The correct way to measure insulation resistance with a megger
1. Before the test, remove the resistance of the power cable under test and all external connections, and ground it for discharge. The discharge time shall not be less than 1 minute. For power cables with larger capacitance, the discharge time shall not be less than 2 minutes to ensure safety and accurate test results.
2. Use a dry, clean soft cloth to wipe off the dirt on the power cable terminal sleeve or core wire and its insulation surface to reduce surface leakage.
3. Place the meter in a stable and horizontal position to prevent the meter from shaking due to uneven force during operation, resulting in inaccurate readings.
4. Under no-load conditions, turn the oscillating watch handle to 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 core wire to be measured to the terminal (L) of the oscillator, short-circuit the other core wires to the ground (lead package) and then connect them to the ground terminal (E) of the oscillator. In order to avoid the influence of leakage current on the surface of the power cable insulation, it is also necessary to use a shaken screen to knock on the wall (G) to completely knock the surface insulation away from the indication of the meter.
6. When the handle of the oscillator is rotated at a constant speed (120r/min), the pointer will gradually rise. After 1 minute, record the insulation resistance value. This stipulation is because it is considered that there are three currents that decay with time in the insulation. Theoretically speaking, the value of the conductive current (i.e. leakage current) should be read out after all three currents have decayed to calculate the insulation resistance. However, due to the time being too long, the workload of dissecting the test is large and the time required for the test system is taken into account. Stability, so it is clearly stipulated in the standard of the test method that the reading will be taken when it reaches one minute after the current is turned on. This regulation not only ensures that most of the non-conductive current has disappeared, but also unifies the test time, so that the reading has Repeatability and comparability, while improving testing efficiency.
7. When the insulation resistance measurement of the power cable is completed or the measurement needs to be repeated, the power cable under test must be grounded and discharged for at least two minutes.
8. Since the insulation and resistance of power cable lines are affected by many external conditions, notes or tables should be filled in carefully during the test to facilitate analysis of the test results. The insulation resistance of the L-end lead of the megger is in parallel with the insulation resistance of the power cable. Therefore, the insulation resistance of the lead is required to be high and should not be dragged on the ground or close to the E-end lead. If the lead must pass through When other supports are connected to the core wires of the power cable, the supports must be well insulated, otherwise the accuracy of the measurement will be affected. During the test, the rotational speed of the megohmmeter should be maintained at the rated value as much as possible and maintain a uniform rotational speed. The rotational speed should not be less than 80% of the normal rotational speed.
