Insulation resistance is the electrical resistance of a material that prevents the flow of current, quantifying its effectiveness as an insulator. Materials that conduct electricity easily, like copper and aluminum, are called conductors and have very low resistance. Conversely, materials that do not conduct electricity well, such as rubber and glass, are called insulators (or dielectrics) and exhibit extremely high resistance.
In electrical equipment and electronic circuits, current must be confined to intended conductive paths. If current strays into unintended areas, it can prevent the circuit from operating correctly and pose a risk of short circuits, leakage currents, and electric shock. Therefore, proper insulation is essential to ensure safety and functionality.
The procedure for measuring the resistance of an insulator to check for degradation or defects is known as insulation resistance measurement. Because the resistance values are typically very high (often in the megaohm or gigaohm range), a specialized instrument called an insulation tester or megohmmeter is used for this purpose.
An insulation tester operates by applying a specific high DC voltage (e.g., 250 V, 500 V, 1000 V) to the component under test and measuring the resulting leakage current. The resistance is then calculated using Ohm's Law (R = V/I). The appropriate test voltage is selected based on the rated voltage of the equipment being tested.
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