The correct way to measure insulation resistance with a megohmmeter
1. Remove the resistance of the power cable under test and all external connections before the test, and discharge it to the ground. The discharge time should not be less than 1 minute, and the power cable with a large capacitance should not be less than 2 minutes to ensure safety and accuracy of test results.
2. Use a dry and clean soft cloth to wipe off the dirt on the power cable terminal sleeve or core wire and its insulating surface to reduce surface leakage.
3. Place the meter in a stable and horizontal position, so as not to shake the meter with uneven force during operation, resulting in inaccurate readings.
4. Under no-load conditions, turn the handle of the watch to reach the rated speed (120r/min) and adjust the pointer to "∞".
5. For multi-core power cables, test the insulation resistance of each phase core wire separately. At this time, connect the lead wire of the tested core wire to the terminal (L) of the megger, short the other core wires to the ground (lead pack) and then connect them to the ground terminal (E) of the megger. In order to avoid the impact of leakage current on the surface of the power cable insulation, it is also necessary to knock the wall (G) with a shaker screen to completely knock the surface insulation out of the indication of the shaker meter.
6. Turn the handle of the watch at a constant speed (120r/min, the pointer will rise gradually, and after 1min, record the insulation resistance value. This regulation is because there are three kinds of currents that decay with time in the insulation, from Theoretically, after the three kinds of currents have all decayed, the value of the conduction current (that is, the leakage current) can be read to calculate the insulation resistance. However, due to the long time, the workload of the dissection test is large, and the test system is time-consuming. Stability, so it is clearly stipulated in the standard of the test method that the reading is taken when the current is turned on for one minute. This regulation not only ensures that most of the non-conductive current has disappeared, but also makes the test time uniform, so that the reading has a certain Repeatability and comparability, while improving test efficiency.
7. When the insulation resistance of the power cable is measured or needs to be repeated, the power cable under test must be grounded and discharged for at least two minutes.
8. Since the insulation resistance of the power cable line is also affected by many external conditions, the records or tables should be carefully filled in the test to facilitate the analysis of the test results. The insulation resistance of the L-terminal lead wire of the megger is connected in parallel with the insulation resistance of the power cable. Therefore, the insulation resistance of the lead wire is required to be high, and it should not be dragged on the ground, nor should it be close to the E-end lead wire. When other supports are connected to the core wire of the power cable, the supports must be well insulated, otherwise the measurement accuracy will be affected. During the test, the speed of the meter should be kept at the rated value as far as possible, and maintain a uniform speed, and the speed of rotation should not be lower than 80% of the speed of the meter.
