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Before performing any measurements, it's crucial to ensure the device under test is completely powered off and that the ground is short-circuited. Never perform measurements while the device is energized, as this can pose serious risks to both personnel and equipment.

If the device might generate high voltage, make sure all such possibilities are eliminated before starting the measurement. This step is essential for safety and accurate results.

It’s also important to clean the surface of the object being tested. This helps reduce contact resistance and ensures more accurate readings during the process.

Before using a megohmmeter, always check its condition. Focus on testing the "0" and "∞" points. When the handle is turned to reach the rated speed, the meter should show "0" when shorted and "∞" when open. This confirms the device is functioning properly.

Place the megohmmeter on a stable surface, away from strong electrical currents or magnetic fields. These external factors can interfere with the measurement accuracy.

Once all preparations are done, you can proceed with the measurement. Pay close attention to the correct wiring of the megohmmeter. Incorrect connections can lead to errors or even inaccurate results.

A typical megohmmeter has three terminals: "L" (line), "E" (earth), and "G" (guard or shield). Normally, the insulation resistance is measured between "L" and "E". However, if the insulator's surface is prone to leakage, the "G" terminal should be connected to the shield ring or the part not being measured. This allows the leakage current to return directly to the generator through the "G" terminal, avoiding the measuring mechanism. This significantly reduces the impact of surface leakage currents.

This is especially important when measuring the insulation resistance between the core of a cable and its outer surface. High humidity or dirt on the cable’s surface can cause large leakage currents. To prevent this, a metal shielding ring is often added around the outer surface of the cable, and the "G" terminal of the megohmmeter is connected to it.

When measuring the insulation resistance of electrical equipment, always make sure the "L" and "E" terminals are correctly connected. The "L" terminal should be connected to the conductor of the device, while the "E" terminal should be grounded. The "G" terminal connects to the insulated part of the device. Reversing "L" and "E" can lead to significant errors, as the leakage current may flow through the wrong path, affecting the reading.

Additionally, the "E" terminal is typically less insulated from the outer casing than the "L" terminal. If the terminals are reversed, the insulation resistance of the "E" terminal to ground could be in parallel with the measured resistance, leading to an incorrect, lower reading.

In conclusion, using a megohmmeter correctly is essential for obtaining accurate and reliable insulation resistance measurements. Any mistakes in setup or connection can compromise the results and lead to unsafe conditions.