Choosing qualified wire and cable products is related to people's production and life safety. For ordinary consumers, in addition to mastering how to choose qualified wire and cable products, they also need to understand the conventional monitoring knowledge of wires and cables. In the quality inspection of wires and cables, there are mainly the following important indicators.
Insulation resistance test:
The type of insulation resistance reflects an important indicator of the insulation characteristics of wire and cable products, which is closely related to the electrical strength of the product, dielectric loss, and the gradual deterioration of insulation materials in working conditions. For communication cables, if the insulation resistance of the line is too low, it can also increase circuit attenuation, crosstalk between circuits, and long-distance power leakage on conductive core wires. Therefore, the insulation resistance should be higher than the specified value.
During the process of measuring insulation resistance, some defects can be found, such as the insulation layer being dry and impermeable or the sheath being damaged by moisture; The insulation layer is contaminated and contains conductive impurities; Various reasons cause insulation layer cracking. In the operation of wires and cables, insulation resistance and leakage current are often tested as the main basis for whether they can continue to operate safely.
At present, for the measurement of insulation resistance of wires and cables, in addition to ohmmeters (trameters), the commonly used method is the galvanometer comparison method, the high resistance meter method, and the current method.
Measurement of capacitance and loss factor:
When an AC voltage is applied to a cable, a current flows through it. When the amplitude and frequency of the voltage are constant, the magnitude of the capacitive current is proportional to the capacitance (Cx) of the cable. For ultra-high voltage cables, the current of this capacitor may reach a value comparable to the rated current, becoming an important factor limiting cable capacity and transmission distance. Therefore, the capacitance of the cable is also one of the main electrical performance parameters of the cable.
In an AC electric field, the insulator in the cable will form dielectric loss due to leakage current and various polarizations. Represented by the dielectric loss factor or tangent of the loss angle (TAN), it not only wastes electrical energy, but also causes the dielectric (insulator) to heat up and accelerate insulation aging. Therefore, TAN is also one of the main parameters of the cable.
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 PCs, so these cables cannot measure partial discharge during factory testing. For extruded cables, there is not only a possibility of partial discharge, but also serious damage to plastics and rubber caused by partial discharge. As the voltage level and working field strength increase, this problem becomes more serious. Therefore, for high-voltage extruded cables, partial discharge measurements must be conducted during factory tests.
There are many methods for measuring partial discharge, which can measure discharge pulses based on the instantaneous charge exchange generated by discharge (electrical measurement method); The voltage can also be measured based on the ultrasonic waves generated by discharge (acoustic measurement method); The intensity of light can also be measured based on the light generated by discharge (photometry). For cables, the electrical measurement method is generally used.
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