DI-ELECTRIC STRENGTH
Dielectric Strength is a measure of the electrical strength of a material as an insulator. Dielectric strength is defined as the maximum voltage required to produce a dielectric breakdown through the material and is expressed as Volts per unit thickness. A higher dielectric strength represents a better quality of insulator.
Dielectric strength is calculated by dividing the breakdown voltage by the thickness of the sample. The data is expressed in Volts/mm. The location of the failure is also recorded. A higher dielectric strength represents a better quality of insulator.
Test Method: ASTM D149
Instrument Used: Break Down Voltage Tester.
DI-ELECTRIC CONSTANT
Dielectric Constant is used to determine the ability of an insulator to store electrical energy. The dielectric constant is the ratio of the capacitance induced by two metallic plates with an insulator between them to the capacitance of the same plates with air or a vacuum between them. Dissipation factor is defined as the reciprocal of the ratio between the insulating materials capacitive reactance to its resistance at a specified frequency. It measures the inefficiency of an insulating material. If a material were to be used for strictly insulating purposes, it would be better to have a lower dielectric constant.When a material is to be used in electric applications where high capacitance is needed, a higher dielectric constant is required. The test can be conducted at different frequencies, often between the 10Hz and 2MHz range – the specific frequency is determined by the customer.
To measure Dielectric Constant of powders the sample is heated in a heating chamber until it melts completely and forms liquid then using a liquid cell the dielectric constant is measured at the required frequency.
Test Method: ASTM D150.
Instrument Used:Dielectric Constant Meter.
TAN D
A pure insulator when is connected across line and earth, it behaves as a capacitor. In an ideal insulator, as the insulating material which acts as dielectric too, is 100 % pure, the electric current passing through the insulator, only have capacitive component. There is no resistive component of the current, flowing from line to earth through insulator as in ideal insulating material, there is zero percent impurity.In pure capacitor, the capacitive electric current leads the applied voltage by 90 o .In practice, the insulator cannot be made 100% pure. Also due to the aging of insulators, the impurities like dirt and moisture enter into it. These impurities provide the conductive path to the current. Consequently, an electric leakage current flowing from line to earth through the insulator has a resistive component.
A very low-frequency test voltage is applied across the equipment whose insulation to be tested. First, the normal voltage is applied. If the value of tan delta appears good enough, the applied voltage is raised to 1.5 to 2 times of normal voltage, of the equipment. The tan delta controller unit takes measurement of tan delta values. A loss angle analyzer is connected with tan delta measuring unit to compare the tan delta values at normal voltage and higher voltages and analyze the results.During the test, it is essential to apply test voltage at a very low frequency.
Test Method: ASTM D150.
Instrument used: Tan D Meter.
SURFACE AND VOLUME RESISTIVITY
Surface resistivity is the resistance to leakage current along the surface of an insulating material. Volume resistivity is the resistance to leakage current through the body of an insulating material. The higher the surface/volume resistivity, the lower the leakage current and the less conductive the material is.
A standard size specimen is placed between two electrodes. For sixty seconds, a voltage is applied and the resistance is measured. Surface or volume resistivity is calculated, and apparent value is given (60 seconds electrification time).
Test Method: ASTM D257.
Instruments used: Sivananda Million Megohm meter.
ELECTRICAL CONDUCTIVITY
Electrical resistivity (also called specific electrical resistance or volume resistivity) and its inverse, electrical conductivity, is a fundamental property of a material that quantifies how strongly it resists or conducts electric current. A low resistivity indicates a material that readily allows electric current. Resistivity is commonly represented by the Greek letter ρ (rho). The SI unit of electrical resistivity is the ohm-meter (Ω⋅m).For example, if a 1m×1m×1m solid cube of material has sheet contacts on two opposite faces, and the resistance between these contacts is 1 Ω, then the resistivity of the material is 1 Ω⋅m.
Electrical conductivity or specific conductance is the reciprocal of electrical resistivity. It represents a material’s ability to conduct electric current. It is commonly signified by the Greek letter σ (sigma), but κ (kappa) (especially in electrical engineering) and γ (gamma) are sometimes used.
Electrical Conductivity testing works on the principle of eddy currents. A probe induces eddy currents at fixed frequency in the test part.These currents affect the electrical impedance of the test probe. The change in impendence is proportional to the electrical conductivity of the test part.Thus, conductivity measurement is possible by measuring the corresponding change in probe impedance. Some of the typical applications are: measurement of absolute conductivity of electrical hardware and electrical conductors made of copper, aluminium and their alloys; determination of homogeneity of metal near surface; estimation of properties which have correlation to electrical conductivity such as ageing, hardness, etc.
Test Method: ASTM E1004
Instruments used: Technofour Electrical Conductivity Meter.
