At present, there are mainly two methods for measuring the DC resistance of wire and cable: bridge method and current method. The bridge method has a single-arm bridge method and a double-arm bridge method. The single-arm bridge method measures a resistance of 1 ohm or more, the double-arm bridge method measures a resistance of 1 ohm or less, and the current rule can be based on the measurement of resistance. Different use of different currents, the measurement range is relatively broad. Among them, when using two-arm bridges and current measuring resistors, four-terminal measuring fixtures are often used to eliminate the effect of test resistance and contact resistance on the measurement.

Insulation resistance test, that is, testing the insulation performance of wire and cable. The insulation resistance of wire and cable measures the leakage current of the wire and cable under normal conditions. The standard specifies that the measurement voltage of the insulation resistance of the low-voltage wire and cable is 100V, 250V, 500V and 1000V. The quality inspection institution uses mostly 100V and 500V. Mostly the minimum insulation value of the wire specified in the IOM standard, so the measured value is also converted into the insulation resistance value per kilometer.

Power frequency withstand voltage test, that is AC voltage test. According to the standard, the test voltage should be an approximate sine wave of 49-61 Hz. For wire and cable with a rated voltage of 450/750, the insulation thickness is 0.6 ram and the following is 1500 V. For insulation thickness of 0.6 mm or more, 200 V is used, and the pressurization time is 5 min. No breakdown or flashover occurs. The sample passes.

Aging test is the stability test that can maintain stable performance under stress (mechanical, electrical and thermal). The heat aging test is to test the aging characteristics of the sample under the effect of heat. The sample is placed in the environment above the rated working temperature. After a specified period of time, the change of some sensitive properties before and after the aging is measured to evaluate the aging characteristics. . Can also be used to increase the temperature to accelerate the aging test, coupled with moisture, vibration, electric field and other heat, machine, electricity and other stress to form an aging cycle, after each aging cycle, determine certain selected sensitive performance parameters. Until the performance drops to the value of the indicated lifetime. This results in a shorter lifetime L (time for sample heating) at higher temperatures.

The thermal stability test is that the cable is heated by the current while it is also subjected to a certain voltage. After a certain period of heating, some sensitive performance parameters are measured to evaluate the stability of the insulation. The insulation stability test is divided into long-term stability test or short-term accelerated aging test.

In addition to the above-mentioned performance tests, the appearance size and marking of wire and cable are also an aspect of the assessment. The size of wire and cable is mainly the insulation thickness and external dimensions of wire and cable. In the random inspection of wire and cable, it was found that wire and cable with unqualified marks accounted for 70% of unqualified wires and cables. The sign stipulates that the wire and cable signs shall have continuity and rub resistance, and have a certain degree of clarity; the product sign shall have model numbers, specifications, and standard numbers; the continuity of the marks shall be the end of a complete mark and the next standard. The distance between the starting end; the sheathing should not exceed 550 mm; the insulation should not exceed 200 mm; the digital markings should be repeated at equal intervals along the insulating core, the adjacent two sets of digital markings should be reversed to each other, and the adjacent two sets of digital markings The spacing should not be greater than 50 ram.

There are thousands of wire and cable manufacturers in China, and the quality is uneven. A large number of black manufacturers produce inferior wires and cables, and even counterfeiting well-known brands to defraud commercial interests. Fire and other safety accidents due to quality problems of electric wires and cables have occurred from time to time. Therefore, the inspection of wires and cables is extremely important.

In view of this, as early as in previous years, China has formulated detailed standards for wire and cable from production to application and testing. Currently, the quality agencies of all electrical testing projects in China have basically carried out wire and cable product testing projects. At present, the unqualified wire and cable on the market involves all aspects of the standard requirements, including the unqualified dimensions and signs, and has not passed the electrical performance test, especially the DC resistance and insulation resistance tests.

Quality Factor Analysis First of all, raw materials are the basis of product quality. Only when the raw materials (copper wire, aluminum wire, and rubber compound) are shut down to ensure that the quality of the used raw materials meets the requirements, can quality problems be prevented from occurring at the root. At this point, the production company may implement such methods as self-inspection, commissioned laboratory inspection, or verification report provided by the supplier by the factory.

Second, in the production process, the copper wire must be grounded and kept in line with the spark machine's grounding to ensure that the spark test is effective, and at the same time, to ensure the production personnel's protection against electric shock, the pay-off wire or take-up reel can be used to prevent the copper wire from being grounded. The problem occurred.

Again, extrusion is the key link, and the appropriate melting temperature should be chosen based on the different compounds. Usually, the temperature of the nozzle of the extruder is low, the temperature in the middle section is basically the same, and the head temperature is the highest. During the production process, the temperature of each section should be monitored and recorded regularly to prevent excessive or under-melting of the rubber material. Over-melting and under-firing can lead to a reduction in the insulation of wires and cables.

The last is the spark test. At present, after wire and cable extrusion, the factory uses water to cool the wire. Therefore, before the spark test, the moisture on the surface of the wire and cable must be removed to avoid flashover caused by moisture. For example, the air is removed by wind blowing. Moisture.

Spark test is a key link in the production of wire and cable. Select the appropriate test voltage according to the insulation thickness of the wire. Because, too low voltage can not guarantee the quality of the wire meets the requirements, if it is too high, it will cause damage to the insulation performance of the wire. The spark test must ensure that the wire and cable stay in the high voltage test area for not less than 5 seconds, can be controlled by adjusting the take-up speed. During the test, static electricity is usually generated on the surface of the wire and cable. Therefore, static electricity on the surface of the wire and cable should be removed immediately after the spark test to avoid damage to the production personnel. At the same time, in order to ensure that the test results are valid, the spark test must be periodically calibrated and checked for effective operation.

In addition, linear adjustment is also essential. Adjust the production equipment to ensure that the wires and cables are in a straight line in the production process. In particular, the conductors on the head should be as concentric as possible to make the conductors be concentric with the inlet and outlet, so that the average insulation thickness of the wires is closer to the thinnest point, which is beneficial to Improve wire and cable insulation and save raw materials.

Test items and methods Wire and cable are national CCC compulsory certification products. The national standards have detailed descriptions of their inspection items, mainly including electrical properties, mechanical properties, and flame retardant properties. The quality inspection agency mainly tests the electrical properties of wires and cables.

The electrical performance test mainly includes conductor direct current resistance test, insulation resistance test, power frequency withstand voltage test, impulse voltage test, etc. At present, the electrical performance tests carried out by domestic quality inspection institutions include conductor direct current resistance, insulation resistance and power frequency withstand voltage test.

DC resistance test, that is, measuring the conductive properties of wire and cable. This test can reflect the conductor performance of the wire and cable and the size of the outer diameter of the wire. When the cross-sectional area is constant, the material of the conductor determines the direct current resistance of the conductor. The corresponding standard specifies the maximum value of the resistance of the conductor at different temperatures. That is to say, the standard is based on the conductor resistance per kilometer. . Taking 20% ​​of the maximum conductor resistance as an example, the DC resistance value of the wire and cable measured at room temperature must first be converted into a DC resistance value of 20 kilometers per kilometer. If R20 is less than the standard value, the wire and cable sample is qualified, otherwise it is unqualified.

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