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Is the actual cross-sectional area of the cable different from that marked at the factory? Is it qualified?

A novice of the third-party wire and cable detection encountered a problem: the cable provided by a manufacturer, such as a 25 square line, only has a little more than 21 square according to his calculation, but the DC resistance just reached the standard, the customer who asked him to test was not happy, and wanted to return the goods, he wanted to determine that it was unqualified, but the manufacturer said that as long as the DC resistance was qualified according to the new national standard, it was OK.


This novice, very sad, sandwiches between customers and manufacturers are not good!


Don't say anything, this beast first throws up the treasure of GB / T 3956-2008:




In addition, gb12706.1-2008 also specifies the cable test items. For cable conductors, the test items only have DC resistance.


If the above story does not stipulate the cable conductor in the contract, it only needs the DC resistance of the cable to meet the requirements of the national standard! Isn't that so? Small braid is too young, of course, small braid only says that low-voltage power cable is suitable, whether other types of cable are suitable or not is more than small Braid's IQ!


The maximum standard resistance of 25 square copper conductor is 0.727 ohm / km, and the cross section obtained by inverse calculation is only 23.72 square meters, only equivalent to 95% of 25 square meters (under the stable state of 0.017241 copper resistivity). The above novice measured 21 square meters, equivalent to 84% of the nominal section 25. The resistance is still qualified, indicating that the copper material is very good. It may be mixed with silver. Ha ha, joking!


In addition, I want to say that the resistance of the 25 square conductor made of inferior copper (mixed with iron and other impurities) may exceed the standard even if the actual cross section reaches 25 square or greater. This is why the current national standard does not examine the actual cross-section of conductors, only the minimum number of single wires and the maximum DC resistance.


In the old standard GB 5023-1985, the diameter and number of single wires of BV and other plastic wires are clearly specified. However, it is now invalid. This standard can only be used as reference, but can not be used as the basis for assessment and inspection.