To ensure accurate measurement of insulation resistance in electrical equipment and various objects with different voltage levels, it's essential to choose a megohmmeter that matches the voltage range. For devices operating 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 should be selected. It’s also important to make sure the insulation resistance of the device under test falls within the meter's measurement range.
Before starting any measurement, it's crucial to test the megohmmeter for both open circuit and short circuit conditions. In an open circuit test, disconnect the test leads and turn the handle at around 120 rpm. The needle should point to infinity. For a short circuit test, connect the two leads together and slowly turn the handle to 120 rpm; the needle should read zero. This confirms the instrument is functioning properly.
A megohmmeter typically has three terminals: L (line), E (earth), and G (guard). The L terminal connects to the conductor being tested, while E is connected to the device’s casing or another insulated conductor. The G terminal helps reduce errors caused by surface leakage current by connecting to the insulation near the L terminal and wrapping it with 3–5 turns of bare copper wire.
During the measurement, maintain a steady shaking speed of about 120 rpm. Start by connecting the E and G terminals first, then the L terminal. Wait for one minute until the pointer stabilizes before taking the reading. After recording the result, always remove the L lead first before stopping the handle. This is especially important when testing capacitive devices like capacitors, as failing to do so can damage the meter due to back-feeding from the device.
The test leads must be of high quality and used exclusively for this purpose. Avoid using regular soft copper wires, as they may introduce leakage current and affect accuracy. Instead, use a single-core insulated copper wire with high dielectric strength.
Before each test, ensure the object under test is fully discharged, particularly if it contains capacitors. Discharging is critical to prevent electric shocks and ensure safe measurements.
Finally, perform all tests in a dry, non-magnetic environment. Avoid conducting tests during rain or in areas with high humidity, as moisture can significantly impact the results. Always follow safety procedures to protect both the equipment and the operator.
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