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What kinds of factors affect the insulation resistance of wire and cable

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What kinds of factors affect the insulation resistance of wire and cable

作者: Release Date: 2019-11-26 Author: Click:

What kinds of factors affect the insulation resistance of wire and cable


The test of insulation resistance in wire and cable is mainly to measure the volume resistance of the insulation. The surface resistance is not calculated and measured. However, there are many factors that affect the insulation resistance value of the cable. In practical applications, the following four factors are mainly The influence of its resistivity is relatively large.


Temperature effect


As the temperature of the cable increases, the insulation resistance coefficient of wire and cable manufacturers decreases. This is because as the thermal movement increases, the generation and migration of ions change accordingly. Under the action of the voltage in the circuit, the ions move As the resulting conducted current increases, its insulation resistance decreases. After testing between insulation and temperature, both theory and practice have shown that the insulation resistance of the cable decreases exponentially with increasing temperature, but the conductor resistance of the cable increases exponentially with increasing temperature.


Influence of electric field strength


When the electric field strength 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 strength is relatively high, the ion mobility increases with the electric field strength. The increasing trend gradually changes from a linear relationship to an exponential relationship, and a large amount of electron migration will occur when it is close to breakdown. At this time, the insulation resistance coefficient is greatly reduced. National standards on cable tests stipulate that the withstand voltage test voltage of various cable products should be at a stage where the mobility of ions increases and decreases in proportion to the strength of the electric field, so only attention is paid to the strength of the electric field and the insulation resistance coefficient. The influence of the electric field is not reflected. When the test piece sample is subjected to a breakdown test, the influence of the electric field on the insulation resistance coefficient is clearly reflected.

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Effects of humidity


Because small-size cable conductors have greater resistance, and the size of water molecules is much smaller than that of polymers. Under the action of heat, the macromolecules and the constituent links of the polymer move relatively, and water molecules will take advantage of this opportunity. Hidden in the polymer, increase the conductive ions in the polymer, and reduce the insulation resistance. Insulation resistance is one of the main electrical properties of insulation materials, and is an important indicator of wire and cable products or materials. Generally, the insulation resistance is required to be not less than a certain value. If the insulation resistance value of the cable is too low, it must be because the leakage current in the cable line is increasing, leading to the waste of electrical energy in the line. At the same time, the electrical energy will be converted into thermal energy, which creates a necessary condition for thermal breakdown and increases. Probability of thermal breakdown. Various countries have standards for water and cable immersion tests. For example, the rubber sheet must be immersed in water for 24 hours before testing the insulation resistance during the test. The purpose is to test the cable when it is exposed to moisture and water during use. Of electrical performance.


In testing the insulation of DC cables, wire and cable manufacturers can use stepped insulation with different resistivity insulation materials to improve the utilization of the insulation material. However, the improvement of the surface resistivity is also very important for the overall insulation of the cable. When there is dirt and moisture on the surface, due to its uneven distribution, some surfaces may be subjected to higher voltages, which may cause partial discharges or creeping discharges, causing serious failures. However, sometimes the surface resistance is also artificially reduced, such as for cables. Conductive cores, because the surface is not completely smooth due to uneven electric field distribution, high electric field strength will cause corona discharge, after the surface of the conductive core is covered with conductive rubber or semiconducting plastic, the electric field strength on the core surface will be reduced to eliminate electricity. Halo, in addition, the degree of moisture of the material can be judged according to the change of the resistivity of the insulating material.


Influence of material purity


Wire and cable manufacturers have impurities mixed in the cable material, which increases the conductive material point in the cable material, which causes the cable's insulation resistance to decrease. Therefore, the insulation resistance of some rubber materials and plastic materials will reflect the cable material side What is the purity? Verify that it meets the standards. In the production process of wires and cables, the process does not strictly follow the operating procedures, mixed impurities, and materials are blistered by moisture. Cracks, insulation scratches, etc. will reduce the insulation resistance of the product. Therefore, to check the insulation resistance, it is necessary to check whether there is a problem during the process operation. During the use of wires and cables, measuring the change in insulation resistance can also check the insulation damage. Prevent accidents.

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