Three important indicators of wire and cable quality testing

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 knowing how to select qualified wire and cable products, it is necessary to understand the routine monitoring knowledge of wire and cable. In the quality inspection of wire and cable, there are mainly the following important indicators.

 

Wire and Cable

 

Insulation resistance test:

 

Insulation resistance reflects an important indicator of the insulation properties of wire and cable products. It is closely related to the electrical strength of the product, the dielectric loss, and the gradual deterioration of the insulation under working conditions. For communication cables, too low insulation resistance between lines will increase loop attenuation, crosstalk between circuits, and long-distance power supply leakage on conductive cores. Therefore, insulation resistance should be higher than the specified value.

 

Determining the insulation resistance can find defects in the process, such as insulation drying or sheath damage and moisture; insulation is contaminated and conductive impurities are mixed; cracking of the insulation layer caused by various reasons. In the operation of wires and cables, it is often necessary to detect the insulation resistance and leakage current as the main basis for whether or not to continue safe operation.

 

At present, the measurement of insulation resistance of wire and cable, in addition to the use of ohmmeter (shaking table), is commonly used galvanometer comparison method high resistance meter method (voltage - current method).

 

Measurement of capacitance and loss factor:

 

When the cable is connected to the AC voltage, current flows. When the amplitude and frequency of the voltage are constant, the magnitude of the capacitor current is proportional to the capacitance of the cable (Cx). For ultra-high voltage cables, the current of such capacitors may reach a value comparable to the rated current, which is 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 alternating electric field, the insulator in the cable, due to leakage current and various polarizations, forms a dielectric loss, expressed as a dielectric loss factor or loss tangent (TAN), which not only wastes electrical energy, but also causes the dielectric (insulator) ) Heating, accelerating insulation aging, so 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 dropout can be found, so capacitance and TAN measurement are performed in cable manufacturing or cable operation. For high-voltage cables, Cx and TAN are measured under their operating conditions, ie, power frequency high voltage. High-voltage Xilin bridges are usually used. This year, the current-to-transformer bridges have also begun to be used.

 

Partial discharge measurement:

 

There is basically no local power generation for oil-filled cables; even if the oil-paper cable has partial discharge, it is usually very weak, such as several PCs, so these cables may not be tested for partial discharge during the factory test. For the extruded cable, not only the possibility of partial discharge is large, but also the damage of the partial discharge to the plastic and the rubber is serious. As the voltage level increases, the work field strength increases, and the problem becomes more serious, so the high voltage Extruded cables are tested for partial discharge during factory testing.

 

There are many methods for measuring partial discharge, which can measure the discharge pulse according to the instantaneous charge exchange generated by the discharge (electrical measurement method); it can also measure the voltage according to the ultrasonic wave generated during discharge (acoustic measurement method); , measuring the intensity of light (photometry). Basically, electrical measurements are used for cables.