The insulation resistance of wire and cable products is mainly to measure the volume resistance of cable insulation. The surface resistance is not measured, and there are many factors affecting the insulation resistance value of the cable. In practice, there are mainly four factors that have a great influence on the insulation resistance coefficient.
Temperature effect
As the temperature rises, the insulation resistance coefficient decreases. This is because the thermal motion increases, and the generation and migration of ions increase. Under the action of the voltage, the conduction current formed by the ion motion increases, and the insulation resistance decreases.
Theory and practice show that the insulation resistance coefficient decreases exponentially with increasing temperature, while the conductance increases exponentially with increasing temperature.
Effect of electric field strength
When the electric field strength is in a relatively low range, the mobility of ions increases with the increase of the electric field strength. In a proportional relationship, the ion current and the electric field intensity obey Ohm's law. When the electric field strength is relatively high, the ion mobility increases with the electric field strength. The trend of increase gradually changes from a linear relationship to an exponential relationship. When a breakdown occurs, a large amount of electron migration occurs, and the insulation resistance coefficient is greatly reduced.
The withstand voltage test voltages of various wire and cable products specified in the standard are in the stage where the ion mobility increases proportionally with the electric field strength, so the influence of the electric field strength on the insulation resistance coefficient cannot be reflected. When the test piece sample is subjected to the breakdown test The effect of the electric field on the insulation resistance coefficient is clearly reflected.
Humidity effect
Because the small conductance is large, and the size of the water molecules is much smaller than the molecules of the polymer, the polymer macromolecules and the constituent links are relatively moved under the action of heat, and the water molecules are easily penetrated into the polymer to make the polymer The number of conductive ions increases, and the insulation resistance decreases.
The standard stipulates the water immersion test of various wires and cables. For example, the rubber test piece is immersed for 24 hours before measuring the insulation resistance. The purpose is to encounter the influence of moisture and water on the electrical performance during use.
Insulation resistance is one of the main electrical properties of insulating materials. It is an important indicator of wire and cable products or materials. Generally, insulation resistance is required to be not lower than a certain value. If the insulation resistance value is too low, the leakage current along the wire and cable line will inevitably increase, resulting in waste of electrical energy, and the electrical energy becomes thermal energy, preparing conditions for thermal breakdown, increasing the possibility of thermal breakdown.
In DC cable insulation, it is also possible to use a material with different resistivity to form a stepped insulation to improve the utilization of the material. It is also important to increase the surface resistivity for cable insulation. When there is dirt and moisture on the surface, it is not distributed. Uniformity may cause some surfaces to withstand higher voltages, causing partial discharge or creeping discharge, causing serious failures, but sometimes artificial methods are used to reduce surface resistance, such as conductive cores of cables, because the electric field is not completely smooth. Uneven distribution, high electric field strength will cause corona discharge, after the surface of the conductive core is coated with conductive rubber or semi-conductive plastic, the electric field strength of the core surface will be reduced to eliminate corona, and, according to the resistivity of the insulating material, Change the situation to determine the degree of moisture in the material.
Effect of material purity
Impurities are mixed into the material, which increases the conductive dots in the material and reduces the insulation resistance. Therefore, the insulation resistance of a certain rubber and plastic material will reflect the purity of the material, and verify whether it meets the standard requirements in wire and cable. During the production process, the process does not strictly follow the operating procedures, mixing impurities, materials are damp and damp, insulation eccentricity or outer diameter is smaller than the standard, insulation delamination or cracks, insulation scratches, etc., will make the insulation resistance of the product Reduce, so check the insulation resistance must check whether there is a problem during the process operation, in the process of using the wire and cable, measuring the change of the insulation resistance can also check the insulation damage to prevent accidents.