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This post was last edited by yuchenchf on 2017-1-18 09:43 During the use of insulation resistance testers, various questions often arise. Solving these questions will help electricians better remember the precautions, understand the important points that should be paid attention to in the work, avoid causing personal injury, and obtain more accurate data. The following are several questions that our technical staff are often asked. They are summarized as follows:: 1. Can I use a megohmmeter to directly measure the charged product under test? What is the impact on the results and why? answer: For the sake of personal safety and normal testing, in principle, it is not allowed to measure the charged test object. If you want to measure the charged test object, it will not cause damage to the instrument (in a short period of time), but the test results are inaccurate, because after being charged, the test object is connected with other test objects, so the results obtained cannot truly reflect the actual data, but the parallel or series resistance together with other test objects. 2. In a high-voltage and high-resistance test environment, why is the instrument required to be connected to the "G" terminal? answer: When a higher rated voltage is applied to both ends of the product under test and the insulation resistance is high, the surface of the product under test is wet and the leakage caused by contamination is large, resulting in a large indication error. The "G" end of the instrument bypasses the leakage current on the surface of the product under test, so that the leakage current does not pass through the test circuit of the instrument, eliminating errors caused by leakage current. 3. Why can an electronic megohmmeter produce higher DC high voltage when powered by several batteries? Answer: This is based on the principle of DC conversion. After the boost circuit processing, the lower power supply voltage is increased to a higher output DC voltage. Although the high voltage generated is high, the output power is small. For example, a few batteries of an electric baton can generate high voltage of tens of thousands of volts. 4. When measuring the rated DC high voltage output at both ends of "L" and "E" of certain models of insulation instruments, use the DCV setting of the pointer multimeter to measure the voltage at both ends of L and E. Why does the voltage drop a lot, but the digital multimeter does not? answer: Use an ordinary pointer multimeter to directly measure the rated DC voltage output by the megohmmeter at its "L" and "E" terminals. The measurement result will be much smaller than the nominal rated voltage value (outside the error range), but not with a digital multimeter. This is because the internal resistance of the analog multimeter is small, while the internal resistance of the digital multimeter is relatively large. The internal resistance of the pointer multimeter is small, and the output voltage at the LE terminal of the megohmmeter is much lower than the output voltage during normal operation. However, it is wrong to use a multimeter to directly measure the output voltage of a megohmmeter. You should use an electrostatic high-voltage meter with a large internal resistance or use a voltage divider or other method with a large enough load resistance to measure it. 5. Why when measuring insulation, it is not only required to measure the simple resistance value, but also the absorption ratio and polarization index. What is the significance? answer: In the insulation test, the insulation resistance value at a certain moment cannot fully reflect the insulation performance of the sample. This is due to the following two reasons. On the one hand, insulating materials with the same performance have a small insulation resistance when the volume is large, and a large insulation resistance when the volume is small. On the other hand, insulating materials have charge absorption ratio processes and polarization processes after high voltage is applied. Therefore, the power system requires that the absorption ratio - that is, the ratio of R60s and R15s, and the polarization index - that is, the ratio of R10min and R1min should be measured during the insulation test of many occasions such as main transformers, cables, motors, etc., and use this data to determine the quality of the insulation condition. 6. When measuring insulation resistance with an insulation resistance tester, what factors may cause inaccurate measurement data? Why? answer: (1) The battery voltage is insufficient. The battery voltage is too low, causing the circuit to not work properly, so the measured readings are inaccurate. (2) The test line connection method is incorrect. By mistake, the "L", "G", and "E" terminals are connected incorrectly, or the "G" and "L" terminals and the "G" and "E" terminals are connected to both ends of the product under test. (3) The "G" terminal is not connected. The product under test may suffer from errors caused by current leakage due to factors such as contamination and moisture, resulting in inaccurate testing. At this time, the "G" terminal must be connected to prevent errors caused by leakage current. (4) The interference is too large. If the product under test is subject to excessive electromagnetic interference from the environment, the meter reading will jump. Or the pointer moves. causing inaccurate readings. (5) Human reading error. When measuring with a pointer megohmmeter, the indication value is inaccurate due to artificial viewing angle error or scale error. (6) Instrument error. The instrument itself has too large an error and needs to be recalibrated.