HIGH VOLUME & SURFACE RESISTIVITY ,INSULATION RESISTANCE
Insulating material are used to isolate the live electric parts i.e. which are at an elevated electric potential. Ideally, the insulating material should be totally non-conductive i.e. the resistivity should be infinity. However, materials employed in practice do carry a certain, usually very weak current (leakage current) when a direct voltage is applied. Thus the resistivity of electrical insulating material is finite, although of extremely high value. The higher is the resistivity of the material, the better is its quality.
The leakage current passing through the insulated portion at a stable process of conduction i.e. a sufficiently long time after a direct voltage is applied, is also constant and is known as residual current. The resistance R in of an insulation equals to the ratio of direct voltage V applied to this insulation to the residual current Im passing through the portion i.e. R in =V / I in Ohms.
The total current passing through the insulation may be considered as consisting of two components of current - one which flow through the volume of insulation Iv, and the other passing over the surface of insulation Is. Unlike conduction current in metals, this component currents through volume and surface, in case of insulating materials has special significance. Hence the resistance is looked upon differently corresponding to volume and surface leakage current. Resistance which is obtained by the ratio of applied voltage V to volume leakage current Iv is known as volume resistance and that due to surface leakage current Is is known as surface resistance. The volume and surface resistance as obtained are dependent on the electrode geometry and physical dimension of the insulating material under test. Hence to define the basic insulation properties, the following two quantities are used - (i) Volume Resistivity (ii) Surface resistivity.
i) Volume Resistivity
It is the resistance of cube of 1 m side of an electrical insulating material in which current flows from one face to the opposite face (any leakage current passing over the surface of the cube is prevented from entering into the measuring circuit). The unit of volume resistivity is Ohm-m. It is also expressed in Ohm-cm unit.
ii) Surface Resistivity
It is the resistance of an insulating surface of a square area (sides may be of any length) measured between two parallel edges. Current is supposed to flow only through the defined square and not from any other paths. The unit of surface resistivity is Ohm since it is independent of the size of the square.
The Volume and surface resistivities as described above are the measures of the basic properties of insulating material only. In case of an electric apparatus or equipment, the isolation between the live parts provided by the insulation is measured by means of the parameter - Insulation Resistance (IR). This is the ratio of the voltage applied between the metal parts and the total leakage current flowing between them. This current has both the components of surface and volume leakage current. Normally it will be difficult to separate these two components in the process of measurement and hence an overall parameter IR is used to denote the quality of insulation.
The magnitude of the resistivity is not strictly definite and invariable but depends on number of factors such as presence of impurities , temperature , humidity , applied voltage time of holding and presence of residual charges etc.Foreign matters like carbon dust oil grease all lower surface resistance value.
Magnitude of voltage has a bearing on IR value. Generally higher test voltage result in lowering of IR , this is due to fracture 0r imperfection in the insulating materials.
In humid conditions moisture is absorbed by most of the insulation materials ,which drastically reduces volume resistance value. Moisture significantly reduces the resistivity of fibrous insulating materials.In order to protect dried hygroscopic materials from moisture they have to be impregnated or coated with non hygroscopic varnishes,compounds etc.
Even if insulating material does not absorb moisture, High humidity lowers the surface resistance by depositing moisture film on the surface of insulating materials. Insulation resistance of an equipment is generally improved by drying and providing a means to prevent condensation of moisture.Normally rise in temperature increases mobility of electron and thereby reduces IR.As a thumb rule for every rise of 18 0 C temperature IR halves. The equipments which may exhibit very high IR value may show large fall in the value at operating elevated temperature.
When voltage is applied the leakage current decreases with time asysptotically towards a limiting value. This is caused due to phenomenon known as dielectric absorption and sweep of mobile ions to the electrode. Interfacial polarisation is cause of dielectric absorption.
Time required for dielectric absorption varies from few seconds to several hours. Insulating materials having IR less than 1x10 11 ohm cm may stabilize in a minute but materials having IR exceeding 1 x 10 13 ohm cm may take several hours. The conventional time of electrification has been arbitrarily taken as one minute
The IR value is not directly related to the dielectric strength.
