughness, scratches, contamination of the insulation material will affect the surface resistance, and then affect the overall insulation resistance. At the same time, the thickness of the insulation layer directly affects the resistance value, and an appropriate increase in the thickness of the insulation layer can improve the insulation resistance.

2. Environmental condition

   a. Temperature: Insulation resistance usually decreases with increasing temperature, which is due to the intensification of the thermal movement of ions or electrons inside the material at high temperatures, increasing the conductivity. Understanding the temperature characteristics of insulation materials helps to determine the safe operating range of equipment in different environments.

   b. Humidity: Humidity has a particularly significant effect on insulation resistance. Moisture will reduce the resistivity of insulation materials, especially for hygroscopic materials, such as paper, cotton, wood and so on. When the humidity increases, the moisture forms a conductive channel inside the material, resulting in a significant decrease in insulation resistance. Therefore, moisture protection measures are essential to maintain good insulation properties.

   c. Electric field strength: In high voltage environment, the electric field strength is too high may lead to ionization of the insulation material, corona discharge and other phenomena, reduce its insulation resistance. Reasonable design of insulation structure of electrical equipment to avoid local electric field concentration is helpful to maintain stable insulation resistance.

3. Test condition

   A. Test voltage: The insulation resistance value increases with the increase of the applied voltage, but when the voltage exceeds a certain value, it may cause partial discharge or breakdown, but the insulation resistance will decrease. Therefore, choosing the right test voltage is crucial to accurately evaluate insulation performance.

   b. Test time: Insulation resistance measurement usually requires waiting for a period of time (such as 1 minute or 10 minutes) after applying voltage to allow the current to reach a steady state. Too short a test time may result in a low reading that does not truly reflect the insulation performance.

Practical suggestions for raising insulation resistance

1. Preferred materials: Select insulation materials with high insulation performance, appropriate dielectric constant, good temperature stability, and low moisture absorption based on the requirements of the working environment, temperature resistance, and humidity resistance of the device.

2. Optimization design: Reasonable design of the insulation structure, such as the use of multi-layer insulation, increase the insulation distance, the use of shielding layer and other methods to reduce the local electric field strength, to prevent corona discharge. At the same time, ensure the uniformity and sufficient thickness of the insulation layer.

3. Strict process control: In the manufacturing process, strictly control the processing, assembly and treatment of insulation materials to avoid scratches, pollution and other defects to ensure surface quality. For equipment that needs to be used outdoors for a long time, effective moisture-proof and anti-corrosion measures should be taken.

4. Regular testing and maintenance: According to the equipment manual and related standards, regularly test the insulation resistance to monitor the change trend of insulation performance. If the insulation resistance drops abnormally, rectify the cause and repair or replace the insulation resistance in time. In harsh environments such as high humidity and high temperature, the detection cycle should be appropriately shortened.

5. Standard test method: Insulation resistance test is carried out in strict accordance with relevant standards (such as IEC 60060, GB/T 3333), and appropriate test voltage and time are selected to ensure the accuracy and comparability of test results.

# The factors that affect insulation resistance # are numerous and intertwined, including material properties, environmental conditions, test conditions, etc. Understanding the mechanism of these factors, combined with practical application requirements, and adopting targeted material selection, design, process control, testing and maintenance strategies can effectively improve the insulation performance of electrical equipment and ensure the safe and stable operation of the power system.