Choosing qualified wire and cable products is crucial for the safety of people's production and daily life. For ordinary consumers, it is not only necessary to know how to select qualified wire and cable products but also to understand the routine monitoring knowledge of wire and cable. In terms of wire and cable quality testing, there are several key indicators.
Insulation Resistance Testing:
Insulation resistance is a critical indicator reflecting 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 the insulation material under working conditions. For communication cables, a low inter-wire insulation resistance can also increase circuit attenuation, cross talk between circuits, and long-distance power supply leakage on the conductor cores, thus necessitating that the insulation resistance should be higher than the specified value.
Measuring insulation resistance can reveal defects in the manufacturing process, such as inadequate insulation drying or moisture-damaged sheath; contamination of insulation and the presence of conductive impurities; and cracking of the insulation layer due to various causes. During the operation of wires and cables, it is often necessary to test insulation resistance and leakage current, which serve as the primary basis for determining whether they can continue to operate safely.
Currently, the measurement of insulation resistance in wire and cable is commonly done using an ohmmeter (megger), as well as the galvanometer comparison method and the high resistance meter method (voltage-current method).
Measurement of Capacitance and Loss Factor
Cables, when subjected to AC voltage, allow current to flow through them. When the magnitude and frequency of the voltage are constant, the size of the capacitive current is directly proportional to the cable's capacitance (Cx). For extra-high voltage cables, this capacitive current may reach values comparable to the rated current, becoming a significant factor limiting cable capacity and transmission distance. Therefore, cable capacitance is also one of the main electrical performance parameters of cables.
In an alternating current field, insulators in cables experience dielectric losses due to leakage currents and various polarizations. These losses are represented by the dielectric loss factor or the tangent of the loss angle (TAN). Not only do they waste electrical energy, but they also cause the medium (insulator) to heat up, accelerating the aging of the insulation. Therefore, TAN is also one of the main parameters of cables.
Through the measurement of capacitance and loss tangent, it is possible to detect various insulation degradation phenomena such as moisture in insulation, peeling of insulation layers, and shedding of shielding layers. Therefore, capacitance and TAN measurements are conducted during both cable manufacturing and cable operation. For high-voltage cables, the measurements of Cx and TAN are performed under their operating conditions, i.e., under industrial frequency high voltage. High-voltage Wien bridges are commonly used, and in recent years, current ratio transformer bridges have also started to be employed.
Partial Discharge Measurement
For oil-filled cables, there is basically no local generation; even for paper-insulated cables with local discharges, they are typically very weak, such as a few pC, so these cables can be exempt from local discharge testing during factory inspections. For extruded cables, not only is the possibility of local discharges high, but the damage to plastics and rubbers caused by local discharges is also severe. As the voltage level and working field strength increase, this issue becomes even more pronounced. Therefore, for high-voltage extruded cables, local discharge measurements must be conducted during factory inspections.
There are numerous methods to measure partial discharge, such as measuring the discharge pulses (electrical measurement) based on the instantaneous charge exchange generated by the discharge, measuring the voltage (acoustic measurement) based on the ultrasonic waves produced during the discharge, or measuring the light intensity (optical measurement) based on the light generated by the discharge. For cables, the electrical measurement method is predominantly used.
