Problems and Improvement Methods in Testing DC Resistance of Wire and Cable

Wire and cable conductor DC resistance is an important performance index of wire and cable. At present, most of them use dual-arm DC bridge for testing. However, there are many inadequacies in equipment and test methods in the actual test process. details as follows:

1, the problem of specimen clamping

The two-arm bridge is used to measure low or ultra-low value resistances below 1 ohm. Gb/t 3048.4—2007 "Test methods for electrical properties of electric wires and cables - Part 4: Conductor direct current resistance test" 3.4: For a four-terminal measuring fixture, the spacing between each potential contact and the corresponding current contact shall not be less than the specimen 1.5 times the circumference of the section. However, the potential electrode and current electrode of the bridge fixture provided by the instrument manufacturer are usually connected together by insulating material and fixed on the base, and the distance between the two cannot be adjusted to meet the requirements for measurement of different cross-section specimens. This kind of fixture can only satisfy the accurate measurement of the sample with 1.5 times the perimeter of the section within the distance between two electrodes. The stability and accuracy of the measurement of the sample with the conductor cross-sectional area exceeding this range are difficult to guarantee. For example, assuming that the distance between the fixture potential contact and the current contact is 60mm, then the corresponding conductor can accurately measure the maximum perimeter of the conductor is 40mm, from which the resulting conductor cross-sectional area is approximately 127mm2. That is, accuracy of the conductor resistance with a cross-sectional area of ​​127 mm2 or more is not guaranteed when the fixture is used.

In addition, when the fixture clamps conductors with irregular cross-sections, such as large cross-section twisted conductors, the cross section of the conductor is usually fan-shaped, arc-shaped, or triangular. When clamping, there will be good contact of the current contact chuck, and poor contact of the potential contact chuck, even when the current contact chuck has been tightened, and the potential contact chuck is still difficult to contact with the conductor, then it cannot be measured normally. When used in this condition for a long period of time, the fixture will wear out and deform, still causing the above situation.

In response to the above problems, I propose the following improvements. The distance between the current contact chuck and the potential contact chuck on the conductor clamp is designed to be adjustable, and the measurement personnel usually measures the resistance of the conductor with a length of 1m. Then the potential contact chuck on the conductor clamp can be fixed at 1m, and the current contact clamp can be fixed. Designed to move freely on the base and lengthen the base properly. In this way, it can be satisfied that the distance between each potential contact and the corresponding current contact is not less than 1.5 times the circumference of the sample section. Then, how to solve the problem that the potential contact chuck and the conductor can not be well contacted? It is also very easy to realize: The current contact chuck and the potential contact chuck on the fixture can be designed as independent clamping and releasing structures. This can not only ensure that the current contact chuck and the potential contact chuck are in close contact with the conductor at the same time, but also can solve the problem that the clip cannot be clamped due to the long-term use of the chuck wear and deformation.

2, the impact of temperature on the measurement

As we all know, the temperature has a significant effect on the electrical resistance of the conductor. Gb/t 3048-2007 specifies the temperature range of the conductor's DC resistance measurement. When the temperature does not meet the standard requirements, testers often artificially change the ambient temperature, which is usually adjusted by using an air conditioner in the test environment. When using air conditioners, it is often found that the galvanometer will slide slowly and it will be difficult to stabilize. Because the temperature is changing, the conductor resistance will change slowly. Since the galvanometer is very sensitive, it is not allowed to change the ambient temperature during the measurement. Therefore, the ambient temperature must be stabilized before measuring. Moreover, the sample must be kept in the measurement environment for a long enough time to bring its own temperature and ambient temperature into balance, because when the measurement personnel adjusts the ambient temperature, the sample temperature does not change synchronously with the ambient temperature, so when measuring Temperature drift may still occur. And the air conditioner does not respond to the measuring device, so as to prevent the galvanometer from being difficult to stabilize. In short, the measurement must be performed under a stable temperature environment.

3, the impact of contact resistance and measurement current on the measurement

Because there is contact resistance between the connecting wire and the terminal button in the measuring circuit, it will have a certain influence on the measurement. Therefore, it must be ensured that the connecting wire and the end button are in reliable contact. Measuring personnel often use the same amount of current to measure the conductor resistance of different cross-sectional areas, I think this is unscientific. The minimum measurement current should be used under the premise of ensuring the sensitivity. If the current is too large, the conductor will easily heat up, which will increase the resistance. The measurement should be completed within the shortest possible time. If the time is too long, the conductor will also generate heat.