# The difference between national standard, American standard and European standard cable temperature resistance grade

03-06

1. UL standard

In UL standards, the common temperature resistance grades are 60°C, 70°C, 80°C, 90°C, 105°C, 125°C and 150°C. In fact, these so-called temperature ratings are called rating temperatures in UL standards. It is not the long-term operating temperature of the conductor.

▎Rated working temperature

The determination of the rated temperature in the UL standard is determined according to formula 1.1 (see UL 2556-2007 Chapter 4.3 Long-term aging of materials). The specific process is to first assume a temperature resistance level of the material, such as 105°C, and then calculate the test temperature of the oven as 112°C according to formula 1.1, and place the sample at this test temperature for 90 days, 120 days and 150 days to obtain the temperature of the sample. Elongation rate of change and aging days data, then calculate the linear relationship between aging days and elongation at break with the least squares method, then calculate the elongation at break of the sample aging based on this linear relationship, oven temperature (112 ° C ) for 300 days.

If the rate of change in elongation at break is less than 50%, the material is considered capable of reaching the assumed rated temperature. If the rate of change in elongation at break is greater than 50%, the material is considered to be below the assumed rated temperature. It is necessary to reset a rated temperature and continue the above test.

It can be seen that in the UL standard system, if the reverse deduction method is used, it can be considered as follows: the material is aged at a certain temperature A°C for 300 days, and the change rate of the elongation rate does not exceed 50%, then subtract 5.463 from the temperature A, and then divide If the temperature B°C is obtained by 1.02, it can be judged that the material can reach the rated temperature of the temperature B°C.

This rated temperature is by no means the long-term maximum operating temperature of the conductor allowed by the insulation layer. Because the "long-term" in the long-term maximum operating temperature should actually be the life of the cable at this operating temperature, at least in years. For example, in the photovoltaic cable standard EN50618, the life of the cable is designed to be 25 years, while the rated temperature in the UL standard is generally higher than the long-term maximum operating temperature of the conductor.

▎Short-term aging temperature

The short-term aging temperature of the material, that is, the most common 7 days, 10 days, etc. in our standard, such as 105 ℃ material, the aging condition is 136 ℃ × 7 days. So what does this have to do with temperature ratings? In the UL standard, the short-term aging temperature is derived from the long-term use experience of materials, but some methods are also summarized to confirm. Determine the short-term aging temperature of the material as described in Chapter 4.3.5.6 and Appendix D of the UL2556-2007 standard. First, select the rated temperature, aging temperature, and aging time according to Table 1-1.

If the rate of change in elongation of a material tested under the above conditions is greater than 50%, it is considered that the material can be determined under these conditions. Aging temperature, if the change rate of elongation is greater than 50%, the rated temperature and short-term aging temperature should be lowered by a grade.

In addition, Chapter 14 of UL758-2010 also summarizes a simple formula to determine the short-term aging temperature.

2. EN/IEC standard

In the EN/IEC standard, the rated temperature (rating temperature) is rarely seen like the UL standard, and replaced by the long-term operating temperature (operation temperature) or temperature index of the conductor. So what is the difference between these two temperatures?

In fact, in the EN/IEC standard system, the evaluation of cable temperature resistance is mainly based on EN 60216 or IEC 60216, which mainly evaluates the thermal life of insulating materials. The evaluation method is to carry out aging tests on materials at different temperatures, and take the change rate of elongation at break as 50% as the aging end point to obtain the aging days of materials at different temperatures. Then, the aging days were linearly correlated with the aging temperature by linear regression to obtain a linear relationship curve. Then determine the maximum working temperature according to the life of the cable, or determine the life of the cable according to the long-term working temperature.

The temperature index refers to the temperature corresponding to the change rate of elongation at break of 50% after thermal aging of insulating materials for 20,000 hours. Based on photovoltaic cable standard EN 50618:2014

For example, the design life of the cable is 25 years, the long-term working temperature is 90°C, and the temperature index is 120°C. The short-term aging temperature of insulating materials is also derived from the above linear relationship.

Therefore, the aging temperature of insulating materials in EN 50618:2014 is 150°C. This aging temperature is very close to the aging temperature of 158°C for materials rated at 125°C in the UL family of standards.

It is not difficult to see from the above analysis that the long-term working temperature of the same conductor may require different aging temperatures due to the different design life of the cable. Under the same long-term working temperature, the shorter the design life of the cable, the lower the short-term aging temperature of the insulating material can be required.

For example, the long-term maximum working temperature of XLPE insulation material required in IEC 60502-1:2004 is 90°C, while the aging temperature of this material is 135°C. 135°C here is very close to the aging temperature of 136°C in the UL standard rated at 105°C, but very different from the insulation aging temperature in EN 50618:2014, which also has a long-term maximum operating temperature of 90°C temperature. Although the design life of the cable is not found in 60502-1:2004, the design life of the two cables must be different.