English Translation: Working Principle and Application of Insulation Resistance Tester
An insulation resistance tester is a core instrument in the field of electrical safety inspection. It is mainly used to measure the insulation resistance values of electrical equipment, circuits and insulating materials, determine whether their insulation performance complies with safety standards, and prevent potential safety hazards such as electric leakage, short circuits and insulation breakdown. Its working principle is based on Ohm's Law combined with high-voltage DC power supply technology. By applying a specific high voltage, measuring leakage current and calculating the insulation resistance, it forms a closed-loop process of high-voltage excitation, micro-current detection and precise calculation.
I. Core Foundation: Nature and Measurement Significance of Insulation Resistance
The insulating parts of electrical equipment (such as cable insulation layers, motor winding insulation, transformer insulating oil, etc.) are theoretically non-conductive. However, in practice, conductive paths are formed due to material aging, moisture absorption, contamination, wear and other factors, generating weak leakage current. Insulation resistance is an indicator to measure this weak conductivity — the higher the resistance value, the better the insulation performance, the smaller the leakage current, and the safer the equipment operation. On the contrary, an excessively low resistance value indicates insulation defects, which are likely to cause electric leakage, electric shock or equipment damage.
Unlike ordinary multimeters that measure resistance, an insulation resistance tester applies high-voltage DC voltage (usually 500V, 1000V, 2500V, 5000V, etc.) to simulate the insulation pressure borne by equipment during normal operation. This fits actual working conditions better, making the measurement results more referential. Ordinary multimeters apply extremely low voltage (a few volts), which cannot excite the weak leakage current of the insulation layer and fail to accurately reflect the true insulation state.
II. Core Principle: High-Voltage Excitation and Leakage Current Measurement
The core logic of an insulation resistance tester is applying high voltage → detecting leakage current → calculating insulation resistance, which strictly follows Ohm's Law (R=U/I). In the formula: U refers to the high-voltage DC voltage applied to both ends of the insulation layer, I is the weak leakage current generated by the insulation layer, and R is the insulation resistance. The detailed breakdown is as follows:
1. High-Voltage DC Power Supply Module (One of the Core Components)
The instrument is integrated with a high-voltage generator that converts mains electricity (220V AC) into stable adjustable high-voltage DC voltage according to test requirements. DC voltage is adopted because AC voltage generates capacitive current and polarization current in the insulation layer, interfering with the measurement accuracy of leakage current. In contrast, DC voltage only produces leakage current, enabling more accurate reflection of the true insulation resistance value.
The output voltage of the high-voltage generator must be stable and controllable, and equipped with overvoltage protection and short-circuit protection functions to avoid damage to the tested equipment or the instrument itself caused by high voltage. For example, 500V DC high voltage is commonly used to measure low-voltage electrical equipment (such as household appliances), while 2500V or higher high voltage is required for high-voltage equipment (such as transformers and high-voltage cables).
2. Micro-Leakage Current Detection Module (The Second Core Component)
When high-voltage DC voltage is applied to both ends of the tested insulator, the insulation layer generates weak leakage current (usually microampere μA level, even nanoampere nA level). This extremely faint current cannot be accurately captured by ordinary measuring instruments, so the tester is equipped with a high-precision micro-current amplifier.
The micro-current amplifier amplifies the weak leakage current to a measurable and calculable range, while filtering out external interference (such as electromagnetic interference and ambient stray current) to ensure measurement accuracy. The detection module is also provided with a Guard Terminal to eliminate interference from surface leakage current of the tested equipment — surface leakage current is generated by moisture and contamination on the insulator surface rather than internal insulation leakage. The guard terminal diverts it to the earth terminal to avoid inclusion in measurement results.
3. Resistance Calculation and Display Module
According to Ohm's Law, the operational circuit inside the instrument automatically calculates the insulation resistance R=U/I through the known high voltage U and detected leakage current I. Since the insulation resistance value is usually extremely large (from several megohms MΩ to thousands of megohms GΩ), the instrument adopts a logarithmic scale or segmented display for easy reading. Meanwhile, the instrument integrates a timing function (e.g., 1-minute, 10-minute measurement). The leakage current of some insulating materials (such as insulating oil and cables) changes with time. Measuring insulation resistance at different time periods can judge the stability of insulating materials (such as dielectric absorption ratio and polarization index) and further evaluate insulation performance.
III. Working Process (Taking Portable Insulation Resistance Tester KG3200 as an Example)
Power-on Calibration: After startup, the instrument performs automatic internal calibration to ensure the accuracy of high-voltage output and current detection, and enters the measurement mode upon completion of calibration.
Wiring Operation: Connect the High-Voltage Terminal (HV) of the tester to the live end of the tested equipment, the Earth Terminal (E) to the grounding end of the tested equipment, and the Guard Terminal (G) to the shell or non-measuring conductive part of the tested equipment (for shielding surface leakage if needed).
Parameter Setting: Set the applied high-voltage (e.g., 500V) and measurement time (e.g., 1 minute) according to the type of tested equipment and standard requirements.
High-Voltage Application and Measurement: Press the measurement key; the high-voltage generator outputs the set DC high voltage to both ends of the tested insulator. The micro-current amplifier detects the leakage current and transmits the signal to the operational circuit.
Result Calculation and Display: The operational circuit calculates the insulation resistance value via U and I, displays it on the screen, and records parameters such as measurement time and voltage. An alarm prompt will be triggered if the resistance value is lower than the set threshold.
End of Measurement: Release the measurement key to stop high-voltage output. The instrument discharges to release residual charge on the tested equipment and prevent electric shock, completing the measurement.
IV. Precautions (Related to Working Principle)
Before measurement, ensure the tested equipment is powered off and discharged to avoid measurement errors or electric shock hazards caused by live equipment. Remove dust and moisture on the insulation surface to reduce interference from surface leakage current.
Distinguish the high-voltage terminal, earth terminal and guard terminal during wiring; wrong connection will lead to distorted measurement results or even instrument damage.
Do not touch the tested equipment and high-voltage binding posts of the tester when high voltage is applied to prevent high-voltage electric shock. Wait for the instrument to complete discharge before disconnecting wires after measurement.
Select an appropriate high-voltage gear according to the rated voltage of the tested equipment. Excessively high voltage may damage the tested equipment, while excessively low voltage fails to accurately measure insulation resistance.
The core principle of an insulation resistance tester is high-voltage DC excitation + weak micro-leakage current detection + Ohm's Law calculation. By simulating the insulation pressure under actual working conditions, it accurately measures insulation resistance values to judge insulation performance. Its key lies in the stability of the high-voltage module, the accuracy of micro-current detection and the anti-interference capability of shielding technology. Although different types of testers differ in functions and accuracy, their core logic always focuses on capturing weak leakage current and calculating insulation resistance. Mastering its principle enables more accurate instrument operation and measurement result interpretation, providing a guarantee for the safe operation of electrical equipment.