The selection of qualified wire and cable products is related to the safety of people's production and life. For ordinary consumers, in addition to mastering how to select qualified wire and cable products, it is also necessary to understand the regular monitoring knowledge of wire and cable. In the quality inspection of wires and cables, there are mainly the following heavyTo indicators.
Test of insulation resistance:
The insulation resistance type reflects the important index of the insulation characteristics of wire and cable products, which is closely related to the electrical strength of the product, the dielectric loss, and the gradual deterioration of the insulation material in the working state. For communication cables, too low insulation resistance between lines will also cause loop attenuation, crosstalk between loops, and leakage of long-distance power supply on the conductive core. Therefore, the insulation resistance is required to be higher than the specified value.
Determination of insulation resistance can be found in the process defects, such as dry insulation or sheath damage moisture; insulation pollution and conductive impurities mixed; various reasons caused by the insulation layer cracking. In the operation of wires and cables, insulation resistance and leakage current are often detected as the main basis for safe operation.
At present, the measurement of wire and cable insulation resistance, in addition to the use of ohmmeter (shaking table), the commonly used galvanometer comparison method high resistance meter method (voltage-current method).
measurement of capacitance and loss factor:
When the cable is coupled with an AC voltage, a current flows through it. When the amplitude and frequency of the voltage are constant, the capacitance current is proportional to the capacitance (Cx) of the cable. For the pressure cable, the current of this capacitor may reach a value comparable to the rated current, which becomes an important factor limiting the cable capacity and transmission distance. Therefore, the capacitance of the cable is also one of the main electrical performance parameters of the cable.
In the AC electric field, the insulator in the cable will form a dielectric loss due to the leakage current and various polarization, which is represented by the dielectric loss factor or loss angle tangent (TAN). It not only wastes electric energy, but also causes the dielectric (insulator) to heat up and accelerates the insulation aging. Therefore, TAN is also one of the main parameters of the cable.
Through the measurement of capacitance and loss factor, various insulation deterioration phenomena such as insulation moisture, insulation layer and shielding layer falling off can be found. Therefore, capacitance and TAN measurement are carried out in cable manufacturing or cable operation. For high-voltage cables, Cx and TAN are measured under their working conditions, that is, power frequency and high voltage. High-voltage Xilin bridges are usually used. This year, current ratio transformer bridges have also been used.
Partial discharge measurement:
For oil-filled cables, there is basically no local power generation; even if there is partial discharge in oil-paper cables, it is usually very weak, such as several PC, so these cables can not detect partial discharge in factory tests. For extruded cable, not only the possibility of partial discharge, and partial discharge of plastic, rubber damage is more serious, with the improvement of the voltage level, the working field strength, this problem is more serious, so the high voltage extrusion cable, in the factory test to do partial discharge measurement.
There are many methods for measuring partial discharge, which can be measured according to the instantaneous charge exchange generated by the discharge, and the discharge pulse (electrometric method); it can also be measured according to the ultrasonic wave generated by the discharge, and its voltage (acoustic method); it can also be measured according to the light generated by the discharge. The intensity of the light (optical method). For the cable is basically the use of electrical measurement method.
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