The rapid development of the power industry is a crucial guarantee for a country's economic growth, and wires and cables are indispensable products in the power construction process. Currently, China's wire and cable industry has risen to become a major manufacturing power. However, quality issues in the domestic wire and cable sector are also quite prominent, leading to frequent fires or other safety accidents, which is concerning. It is necessary to conduct inspections of various projects in accordance with national standards and scientific methods during the production process to provide scientific and accurate bases for enhanced supervision.
Currently, the domestic wire and cable manufacturing industry is characterized by a large number of manufacturers and a lack of order, with varying production技术水平 and an imperfect quality assurance system. This results in a significant gap in the quality of Chinese-made wire and cable products compared to those in developed countries. Additionally, some manufacturers, in pursuit of high profits, use substandard raw materials and production processes, further leading to an influx of counterfeit and inferior wire and cable products in the domestic market. In recent years, the state has gradually increased its attention to the inspection issues in the wire and cable industry, and has established some strict production and quality regulations, which have, to some extent, promoted the improvement of the quality of wire and cable products in China.
Performance Issues
A domestic quality inspection agency conducted an investigation on some electrical wire and cable products, revealing that the pass rate of wire and cable products from manufacturers with ISO9000 certification was below 90%. Meanwhile, the pass rate of products from small-scale wire and cable manufacturers without certification was even less than 30%. According to the investigation report issued by the agency, the overall pass rate of wire and cable products sold in various specialized stores on the market was around 70%, while the pass rate of products sold in smaller hardware stores was even less than 10%. The quality of domestic wire and cable products is cause for concern.
Testing Method
In accordance with national standards, the testing items for electrical wires and cables primarily include electrical and mechanical properties. The electrical property tests mainly involve direct resistance measurement, insulation resistance measurement, and working frequency withstand voltage testing, among others.
Direct Resistance Measurement
There are clear provisions in relevant national standards: The direct resistance of wires and cables must be compared based on the conductor resistance per kilometer, and the measured direct resistance data of wires and cables must first be converted to the direct resistance value per kilometer at 20℃. After converting the measured direct resistance value to the direct resistance value at 20℃ conditions, if the value is less than the specified standard, the wire and cable sample is deemed qualified; otherwise, it is considered不合格.
Currently, domestic authorities typically use two methods, the Wheatstone bridge method and the current method, to measure the direct resistance of wires and cables. The Wheatstone bridge method has a narrow measurement range and can be divided into single-bridge and double-bridge methods. The single-bridge method is used when the resistance of the wire or cable is approximately 1 ohm or more; the double-bridge method is used when the resistance is less than 1 ohm. The current method, also known as the micro-ohmmeter method, operates on the principle of using a constant current source to output different constant currents based on the resistance of the wire or cable. The voltage across the wire or cable is then precisely measured, and the data obtained is calculated according to Ohm's Law to determine the direct resistance of the wire or cable. The current method can output different currents, thus having a wider measurement range.
Insulation Resistance Testing.
Insulation resistance values of wire and cable must be converted to insulation resistance per kilometer. Unlike DC resistance, insulation resistance is inversely proportional to the length of the wire and cable. For low-voltage wire and cable insulation resistance testing, there are four measurement voltages available: 100V, 250V, 500V, and 1000V, with 100V and 500V being widely used during quality inspection. There is no specific length requirement for the wire and cable being measured, but for convenience, a length of 10 meters is typically used. The charging time before measurement is usually one minute.
Insulation resistance testing for wire and cable is typically conducted using the voltage-current method, also known as the high-resistance meter method. Wire and cable with metallic sheaths have certain shielding functions; for such products, the insulation resistance measurement usually involves the resistance between the conductor and the metallic sheath, shielding layer, or armor. For wire and cable without a metallic sheath, the insulation resistance must first be measured after submerging the cable into water, then testing the resistance between the conductor and the water. Additionally, the tested sample must be kept at a compatible temperature with the water during the measurement.
A new DC resistance insulation resistance tester, ZZJ3D, has been developed domestically. The tester is easy to operate, with the entire measurement process controlled by a computer, offering precision and stability far superior to traditional testing equipment.
(3) Power Frequency Voltage Withstand Testing.
AC voltage is generally used for dielectric strength testing. National standards stipulate that the applied AC voltage is an approximate sine wave with a frequency between 49Hz and 61Hz; for products with a rated voltage of 450/750V, when the insulation thickness is ≤0.6mm, a high voltage of 1500V is used; when the insulation thickness is ≥0.6mm, a high voltage of 2000V is applied, with a 5-minute duration. If the tested wire and cable sample does not experience breakdown or arcing, it is deemed a qualified product; otherwise, it is not. For instance, a sample with the specification 60227 IEC53 (RVV) 300/500V 32.5 requires dielectric strength testing. This involves connecting the first core to high voltage against water, followed by the second core, and then the third core. Finally, each of the three cores needs to be connected to high voltage against water for one dielectric strength test, totaling four tests.
(M) Mechanical Property Testing.
Mechanical properties primarily refer to the tensile strength of wires and cables before and after aging. Relevant national standards stipulate that: to obtain aging samples of wires and cables using a forced ventilation aging box, the sampling should be as close as possible to the unaged portion during testing. The measurement of mechanical properties is generally carried out directly with electronic tensile strength measuring instruments. First, use a micrometer to accurately measure the width and thickness of the middle section of the wire and cable being tested. Then, place the sample in an air-drying oven for artificial aging, and use an electronic tensile tester to measure it. Record the elongation distance and the tensile stress at the point of stretching and breaking of the wire and cable. With this data, you can calculate the tensile strength and elongation at break before and after aging of the tested wire and cable. Compare these results with the product standards of the product to determine if it is qualified.
(5) Other Testing Items and Methods.
In addition to the aforementioned main testing items, there are also tests for insulation thickness, dimensions and markings, and sheath thickness. These can generally be carried out using relatively simple measuring instruments or manual inspection. Insulation thickness refers to the thickness after all protective layers on the insulation are removed, which is measured using a projector and a reading microscope. The average measurement data is then compared to the product standard's specifications; the measured average must be greater than the specified value to be considered a qualified product. The external dimensions can be measured using a projector or wrapping tape. The method for measuring ellipticity is to measure the outer diameter of any two points on the same cross-section of a circular sheath cable, take the difference, and ensure that this difference does not exceed 15% of the cable's standard average outer diameter. The failure rate of marking on China's electrical wires and cables is high. The national standard stipulates that the markings on electrical wires and cables must have continuity and resistance to rubbing, as well as high clarity. The resistance to rubbing is tested by gently wiping the printed area with alcohol-soaked medical gauze back and forth ten times; if the print is clear, it is considered qualified.
Several strategies
(1) Enhance promotional efforts and reinforce quality awareness. Prioritize the education and publicity of regulations in the special rectification campaign, systematically organize enterprises to study relevant laws and regulations, publicly handle major cases, and create a massive public opinion and social supervision atmosphere.
In our ongoing efforts to combat counterfeit and substandard products, we are focusing on regulating production and business operations. Combating counterfeit and substandard electrical wires and cables is an effective way to improve quality. This fight should be approached from three angles: first, by targeting the source, combating companies producing such products; second, by cracking down on dealers selling counterfeit cables, with strict and severe penalties; and third, by educating and penalizing users of counterfeit products. Additionally, while tackling counterfeits, we emphasize the importance of standardization, support, and guidance to ensure businesses operate in a regulated manner and enhance product quality.
(3) Enhancing Supervision and Inspection to Promote Enterprise Reform. Strengthen the supervision and inspection efforts of enterprises to improve product quality from the source. While reinforcing random inspections, place greater emphasis on fostering quality and brand awareness, transforming the industry from a price war to a quality war. During supervision and inspection, impose penalties on substandard products and simultaneously propose corrective measures to ensure steady improvement in product quality until all products meet the required standards.
(4) Assist in technological transformation and enhance technological content. Encourage companies to increase productivity of labor equipment, improve product quality and technological content, and boost independent and innovative research. Establish or revise product standards for companies to organize production according to these standards. Perfect the company's inspection system, strengthen product quality monitoring, and establish a long-term supervision mechanism. Raise the awareness of companies regarding quality work, and manage the company's quality system certification, product quality qualification certification, and safety certification, etc.
