The insulation resistance of wire and cable products is mainly to measure the volume resistance of cable insulation, surface resistance is not measured, and the influence of cable insulation resistance value is more factors, in practice, there are mainly four factors on the insulation resistance coefficient.

The temperature influence

As the temperature rises, the insulation resistance decreases, which is due to the increase of thermal motion and the increase of ion generation and migration. Under the action of voltage, the conduction current formed by ion motion increases, so the insulation resistance decreases. Theory and practice show that the insulation resistance coefficient decreases exponentially with increasing temperature, while the electrical conductivity increases exponentially with increasing temperature.

The effect of electric field intensity

When in low range, electric field intensity of ion migration rate increases with the electric field strength and ionic current and electric field intensity obey ohm's law, when the electric field intensity is higher, ion migration rate increased with the increase of the electric field intensity and the trend of gradually changed from linear relationship to index relationship, when close to breakdown there will be a lot of electron mobility, At this time, the insulation resistance coefficient is greatly reduced.

The voltage test voltage of various wire and cable products stipulated in the standard is at the stage when the mobility of ions increases proportionally with the electric field intensity, so the effect of electric field intensity on insulation resistance coefficient cannot be reflected. When the sample is tested for breakdown, the effect of electric field on insulation resistance coefficient is obviously reflected.

Influence of humidity

Due to the small conductivity, and the size of the water molecule is much smaller than the polymer molecules, under the heat action of polymer macromolecules and chain joint relative movement, water molecules are easy to penetrate into the polymer, the polymer conductive ions increase, and the insulation resistance decreases.

The standard provides a variety of wire and cable immersion test, such as rubber test sheet in the insulation resistance before immersion 24 hours, its purpose is because in use to meet moisture and water on the electrical performance.

Insulation resistance is one of the main electrical properties of insulation materials, is an important index of wire and cable products or materials, generally requires that the 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 lines will inevitably increase, resulting in the waste of electric energy, and electrical energy into heat energy to prepare for thermal breakdown, increasing the possibility of thermal breakdown.

In dc cable insulation, also can use different resistivity of insulating materials composition order, in order to improve the material utilization ratio and improve the surface resistivity of cable insulation is also very important, when the surface dirt and moisture, due to its uneven distribution, could make part of the surface under high voltage, and lead to partial discharge or creeping discharge, causing serious fault, But also sometimes use human methods to reduce the surface resistance, such as the conductive wire core of the cable, because the surface is not completely smooth resulting in uneven distribution of the electric field, high electric field intensity will cause corona discharge, in the conductive wire core surface coated with conductive rubber or semi-conductive plastic will reduce the electric field intensity of the core surface to eliminate corona, and, According to the changes in the resistivity of insulating materials to judge the degree of moisture.

The influence of material purity

The material is mixed with impurities, increasing the conductive particle in the material, so that the insulation resistance decreases, so the size of the insulation resistance of a rubber and plastic material will reflect the level of material purity, verify whether it meets the standard provisions. In the production process of wire and cable, the process does not strictly abide by the operating rules, mixed impurities, materials and damp bubbles, insulation partial core or outer diameter size is less than the standard, insulation layer or cracks, insulation scratch, etc., will make the insulation resistance of the product reduced. Therefore, it is necessary to check the insulation resistance to check whether there are problems in the process of operation. In the process of using wires and cables, measuring the change of insulation resistance can also check the insulation damage to prevent accidents.

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