To measure the insulation resistance of a power cable using a megohmmeter, it's essential to follow these key steps carefully:
1. Before starting the test, disconnect the power cable from all external wiring and discharge it to ground. The discharge time should be at least 1 minute, and for cables with large capacitance, it should be extended to 2 minutes. This ensures both safety and accurate results.
2. Clean the terminal sleeves or core wires and their insulating surfaces using a dry, soft cloth to reduce surface leakage current, which can affect the accuracy of your readings.
3. Place the megohmmeter on a stable, flat surface to avoid any movement that could lead to incorrect measurements during the test.
4. With no load connected, turn the handle of the megohmmeter to its rated speed (usually 120 revolutions per minute) and adjust the pointer to “∞†to ensure the meter is properly calibrated.
5. For multi-core cables, test each phase separately. Connect the core being tested to the "L" terminal of the megohmmeter, short the other cores to ground (connect them to the "E" terminal), and use the "G" terminal to shield the surface insulation, preventing surface leakage from affecting the reading.
6. Rotate the handle at a steady speed. After about one minute, record the insulation resistance value. Although theoretically, you should wait until all currents have stabilized, the standard practice is to take the reading after one minute. This balances accuracy with efficiency, ensuring consistent and repeatable results.
7. If the insulation is poor or if multiple tests are required, make sure to ground and discharge the cable for at least two minutes between tests to prevent residual charge from interfering with subsequent readings.
8. Keep detailed records of the test conditions and results, as environmental factors can influence the insulation resistance. Accurate documentation helps in analyzing and comparing results over time.
Additionally, remember that the insulation resistance of the leads connected to the "L" terminal is in parallel with the cable’s insulation. Therefore, these leads must be well-insulated and not dragged along the ground or tied together with the "E" leads. If they must be connected to other supports or components, those supports must also be properly insulated to avoid errors in measurement.
During the test, maintain a steady speed—ideally at least 80% of the rated speed—to ensure reliable and consistent readings. A stable speed helps in obtaining accurate data and prevents fluctuations that could distort the results.
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