Special factors should be considered in cross-linked cable inspection
In recent years, silane crosslinked polyethylene cable materials (Hereinafter referred to as XLPE)
Because of its simple manufacturing equipment, mature process, convenient operation, low comprehensive cost and other advantages, it has become the leading material for low-voltage crosslinked cable insulation.
At present, XLPE is commonly used, one of which is two-step XLPE. When cable factories produce insulated wire cores, polyethylene grafted with silane (PE)
And the catalyst Masterbatch is mixed in a certain proportion, extruded in a common extruder, and then cross-linked in hot water or steam;
Another one-step XLPE is made by the cable material manufacturer, mixing all the raw materials together according to the proportion by special method, and the cable factory directly completes the grafting and extrusion of the insulating wire core in one step at the same time in the extruder, crosslinking is then completed under natural conditions.
The common point of these two kinds of XLPE is that the crosslinking process is relatively simple without special extrusion equipment. As long as the raw materials and process conditions meet the requirements, they can become insoluble thermosetting plastics.
Compared with thermoplastic PE, its heat-resistant deformation and mechanical properties at high temperature, environmental stress cracking, aging resistance, chemical resistance and other properties have been improved or improved, while the electrical properties remain basically unchanged, and the long-term working temperature of the cable is increased from the original 70 °C to 90 °C, thus improving the short-term current withstand capability of the cable.
To sum up, XLPE low-voltage cables have become the main products of cable manufacturers in recent years.
As a third-party inspection agency, the number of such cables inspected is also increasing year by year. How to accurately provide the test results of thermal extension and aging performance of such products, inspectors are facing some special circumstances. The following is an analysis: first, the problem of abnormal thermal extension of XLPE insulation.
The author often finds during the inspection that the elongation of XLPE cable insulation under the load of 200 ℃ thermal extension test greatly exceeds the requirements specified in the standard, or the sample is put into an oven and blown in a short time, if you use the same retest immediately, the reproducibility of the results is very good. If you follow the routine, as long as the test method is correct and the sampling is correct, you can completely judge the conclusion according to the test results, but for XLPE, there may be great risks in doing so.
Because the crosslinking process of XLPE is a slow chemical change process related to temperature, humidity, time, insulation thickness and other factors, especially natural crosslinking XLPE insulation materials, it is also affected by the above factors. The time for completing the crosslinking will be quite different. It is entirely possible that the natural crosslinking has not been completed within the specified test period.
Once the natural crosslinking is completed over time, its performance may meet the requirements specified in the national standards.
For this kind of situation, the author believes that on the premise of reflecting the current situation of the sample, we should not rush to judge, but should provide a condition to promote crosslinking for the sample--
Soak 4 ~ in 90 C 2 C hot water ~ The thermal extension test will be conducted after 5 hours.
Practice has proved that the test results at this time can be used as the basis for judgment.
It is worth mentioning that some manufacturers unilaterally pursue commercial profits and use PE and XLPE to pretend PE to be XLPE, however, no matter what kind of conditions are provided to promote cross-linking, PE will not produce cross-linking changes, and its performance cannot meet the requirements of XLPE at all, which is the same as that stones cannot hatch chickens.
This requires inspectors to have the ability to identify true and false, good and bad XLPE.
In fact, through observation and work accumulation, we can distinguish whether the tested sample belongs to undercrosslinked, inferior XLPE or PE according to the fusing time and fusing point after the sample is put into the oven?
However, as a third-party inspector, it cannot be concluded by experience alone, and must be judged according to real data.
Second, the change rate of XLPE thermal aging test exceeds the standard.
During the test, if you get the sample, immediately prepare the sample and put it into the oven for aging according to the routine, the change rate of tensile strength and elongation at break after aging will often exceed the standard, and you must be careful in judging this result.
This phenomenon is not completely caused by poor aging performance, possibly because XLPE has not been completely crosslinked (
It can be seen from the time curve of thermal extension of XLPE cable material placed in warm water that when the thermal extension is qualified, it does not mean that the sample is completely crosslinked)
After being put into the aging box, XLPE is still completing its crosslinking process, which leads to an increase in tensile strength, a decrease in elongation at break, and the final change rate exceeds the standard.
Due to the long aging time, it is more troublesome to find the problem once the test is over. Therefore, it is necessary to let the sample be completely crosslinked before the aging test.
To sum up, it can be seen that special factors should be considered in judging XLPE thermal extension and thermal aging performance.
The personnel who are engaged in the third-party inspection can neither make a hasty conclusion for the test results, because there is a risk of misjudging qualified products as unqualified;
It is also impossible to avoid these two tests because it is difficult to draw a conclusion. This may cause unqualified products or fake and shoddy products to be missed.
Therefore, it is necessary to exclude the possibility that the sample has not been crosslinked or completely crosslinked before carrying out the above two tests.
We advocate using scientific and reasonable test methods to provide fair and reliable test results. References 1. Xu Yinglin. Wire and Cable Manual . Mechanical Industry Press. 2. Key, etc. Silane crosslinked polyethylene cable insulation. Wire and Cable, 2007 ,(6). (
Author: Wuxi Product Quality Supervision and Inspection Institute, Jiangsu province)