Testing items and methods for wires and cables

The rapid development of the power industry is an important guarantee for a country's economic development, and wires and cables are essential products in the process of power construction. At present, China's wire and cable industry has risen to become the largest manufacturing country. But nowadays, the quality problems of wires and cables in China are also prominent, which can lead to fires or other safety hazards
【 Keywords 】 Wire and cable; testing;
The rapid development of the power industry is an important guarantee for a country's economic development, and wires and cables are essential products in the process of power construction. At present, China's wire and cable industry has risen to become the largest manufacturing country. However, the quality problems of wires and cables in China are also prominent nowadays, and the resulting fires or other safety accidents are not uncommon, which is worrying. It is necessary to carry out testing of various projects in accordance with national standards and scientific methods during the production process, in order to provide scientific and accurate basis for strengthening supervision.
At present, there are many and disorderly manufacturers of wires and cables in China, with uneven levels of production technology and an imperfect quality assurance system in the industry, resulting in a significant gap in the quality of wire and cable products produced in China compared to other developed countries. In addition, some manufacturers use substandard raw materials and production processes in pursuit of high profits, further leading to the proliferation of counterfeit and inferior wire and cable products in the domestic market. In recent years, the country has gradually attached importance to the inspection of wires and cables, and has formulated some strict production and quality regulations for this purpose, which has to some extent promoted the improvement of the quality of wire and cable products in China.
Performance issues
A domestic quality inspection agency once conducted a survey on the wires and cables of some electrical appliances, and the results showed that among the production enterprises that have passed ISO9000 certification, the qualified rate of their wires and cables is also below 90%, while those small-scale wire and cable production enterprises that have not passed certification even have a product qualification rate of less than 30%. According to the investigation report issued by the quality inspection agency, the overall pass rate of wires and cables sold in various specialty stores on the market is generally around 70%, and the pass rate of wires and cables sold in smaller hardware stores is even less than 10%. The quality of wires and cables in China is worrying.
Testing method
According to relevant national standards, the testing items for wires and cables mainly include testing of electrical and mechanical properties. Among them, electrical performance testing mainly includes DC resistance testing, insulation resistance testing, and power frequency withstand voltage testing.
（1） DC resistance detection.
There are clear regulations in relevant national standards that the DC resistance of wires and cables must be compared based on the conductor resistance per kilometer, and the measured DC resistance data of wires and cables must first be converted to the DC resistance value per kilometer at a temperature of 20 ℃. After converting the measured DC resistance value to the DC resistance value at 20 ℃, if the value is less than the specified standard value, the wire and cable sample is considered a qualified product. Otherwise, it is considered an unqualified product.
At present, relevant departments in China usually use two methods, the bridge method and the current method, to determine the DC resistance of wires and cables. The measurement range of the bridge method is relatively narrow and can be divided into single arm bridge method and double arm bridge method. When the resistance value of the wire and cable is about 1 or more, the single arm bridge method is used; When the resistance value of the wire and cable is less than 1, the double arm bridge method is used. The current method, also known as the micro ohmmeter method, works by using a constant current source to output different constant currents based on the resistance value of the wire and cable, and then accurately measuring the voltage at both ends of the measured wire and cable. The measured data can be calculated according to Ohm's law to obtain the DC resistance of the measured wire and cable. The current method can output different currents, so its measurement range is relatively wide.
（2） Insulation resistance testing.
The measured insulation resistance of wires and cables must be converted into the insulation resistance value per kilometer. Unlike DC resistance, the insulation resistance value is inversely proportional to the length of the wire and cable; There are four types of measurement voltages for insulation resistance testing of low-voltage wires and cables: 100V, 250V, 500V, and 1000V. Among them, the detection voltages of 100V and 500V are widely used in quality inspection departments; The length of the measured wires and cables is not clearly specified, but for the convenience of measurement and calculation, it is generally measured at 10m. The charging time before measurement is generally 1 minute.
The insulation resistance detection of wires and cables generally adopts the voltage current method, also known as the high resistance meter method. Some wires and cables have metal protective covers with certain shielding functions. For this type of wire and cable, the insulation resistance measurement mostly measures the insulation resistance between the conductor and the metal cover, shielding layer, or armor layer; For wires and cables without metal sheaths, when measuring their insulation resistance values, the tested wires and cables must first be immersed in water, and then the insulation resistance between the conductor and water must be measured. During the testing, the tested sample must be kept matched with the water temperature.
At present, a DC resistance insulation resistance tester ZZJ3D has been developed in China. The tester is easy to operate, and the entire measurement process can be controlled by a computer. Its accuracy and stability are far higher than traditional detection equipment.
（3） Power frequency withstand voltage testing.
The power frequency withstand voltage is generally tested using AC voltage. According to national standards, the AC voltage used is an approximate sine wave with a frequency between 49Hz and 61Hz; For products with a rated voltage of 450/750V for wires and cables, 1500V high voltage is used when the insulation thickness is ≤ 0.6mm; When the insulation thickness is ≥ 0.6mm, use 2000V high voltage and apply pressure for 5 minutes. If the tested wire and cable sample does not experience breakdown or flashover, it is considered a qualified product. Otherwise, it is considered unqualified. For example, if there is a sample with a specification of 60227IEC53 (RVV) 300/500V32.5 that needs to be tested for voltage resistance, we need to connect the first core to high voltage water, then connect the second core to high voltage water, and finally connect the third core to high voltage water. Finally, all three cores need to be tested for voltage resistance once each, for a total of four tests.
（4） Mechanical performance testing.
Mechanical performance mainly refers to the tensile strength of wires and cables before and after aging. According to relevant national standards, when using a forced ventilation aging chamber to prepare aged wire and cable samples, the sampling should be as close as possible to the unaged parts during testing. The testing of mechanical properties is generally carried out directly using electronic tensile force measuring instruments. First, use a thickness gauge to accurately measure the width and thickness of the middle part of the measured wire and cable. Then, place the sample in a blast drying oven for artificial aging, and use an electronic tensile machine to measure. Record the elongation distance and the magnitude of the tensile stress when the wire and cable are stretched and broken. Using the obtained data, the tensile strength and elongation at break of the measured wire and cable before and after aging can be calculated, and compared with the product standard to determine whether it is qualified.
（5） Other testing items and methods.
In addition to the main testing items mentioned above, there are also insulation thickness testing, size and marking testing, and sheath thickness testing, which can generally be checked using simple measuring instruments or manual inspection. The insulation thickness refers to the thickness after removing all protective layers on the insulation layer. It is measured using a projector and a reading microscope, and the measured data is averaged and compared with the product standard. The measured average value must be greater than the specified value to be a qualified product. The external dimensions can be measured using a projector or wrapping tape. The ellipticity measurement method is to measure the outer diameter of any two points on the same cross-section of a circular sheathed cable, take the difference, and then use the difference to determine that the ratio of the average outer diameter to the cable standard cannot exceed 15%. The unqualified rate of wire and cable markings in our country is very high. According to national standards, wire and cable markings must have continuity, scratch resistance, and high clarity. Scratch resistance requires gently wiping back and forth 10 times with medical degreasing cotton dipped in alcohol, and clear printing is considered qualified.
Several countermeasures
（1） Intensify publicity efforts and strengthen quality awareness. Prioritize the promotion and education of regulations in special rectification efforts, organize enterprises to study relevant laws and regulations in a planned manner, publicly handle major cases, and create a grand atmosphere of propaganda, public opinion, and social supervision.
（2） Crack down on counterfeit goods and standardize production and operation. Cracking down on counterfeit and inferior products is currently an effective way to improve the quality of wires and cables. To combat counterfeiting, we need to start from three aspects: first, we need to grasp the source and crack down on enterprises that produce counterfeit and inferior products; Secondly, we will crack down on distributors who sell counterfeit and inferior wires and cables, and impose strict and severe punishment; The third is to crack down on and educate users who use counterfeit and inferior products, and impose appropriate penalties. In addition, while cracking down on counterfeit and shoddy products, attention should also be paid to standardization, assistance, and guidance to enable enterprises to operate in a standardized manner and improve product quality.
（3） Supervise and inspect, promote enterprise reform. Strengthen the supervision and inspection of enterprises, and improve product quality from the source. While strengthening supervision and spot checks, more attention should be paid to establishing quality awareness and brand awareness, so that enterprises can transform from price wars to quality wars. In supervision and inspection, while punishing non-conforming products, a rectification plan is proposed to steadily improve product quality until it reaches full compliance.
（4） Assist in technological transformation and enhance technological content. Encourage enterprises to improve the productivity of labor equipment, enhance the quality and technological content of products, and increase independent research and innovation. Develop and revise product standards for enterprises to organize production according to the standards. Improve the inspection mechanism of enterprises, strengthen product quality monitoring, and establish a long-term supervision mechanism. Improve the understanding of quality work in enterprises, and do a good job in enterprise quality system certification, product quality qualification certification, and safety certification.
Conclusion: Wire and cable are important products for ensuring the safe and stable development and operation of the power industry. Although relevant national departments have established detailed standards for the testing of wires and cables, there is still a certain gap in product quality between China and developed countries. All production enterprises must strictly follow national standards for production, strengthen testing, and improve the product quality of wires and cables in China.