To ensure accurate measurement of electrical equipment and objects with different voltage levels, it's essential to choose a megohmmeter that matches the required voltage range. For devices below 500V, a 500V megohmmeter is recommended. For equipment rated between 500V and 3000V, use a 1000V megohmmeter, and for voltages above 3000V, a 2500V megohmmeter is necessary. Additionally, make sure the insulation resistance of the device being tested falls within the measurement range of the meter to avoid inaccurate readings.
Before starting any measurement, perform an open-circuit and short-circuit test on the megohmmeter to verify its functionality. In the open-circuit test, disconnect the leads and rotate the handle at approximately 120 rpm. The pointer should point to infinity. For the short-circuit test, connect the two leads together and slowly turn the handle to 120 rpm. The needle should then read zero.
The megohmmeter typically has three terminals: L, E, and G. The L terminal connects to the conductor of the device under test, while E is connected to the device’s casing or another insulated phase conductor. The G terminal helps reduce errors caused by surface leakage currents. It should be connected to the insulating material near the L terminal and wrapped around the object with 3–5 turns of soft bare wire.
When performing the measurement, maintain a steady rotation speed of about 120 rpm. Connect the E and G terminals first, then the L terminal. Wait for one minute until the pointer stabilizes before taking a reading.
After taking the measurement, remove the L lead first, then stop rotating the handle. This is especially important when testing capacitive devices like capacitors, as failing to do so may cause damage to the meter. Immediately after testing, manually discharge the device to ensure safety.
The test leads must be of high quality and exclusively used for this purpose. Avoid using regular copper wires; instead, use single-strand insulated copper wire with high dielectric strength to prevent leakage current from affecting the results.
Always discharge the device being tested thoroughly before measurement, especially if it contains capacitors. Also, take care of the environment—avoid strong magnetic fields and moisture. High humidity can also impact the accuracy of your readings. Conduct measurements in dry conditions whenever possible to ensure reliable results.
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