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 routine monitoring knowledge of wire and cable. In the quality inspection of wires and cables, there are several important indicators.

Insulation resistance test:

Insulation resistance is an important indicator that reflects the insulation characteristics of wire and cable products. It is closely related to the product's electrical strength, dielectric loss, and the gradual deterioration of insulation materials under working conditions. For communication cables, low insulation resistance between wires can increase circuit attenuation, crosstalk between circuits, and long-distance power supply leakage on conductive cores. Therefore, it is required that the insulation resistance should be higher than the specified value.

Measuring insulation resistance can reveal defects in the process, such as insulation that is dry and not transparent, or damage and moisture to the sheath; Insulation is contaminated and conductive impurities are mixed in; Insulation layer cracking caused by various reasons. 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, in addition to using an ohmmeter (megohmmeter), commonly used methods for measuring the insulation resistance of wires and cables include the galvanometer comparison method, high resistance meter method, and current method.

Measurement of capacitance and loss factor:

When AC voltage is applied to a cable, 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 insulation in the cable will form dielectric losses due to leakage current and various polarizations. Represented by the dielectric loss factor or loss angle tangent (TAN), it not only wastes electrical energy, but also heats up the dielectric (insulation) and accelerates insulation aging. Therefore, TAN is also one of the main parameters of the cable.

Through the measurement of capacitance and loss factor, various insulation degradation phenomena such as insulation moisture, insulation layer and shielding layer detachment can be discovered. Therefore, capacitance and TAN measurements are carried out in both cable manufacturing and cable operation. The measurement of Cx and TAN for high-voltage cables is carried out under their working conditions, that is, under high frequency voltage. The commonly used bridge is the high-voltage Xilin bridge, and in recent years, the current ratio transformer bridge has 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 a few PCs, so these cables can be tested without partial discharge during factory testing. For extruded cables, not only is there a high possibility of partial discharge, but partial discharge also causes serious damage to plastics and rubber. With the increase of voltage level and working field strength, this problem becomes even more serious. Therefore, partial discharge measurement is required for high-voltage extruded cables during factory testing.

There are many methods for measuring partial discharge, and the discharge pulse can be measured based on the instantaneous charge exchange generated by the discharge (electrical measurement method); The voltage can also be measured based on the ultrasonic waves generated during discharge (acoustic method); The intensity of light can also be measured based on the light generated by the discharge (optical measurement method). For cables, the electrical measurement method is basically used.