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Electrical Properties of Insulating Materials
Dielectric Constant (Permitivity)
A dielectric is a material which has poor electrical conductivity but inherits an ability to store an electrical charge (due to dielectric polarization). The dielectric constant is a measure of the electrostatic energy stored in the insulating material per unit volume under one unit of voltage gradient (is a measure of the charge retention capacity of a medium). It is dependent also a temperature, moisture, exposure frequency and other factors
This property is defined as the ratio of the electric flux density in the material to that produced in free space by the same electric force, or the ratio of the permittivity of the substance to the permittivity of the free space is the dielectric constant.
Measures how strongly a plastic material opposes the flow of electric current through a volume of cubic specimen. The lower the resistivity the higher the conductivity (electric charges meet weak resistance to circulation). It is also known as electrical resistivity, bulk resisitivity, specific electrical resistance, specific volume resistance or simple resistivity. Volumen resistiviy is measured in units: Ohm.cm
- Below 105cm; the material is considered as conductive
- Above 109cm, the material is considered as an electrical insulator
Surface resistivity is the resistance to leakage current along the surface of an insulating material. The resistance offered by insulating material to the electric current is the composite effect of volume and surface resistances, which always act in parallel:
- Volumen resistance is the resistance to leakage of the current passes through a body of the material. It depends largely on the nature of the material
- Surface resistance, is the resistance to leakage along the surface of the material and is largely a function of surface finish and cleanliness
It is one of the most important factors in selecting a resin for various conformal coating, potting and encapsulation applications in the electrical and electronic industries, where excellent electrical insulation properties are a must. Dielectric strength can be defined as the maximum voltage required to cause a dielectric breakdown through the product. In other words, it is the measure of the insulating strength of a material or is the voltage per unit thickness at wich material conduct electricity. The higher the value, the more electrically insulating material is. Unit for dielectric strength is kV by mm or cm. Most plastics have good dielectric strengths, in order of 100 to 300 kV/cm. The measurement of dielectric strength is usually carried out either by:
- Short-time method
- Slow-rate of rise method
- Step-by-step method
Dielectric loss factor (loss tangent)
Is a measure of the loss of energy in a dielectric material through conduction, slow polarization currents, and other dissipative phenomena.
Distance Through Insulation
When it comes to high voltage safety certifications, the following three distances are of utmost importance to determine insulation ratings of semiconductor components: clearance, creepage, and DTI. Clearance is the shortest distance in air between two conductive parts and creepage is the shortest distance along the surface of a solid insulation material between two conductive parts across the isolation barrier. DTI, on the other hand, is the shortest distance within an insulating material interposed between two conductive parts. In other words, DTI is the distance inside a solid insulation whereas clearance and creepage are distances outside the solid insulation.
Solid Insulation is defined as insulation consisting entirely of solid material. The intrinsic material characteristics of solid insulation directly impact its insulation behavior. As the electric strength of solid insulation is considerably greater than that of air, the distances through solid insulation are much smaller than the clearance so that high electric stresses result. In insulation systems, gaps or voids may occur between electrodes and insulation and between different layers of insulation. Partial discharges can occur in these voids at voltages far below the level of puncture and this influences the service life of the solid insulation. As opposed to air, solid insulation is not a renewable insulating medium and therefore high voltage peaks which may occur infrequently can have a very damaging and irreversible effect on solid insulation. This situation can occur while in service and during routine high-voltage testing. The physical and geographical location of the equipment can affect the insulation system significantly. Environmental factors such as altitude, temperature, vibrations and humidity require consideration to ensure that the insulation remains reliable over the life time of the equipment
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