Dielectric strength reflects the electric strength of an insulating material at power frequencies 48Hz to 62Hz. It is also a measure of the maximum electric field strength a material can withstand without breaking down (failure of its insulating properties). The value is given in kV/mm representing the applied voltage just before breakdown divided by the thickness of the specimen. The surrounding medium can be either oil or air.
A number of factors can affect the values:
- Thickness, homogeneity and moisture content of the specimen
- Dimensions and thermal conductivity of the test electrodes
- Frequency and wave form of the applied voltage
- Ambient temperature, pressure and humidity
- Electrical and thermal characteristics of the ambient medium
SURFACE RESISTIVITY (ASTM D257/IEC60093)
When a voltage is applied to an insulating thermoplastic. some portion of the current will flow along the surface of the plastic moulding if there is another conductor or earth attached to the same surface. Surface resistivity is a measure of the material’s ability to resist that surface current.
It is measured as the resistance when a direct voltage is applied between parallel surface mounted electrodes separated by a distance equal to the contact length of the electrodes. It is reported as Ohms – sometimes ohms per square.
VOLUME RESISTIVITY (ASTM D257/IEC60093)
Volume resistivity is similar to surface resistivity except it is a measure of current leakage through the body of an insulating material. Measured generally in ohms/cm. It varies inversely with temperature and decreases slightly in moist environments.
Materials with volume resistivity values above 108 ohms/cm are considered to be insulators. Those with values of 103 to 108 ohms/cm are partial conductors.
DIELECTRIC CONSTANT OR RELATIVE PERMITTIVITY (ASTM D150/IEC60250)
The ratio of capacitance of a capacitor with test material as the dielectric to the capacitance of a capacitor with a vacuum as the dielectric. Materials being used to insulate electrical components should have a low dielectric constant.
DISSIPATION FACTOR (ASTM D150/IEC60250)
The ratio of the power dissipated in the test material to the power applied. A low dissipation factor is important for plastic insulators in high frequency applications such as radar equipment and microwave parts. Smaller values mean better dielectric materials. A high dissipation factor is important for welding capabilities.
ARC RESISTANCE (ASTM D495)
When an electric current travels across an insulator’s surface, this surface will become damaged over time and become conductive. Arc resistance is a measure of the time in seconds it takes to make an insulating surface conductive. Alternatively, arc resistance is the elapsed time in which the surface of a thermoplastic material will resist the formation of a conducting path.
COMPARATIVE TRACKING INDEX (IEC60112)
The voltage which causes tracking after 50 drops of ammonium chloride solution have fallen on the material. Comparative Tracking Index (CTI) is defined as the maximum voltage at which no failure occurs. CTI tests are undertaken to evaluate the safety of components carrying live parts.
The test procedure is complex and can be influenced by the condition of the electrodes, electrolyte and sample surface, and the applied voltage. Values can be lowered by additives such as pigments (especially carbon black), flame retardants and glass fibres.