Interpretation of insulation resistance measurements

Insulation resistance is defined as the ratio of the direct voltage applied to two electrodes in contact with or embedded in a specimen to the total current between the electrodes. Test values, therefore, are highly dependent on the geometry of the electrode-specimen system and the volume and surface resistances of the material under test. Also, the numerical results obtained are extremely sensitive to the test environment, the insulation resistance of the test equipment, the magnitude and stability of the voltage, and the time of electrification. Since test values are directly comparable only when geometry, humidity, voltage, temperature, material history, cleanliness, conductor configuration, ad infinitum, are identical, it is safe to say that direct application of test values on materials rarely can be utilized.