The standard "ASTM D257-14 Standard test methods for DC resistance or conductivity of insulating materials" developed by the American Society for Testing and Materials (ASTM) describes a number of procedures for determining the direct current (DC) insulation resistance, volume resistivity, volume resistivity, surface resistivity and surface resistivity or corresponding conductivities of electrical insulating materials.
These test methods are not suitable for measuring electrical resistivity/conductivity of moderately conductive materials. To evaluate such materials, reference should be made to “ASTM D4496-21 Standard Test Method for DC Resistance or Conductivity of Moderately Conductive Materials”.
ASTM D257-14 is a test method designed to measure the conductivity, volume, and surface resistivity of electrical insulating materials. This method determines the mechanical, chemical, and heat-resistant properties of the sample. The results are used primarily for quality control and specification purposes. Insulating materials insulate and support electrical system components. It is ideal to have excellent insulation resistance that meets mechanical, chemical, and heat-resistant standards.
The ASTM D257-14 test method is generally applicable to insulating materials but is not suitable for measuring electrical resistivity and conductivity of moderately conductive materials. The method also provides information on voltage stress limits, electrode geometry, sensitivity of sample configuration, and finite electrification.
The ASTM D257-14 test method proceeds by placing a sample, the surface of which has been cleaned with a suitable solvent, between the measuring electrodes. The surface resistance is then calculated and the details of the surface cleanliness are recorded. The method involves determining the resistance and conductivity of the sample or capacitor by measuring the current or voltage drop under specified conditions. The surface and volume resistivity or conductivity are measured separately by means of suitable electrode systems. Knowing the sample and electrode dimensions will aid in calculating the resistivity or conductivity.
When determining insulation resistance or conductivity or surface resistance or conductivity, the dimensions and distance between the electrodes are determined together with the surface resistance and conductivity between them. The electrification time is specified as 60 seconds and a direct voltage of 500 V is applied.
The ASTM D257-14 test method provides information on dielectric breakdown, dissipation factor properties, moisture content, degree of cure, mechanical continuity, and electric field intensity. Volume, surface resistivity, and conductivity are recorded. Resistance and conductivity values vary with temperature and humidity.
The International Electrotechnical Commission (IEC) developed the standard "IEC 60093 Test methods for volume resistivity and surface resistivity of solid electrical insulating materials" similar to ASTM D257-14. According to the IEC 60093 standard, volume resistivity is defined as the resistance to leakage current through the body of an insulating material. It determines the volume resistivity corresponding to the direct current resistance between opposite faces of one cubic meter of material. Surface resistivity can be related to material qualities when contaminated, but it is not a material property in the traditional sense. Surface resistivity is the resistance to leakage current along the surface of an insulating material. The IEC 60093 standard defines surface resistivity as the ratio of the potential gradient and the electrode length per unit of current.
For the purposes of this standard,
The resistance or conductivity of a material sample or a capacitor is determined from measuring the current or voltage drop under specified conditions. Using suitable electrode systems, surface and volume resistivity or conductivity are measured separately. Resistivity or conductivity is calculated when known sample and electrode dimensions are known.
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