Volume and Surface Resistivity
ASTM D257, IEC 60093
Volume resistivity is the resistance to leakage current through the body of an insulating material. The ratio of the potential gradient parallel to the current in a material to the current density. In SI, volume resistivity is numerically equal to the direct-current resistance between opposite faces of a one-meter cube of the material (Ohm-m). Surface resistivity is the resistance to leakage current along the surface of an insulating material. The electrical resistance between two parallel electrodes in contact with the specimen surface and separated by a distance equal to the contact length of the electrodes. The resistivity is therefore the quotient of the potential gradient, in V/m, and the current per unit of electrode length, A/m. Since the four ends of the electrodes define a square, the lengths in the quotient cancel and surface resistivity is reported in ohms, although it is common to see the more descriptive unit of ohms per square.
The resistivity of an insulator is measured by sourcing a known voltage, measuring the resulting current, and calculating the resistance using Ohm’s Law. From the resistance measurement, the resistivity is determined based on the physical dimensions of the test sample. The resistivity is dependent on several factors. First, it is a function of the applied voltage. Sometimes the voltage may be varied intentionally to determine an insulator’s voltage dependence. The resistivity also varies as a function of the length of electrification time. The longer the voltage is applied, the higher the resistivity because the material continues to charge exponentially. Environmental factors also affect an insulator’s resistivity. In general, the higher the humidity, the lower the resistivity. To make accurate comparisons to a specific test, the applied voltage, electrification time, and environmental conditions should be kept constant from one test to the next. According to the ASTM standard, a commonly used test condition is a voltage of 500V applied for 60 seconds. Depending upon the application, the volume or the surface resistivity, or both, is measured.
Surface resistivity is defined as the electrical resistance of the surface of an insulator material. It is measured from electrode to electrode along the surface of the insulator sample. Since the surface length is fixed, the measurement is independent of the physical dimensions (i.e., thickness and diameter) of the insulator sample.
Volume resistivity is defined as the electrical resistance through a cube of insulating material. When expressed in ohm-centimeters, it would be the electrical resistance through a one centimeter cube of insulating material. If expressed in ohm-inches, it would be the electrical resistance through a one-inch cube of insulating material.