Multimeter to measure transformer insulation resistance
1. After disconnecting the power supply and all external connections of the test object, the test object should be grounded and discharged first, especially for large capacitance test objects (such as large transformers, generators, cables, etc.), which should be fully discharged.
2. Before measurement, wipe off the dirt on the surface of the sleeve with a cloth to reduce the impact on the measurement.
3. The megohmmeter should be placed horizontally, and the driving speed of the megohmmeter should reach the specified speed. Before measurement, first test the megohmmeter empty. When the phase line L is open circuited, the pointer should point to "infinity". If the indication is normal, the test can be carried out. In case of significant leakage on the insulation surface of the test sample or testing of important equipment, shielding should be applied to avoid the influence of the surface on the measurement results. The shielded wire should be made of soft copper wire and wrapped tightly around the insulation surface near the phase line end. The shielded wire should not be placed near the E end of the ground wire.
4. After reading the insulation resistance, the phase line L of the test sample should be disconnected first, and then the driving of the megohmmeter (or the DC power supply of the megohmmeter) should be stopped to prevent the charge charged by the capacitance of the test sample during measurement from discharging through the coil of the megohmmeter and damaging the megohmmeter. This is especially important when measuring large capacity test samples.
5. Measure the insulation resistance values at 15s and 60s, and also record the temperature and humidity during the measurement.
6. After each measurement of insulation resistance, the test sample should be fully discharged, with a discharge time greater than the charging time, in order to discharge the remaining charge. Otherwise, during repeated measurements, due to the influence of the remaining charge, the charging current and absorption may be smaller compared to the first measurement, resulting in a false phenomenon of decreased absorption ratio and increased insulation resistance value.
