Method and precautions for measuring insulation resistance with a megohmmeter
1. Before the test, remove the resistance and all external wires of the tested power cable, and ground it for discharge. The discharge time should not be less than 1 minute, and for power cables with large capacitance, it should not be less than 2 minutes to ensure safety and accurate test results.
2. Use a dry and clean soft cloth to wipe away the dirt on the power cable terminal sleeve or core wire and its insulation surface to reduce surface leakage.
3. Place the shaker in a stable and horizontal position to avoid uneven shaking during operation, which may result in inaccurate readings.
4. Under no-load conditions, rotate the handle of the dial gauge 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 of the core wire separately. At this point, connect the outgoing line of the tested core wire to the wiring terminal (L) of the megger, and short circuit the other core wires to the ground (lead pack) before connecting them to the grounding terminal (E) of the megger. In order to avoid the impact of leakage current on the insulation surface of power cables, it is also necessary to use a shakedown screen to knock on the wall (G), completely breaking the surface insulation beyond the indication of the megger.
6. When rotating the handle of the dial gauge at a constant speed of 120r/min, the pointer gradually rises. After waiting for 1 minute, record its insulation resistance value. This regulation is because there are three types of currents in the insulation that decay over time. In theory, the conductive current (i.e. leakage current) value is only read out to calculate insulation resistance after all three currents have decayed. However, due to the long duration, high workload of dissection, and considering the stability of the testing system's time constraints, it is clearly stipulated in the testing method standards that the reading should be taken exactly one minute after the current is applied. This regulation not only ensures that most of the non-conductive current has disappeared, but also unifies the testing time, making the readings repeatable and comparable, while improving testing efficiency.
7. When the insulation resistance measurement of power cables is completed or needs to be repeated, the tested power cables must be grounded and discharged for at least two minutes.
8. Due to the influence of many external conditions on the insulation resistance of power cable lines, it is necessary to carefully fill out records or tables during the test to facilitate the analysis of test results. The insulation resistance of the L-end lead of the megger is parallel to the insulation resistance of the power cable. Therefore, it is required that the insulation resistance of the lead is high and should not be dragged on the ground, nor should it lean against the E-end lead. If the lead must be connected to the power cable core through other supports, the support must have good insulation, otherwise it will affect the accuracy of measurement. During the testing process, the rotational speed of the tachometer should be kept at its rated value as much as possible and maintain a uniform rotational speed. The rotational speed should not be less than 80% of the rated speed.
