Insulation Resistance Measurements
Insulation resistance, IR, is the ratio of a DC applied voltage to a specimen between two electrodes and the total current between them. The two electrodes are either embedded into or in contact with the specimen. The insulation resistance is dependent on both the surface and volume resistance of the material. The DC voltage is applied for a specified period of time before the result-ing current is measured.
This test procedure is described in detail in the American Society for Testing Materials method D257 titled, “DC Resistance or Conductance of Insulating Materials.” According to this procedure, an applied voltage of 500V is applied for 60 seconds and then the resulting current is measured. This time and voltage often varies depending upon the application but must always be consistent to a specific test in order to make accurate comparisons.
Volume and Surface Resistivity Measurements
Volume resistivity is the electrical resistance through a one-centimeter cube of insulating material and is expressed in ohm-centimeters.
Surface resistivity is the electrical resistance between two electrodes on the surface of an insulating material and is expressed in ohms (usually stated as ohms per square).
Both volume and surface resistivity measurements are obtained by taking resistance measurements and then converting them to resistivity values by taking geometric considerations into account.