Main factors affecting cable insulation resistance

The insulation resistance of wire and cable products mainly measures the volume resistance of the cable insulation, and the surface resistance is not measured. There are many factors affecting the resistance value of the wire and cable. In practice, the three main factors affect the insulation resistance coefficient, and its specific details as following:
1, the influence of temperature The temperature rises, the insulation resistance coefficient decreases, this is due to the increase of thermal motion, ion generation and migration have increased, under the action of the voltage, the conduction current formed by ion movement increases, so the insulation resistance decline.
Theory and practice show that the insulation resistance coefficient decreases exponentially with increasing temperature, while conductance increases exponentially with increasing temperature.
2. Effect of electric field intensity When the electric field intensity is in a relatively low range, the mobility of ions increases as the electric field strength increases, in a proportional relationship, the ion current and the electric field strength obey Ohm's law, when the electric field intensity is relatively high, ion migration The rate of increase with the increase of the electric field intensity gradually changes from a linear relationship to an exponential relationship. When the breakdown occurs, a large amount of electron migration occurs, and the insulation resistance coefficient greatly decreases.
The withstand voltage test voltages of all kinds of wire and cable products specified in the standard are in the stage where the mobility of ions 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 used for the breakdown test At that time, the influence of the electric field on the insulation resistance coefficient is clearly reflected.
3, the impact of humidity due to the small conductance, and the size of the water molecules is much smaller than the polymer molecules, under the action of heat polymer macromolecules and the composition of the relative movement of the chain, water molecules to facilitate the penetration of the polymer This increases the amount of conductive ions in the polymer and decreases the insulation resistance.
The standard stipulates various water and cable immersion tests. For example, the rubber test piece is immersed in water for 24 hours before measuring the insulation resistance. Its purpose is to meet the influence of moisture and water on the electrical properties during use. Insulation resistance is the main insulation material. One of the properties is an important indicator of wire and cable products or materials. Generally, insulation resistance is 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 the waste of electrical energy, and at the same time the electrical energy becomes thermal energy, which prepares the conditions for thermal breakdown and increases the possibility of thermal breakdown.
In the insulation of DC cables, it is also possible to use different resistivity materials to form hierarchical insulation to improve the utilization of materials. It is also very important to improve the surface resistivity of the cable insulation. When the surface has dirt and moisture, its distribution is not Uniform, may cause part of the surface to withstand higher voltage, resulting in partial discharge or creepage discharge, causing serious failure, but sometimes also artificially reduce the surface resistance, such as the conductive core of the cable, because the surface is not completely smooth caused by the electric field Non-uniform distribution, high electric field strength will cause corona discharge, after the surface of the conductive core is covered with conductive rubber or semi-conductive plastic, it will reduce the electric field strength of the core surface to eliminate corona, and can be based on the resistivity of the insulating material. Change the situation to determine the degree of moisture in the material.
4, the impact of material purity

Impurities are mixed in the material, which increases the conductive point in the material and reduces the insulation resistance. Therefore, the size of the insulation resistance of a certain rubber and plastic material will reflect the purity of the material, and verify whether it meets the requirements of the standard in the wire and cable. During the production process, the process does not strictly comply with the operating procedures, mixed impurities, materials damp and blisters, insulated core or outside diameter size is less than the standard requirements, insulation layered or cracked, insulation scratches, etc., will make the product insulation resistance Reduce, so check the insulation resistance must check whether there is a problem during the process of operation, in the use of wire and cable, measuring the change in insulation resistance can also check the insulation damage, prevent accidents.

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One-Way Scraping Wiper Linkage

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