ASTM D257-14
Standard Test Methods for DC Resistance or Conductance of Insulating Materials

Standard No.

ASTM D257-14

Release Date

2014

Published By

American Society for Testing and Materials (ASTM)

Status

Be replaced

Replace By

ASTM D257-14(2021)e1

Latest

ASTM D257-14(2021)e1

Scope

5.1 Insulating materials are used to isolate components of an electrical system from each other and from ground, as well as to provide mechanical support for the components. For this purpose, it is generally desirable to have the insulation resistance as high as possible, consistent with acceptable mechanical, chemical, and heat-resisting properties. Since insulation resistance or conductance combines both volume and surface resistance or conductance, its measured value is most useful when the test specimen and electrodes have the same form as is required in actual use. Surface resistance or conductance changes rapidly with humidity, while volume resistance or conductance changes slowly with the total change being greater in some cases.

5.2 Resistivity or conductivity is used to predict, indirectly, the low-frequency dielectric breakdown and dissipation factor properties of some materials. Resistivity or conductivity is often used as an indirect measure of: moisture content, degree of cure, mechanical continuity, or deterioration of various types. The usefulness of these indirect measurements is dependent on the degree of correlation established by supporting theoretical or experimental investigations. A decrease of surface resistance results either in an increase of the dielectric breakdown voltage because the electric field intensity is reduced, or a decrease of the dielectric breakdown voltage because the area under stress is increased.

5.3 All the dielectric resistances or conductances depend on the length of time of electrification and on the value of applied voltage (in addition to the usual environmental variables). These must be known and reported to make the measured value of resistance or conductance meaningful. Within the electrical insulation materials industry, the adjective “apparent” is generally applied to resistivity values obtained under conditions of arbitrarily selected electrification time. See X1.4.

5.4 Volume resistivity or conductivity is calculated from resistance and dimensional data for use as an aid in designing an insulator for a specific application. Studies have shown changes of resistivity or conductivity with temperature and humidity (1, 2, 3, 4).⁴ These changes must be known when designing for operating conditions. Volume resistivity or conductivity determinations are often used in checking the uniformity of an insulating material, either with regard to processing or to detect conductive impurities that affect the quality of the material and that are not readily detectable by other methods.

5.5 Volume resistivities above 10²¹ Ω·cm (10¹⁹ Ω·m), calculated from data obtained on specimens tested under usual laboratory conditions, are of doubtful validity, considering the limitations of commonly used measuring equipment.

5.6 Surface resistance or conductance cannot be measured accurately, only approximated, because some degree of volume resistance or conductance is always involved in the measurement. The measured value is also affected by the surface contamination. Surface contamination, and its rate of accumulation, is affected by many factors including electrostatic charging and interfacial tension. These, in turn, affect the surface resistivity. Surface resistivity or conductivity is considered to be related to material properties when contamination is involved but is not a material property of electrical insulation material......

ASTM D257-14 Referenced Document

• ASTM D1169 Standard Test Method for Specific Resistance (Resistivity) of Electrical Insulating Liquids
• ASTM D150 Standard Test Methods for AC Loss Characteristics and Permittivity (Dielectric Constant) of Solid Electrical Insulation
• ASTM D1711 Standard Terminology Relating to Electrical Insulation
• ASTM D374 Standard Test Methods for Thickness of Solid Electrical Insulation
• ASTM D4496 Standard Test Method for D-C Resistance or Conductance of Moderately Conductive Materials
• ASTM D5032 Standard Practice for Maintaining Constant Relative Humidity by Means of Aqueous Glycerin Solutions
• ASTM D6054 Standard Practice for Conditioning Electrical Insulating Materials for Testing
• ASTM E104 Standard Practice for Maintaining Constant Relative Humidity by Means of Aqueous Solutions

ASTM D257-14 history

• 2021 ASTM D257-14(2021)e1 Standard Test Methods for DC Resistance or Conductance of Insulating Materials
• 2014 ASTM D257-14 Standard Test Methods for DC Resistance or Conductance of Insulating Materials
• 2007 ASTM D257-07 Standard Test Methods for DC Resistance or Conductance of Insulating Materials
• 1999 ASTM D257-99(2005) Standard Test Methods for DC Resistance or Conductance of Insulating Materials
• 1999 ASTM D257-99 Standard Test Methods for DC Resistance or Conductance of Insulating Materials
• 1991 ASTM D257-91 Standard Test Methods for D-C Resistance or Conductance of Insulating Materials