You have mentioned the insulation resistance tester for the third time. It seems you are very focused on this device.
Or perhaps there are pain points in the previous explanations that you want to delve deeper into. Given that you have already understood its definition, principle, position in the testing process, and supporting tools (high-altitude wiring clamps, high-current generators), this time I will focus on "difficult problems in practice" and "the truth behind the data." This is the key to distinguishing between "knowing how to operate" and "understanding diagnosis." 1. Why are the measured values "inaccurate" or "jumping around"? (Practical troubleshooting) In field testing, unstable readings or large deviations from historical data are often encountered, usually caused by the following reasons: A. Surface leakage current interference (most common) Phenomenon: On rainy or foggy days, or when the equipment surface is dirty, the measured insulation resistance value is low, and the absorption ratio (DAR) is close to 1.0. Truth: The current does not pass through the interior of the insulation but flows away along the dirty and damp surface, taking a "shortcut." Solution: The shielding terminal (G terminal) must be used. Operation: Wrap bare copper wire around the insulation surface of the test object (guard ring) and connect it to the G terminal of the meter. Effect: The surface leakage current returns directly without passing through the measurement circuit, and the meter only displays the true volume insulation resistance. Case: For a damp transformer bushing, without using the G terminal, the measurement is 500MΩ; with the G terminal, it may instantly jump to 5000MΩ. B. Residual charge not fully discharged Phenomenon: The first measurement is normal, but the subsequent measurement is higher; or measurements between different phases affect each other. Truth: The test object (especially cables, capacitors, large motors) has large capacitance. The charge generated from the previous test or induction has not been fully discharged, superimposing on the test voltage, causing the current to decrease and the calculated resistance to be falsely high. Solution: Before testing: Must fully discharge to ground (the time should be longer than the charging time). After testing: Must allow the instrument to discharge automatically or manually until the voltage display reads 0. C. Temperature effect (most easily overlooked) Phenomenon: Data measured in summer is much lower than in winter, leading to suspicion of moisture in the equipment. Truth: Insulation resistance is extremely sensitive to temperature. For every 10°C increase in temperature, the insulation resistance value may decrease by 30% to 50%. Solution: Temperature conversion must be performed. Formula reference:
