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What are the most critical indicators of wire and cable quality testing?

Dec.11,2019

The detection of wires and cables has always been an important part of the national standard wires and cables. The higher the quality inspection department within a wire and cable enterprise is, the better the quality of wires and cables produced by this enterprise is and the more trustworthy it is. The stricter the third party's quality inspection department is, the more beneficial it will be to the development of this industry.

Following are some important indicators of wires and cables, which are the most critical indicators to measure wires and cables.

1.Measurement of DC resistance of conductor

The conductor cores of wires and cables mainly transmit electric energy or signals. The resistance of conductor is the main index of its electrical performance. When AC voltage acts, the core resistance is larger than that when DC voltage acts because of skin effect and adjacent effect surface. However, when the eye frequency is 50Hz, the difference between them is very small. Now the standard stipulates that only the DC resistance or resistance of the core can be tested to exceed the standard. Some defects in the production process can be found through the inspection of the prescribed values in this paper, such as wire breakage or partial single-wire breakage, cross-section of wire does not meet the standards, length of product is incorrect, etc. For power cables, it is also possible to check whether it will affect the allowable current carrying capacity in the operation of wire and cable products. The measurement of conductor DC resistance includes single arm DC resistance method and double arm DC bridge method. The accuracy of the latter method is higher than that of the former. The test steps are also more complex than the former.

2.Insulation resistance test

The insulation resistance type is an important index reflecting the insulation characteristics of wire and cable products. It is closely related to the electrical resistance, dielectric loss and the gradual deterioration of insulating materials under working conditions. For communication cables, too low insulation resistance will also increase circuit attenuation, cross talk between circuits and long-distance power supply leakage on conductor cores, so it is required that the insulation resistance should be higher than the prescribed value. Defects in the process can be found by measuring insulation resistance, such as dry insulation or damp sheath damage, pollution of insulation and mixing of conductive impurities, cracking of insulation layer caused by various reasons, etc. In the operation of wires and cables, insulation resistance and leakage current are often detected as the main basis of whether they can continue to operate safely. At present, besides the ohmmeter (shaking table), the common method for measuring insulation resistance of wire and cable is the galvanometer comparison method and the high resistance method (voltage-current method).

3.Measurement of capacitance and loss factor

When the voltage amplitude and frequency are constant, the capacitive current is proportional to the capacitance (Cx) of the cable. For UHV cables, the current of this capacitor may reach a value comparable to the rated current, which becomes an important factor limiting the capacity and transmission distance of the cables. Therefore, the capacitance of the cable is also one of the main electrical performance parameters of the cable. In AC electric field, the insulator in cable will form dielectric loss because of leakage current and polarization. It is expressed by dielectric loss factor or tangent value of loss angle (TANdel). It not only wastes electric energy, but also makes dielectric (insulator) heat and accelerates insulation aging. Therefore, TANdel is also one of the main parameters of cable. One. Through the measurement of capacitance and loss factor, various insulation deterioration phenomena, such as damp insulation, insulation layer and shielding layer falling off, can be found. Therefore, capacitance and TANdel are measured in both cable manufacturing and cable operation. For high voltage cables, the measurement of Cx and TANdelta is carried out under their working conditions, i.e. power frequency and high voltage. Usually the high voltage Xilin bridge is used. This year, the current ratio transformer bridge has also been used.

4.Insulation strength test

The insulation strength of wires and cables refers to the ability of insulating structures and materials to withstand electric field without breakdown. In order to check the quality of wires and cables and ensure the safe operation of products, all types of insulating wires and cables are generally tested for insulation strength. Insulation strength test can be divided into voltage withstanding test and breakdown test. Voltage withstanding test is to apply a certain voltage to the test under certain conditions. After a certain period of time, the breakdown occurs as a criterion to judge whether the test is qualified or not. The voltage of time is generally higher than the rated working voltage of the test product. Specific voltage value and withstanding time are stipulated in the product standard. Through withstanding voltage test, the reliability of the product under working voltage can be tested and serious defects in insulation can be found. Some defects in production process can also be found, such as: serious insulation is not included. Part damage, serious defects on conductors that cause sharp distortion of electric field, penetration defects or large conductive impurities in insulation production, etc. Breakdown test is to measure the breakdown field strength or breakdown voltage until the breakdown occurs under certain test conditions. Through breakdown test, the safety margin between the ability of cable to withstand voltage and the working voltage can be checked. The breakdown field is one of the important parameters in cable design. Cables generally bear AC voltage in operation, but also DC voltage in HVDC transmission system and some special occasions. For high-voltage cables, they may also be attacked by atmospheric voltage (lightning) and operating overvoltage. Therefore, according to the different experimental voltage waveforms, it can be divided into three kinds of insulation strength tests: 1. AC (power frequency) voltage, 2. DC voltage and 3 impulse voltage.

5.Partial discharge measurement

For oil-filled cables, there is basically no partial power generation; even if there is partial discharge in oil-paper cables, it is usually very weak, such as several PCs, so these cables can be unexpected partial discharge in factory tests. For extruded cables, not only the possibility of partial discharge is great, but also the damage of partial discharge to plastics and rubber is serious. With the increase of voltage level and the increase of worksite strength, this problem becomes more serious. Therefore, the partial discharge measurement of high voltage extruded cables should be done in factory test. There are many methods for measuring partial discharge, which can measure the discharge pulse (electrometric method) according to the instantaneous charge exchange generated by discharge, the voltage (acoustic method) according to the ultrasonic wave generated during discharge, and the intensity of light (photometric method) according to the light generated by discharge. Electrical measurement is basically used for cables.

6.Aging and stability test

Aging test is a stability test that can maintain stable performance under the action of stress (mechanical, electrical and thermal). Thermal aging test: Simple thermal aging test is to test the aging characteristics of the test sample under the action of heat. The test sample is placed in an environment higher than the rated working temperature and a certain value. After a specified time, the aging characteristics are evaluated by measuring the changes of some sensitive properties before and after aging. It can also be used to accelerate the aging of the test sample by increasing the temperature, together with the thermal, mechanical and electrical stresses such as damp, vibration, electric field, etc. to form an aging cycle. After each aging cycle, some selected sensitive performance parameters can be determined. Until the performance decreases to the value of the nominal life. In this way, at a higher temperature T, a shorter life L (heating time of the sample) can be obtained. Thermal stability test: Thermal stability test is that the cable is heated by electric current while withstanding a certain voltage. After a certain period of heating, some sensitive performance parameters are determined to evaluate the insulation stability. Insulation stability test can be divided into long-term stability test or short-term accelerated aging test. Responsible enterprises are thinking about how to improve the quality of their products in order to improve their market competitiveness. Product quality is the basis of their production and development, especially in the wire and cable industry.

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