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Before starting any measurement, it is crucial to ensure that the device under test is completely powered off. The ground should be short-circuited to prevent accidental energization, which could lead to serious safety hazards for both personnel and equipment.

If the equipment may generate high voltage, make sure to eliminate this risk before proceeding with the measurement. This step is essential to avoid damage or injury during testing.

It’s also important to clean the surface of the object being tested. A clean surface reduces contact resistance and improves the accuracy of your readings, ensuring reliable results.

Before using a megohmmeter, always check its condition. Test the two key points: "0" (short circuit) and "∞" (open circuit). To do this, turn the handle until the motor reaches its rated speed. When the meter is shorted, it should read "0", and when open, it should show "∞".

Place the megohmmeter on a stable and level surface, away from sources of strong electrical currents or magnetic fields. These external factors can interfere with the accuracy of your measurements.

Once all preparations are complete, you can proceed with the measurement. Pay close attention to the wiring of the megohmmeter, as incorrect connections can cause significant errors in your readings.

The 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 surface of the insulator is highly conductive or contaminated, connect the "G" terminal to the shielded part of the device. This ensures that leakage current flows directly back through the "G" terminal, avoiding the measuring mechanism and providing more accurate results.

This is especially important when measuring the insulation resistance between a cable core and its outer surface. In humid conditions or if the cable is not clean, surface leakage currents can be very high. To prevent these from affecting the internal insulation reading, a metal shielding ring is often added around the cable, and the "G" terminal is connected to it.

When measuring the insulation resistance of electrical equipment, it is vital to connect the "L" and "E" terminals correctly. 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 cause leakage current to flow through the wrong path, leading to inaccurate readings and loss of the shield's effectiveness.

Additionally, the "E" terminal is usually less insulated than the "L" terminal. If reversed, the insulation resistance of the "E" terminal to ground could be in parallel with the measured value, resulting in a lower-than-accurate reading and large measurement errors.

As you can see, proper use of the megohmmeter is essential for accurate and reliable insulation resistance measurements. Following these guidelines will help ensure that your tests are safe and effective.