First, UL standard
In the UL standard, the common temperature resistance levels are 60 ° C, 70 ° C, 80 ° C, 90 ° C, 105 ° C, 125 ° C and 150 ° C. In fact, these so-called temperature resistance levels are called the rated temperature in the UL standard ( Rating temperature). It is not the long-term operating temperature of the conductor.
Rated operating temperature
The confirmation of the rated temperature in the UL standard is determined according to Equation 1.1 (see section 4.3 Long-term aging of materials in UL 2556-2007). The specific process is to first assume a temperature resistance grade of the material, such as 105 ° C, and then calculate the oven test temperature 112 ° C according to the formula 1.1, respectively, at this test temperature, the sample is placed for 90 days, 120 days and 150 days to obtain samples The rate of change of elongation rate and the number of days of aging, and then calculate the linear relationship between the number of aging days and the elongation at break by least squares method, and then calculate the sample after aging for 300 days at this oven temperature (112 ° C) based on the linear relationship. Elongation at break.
If the rate of change of elongation at break is less than 50%, the material is considered to reach this assumed temperature. If the rate of change of elongation at break is greater than 50%, the rated temperature of the material is considered to be below the assumed temperature. It is necessary to re-assume a rated temperature and continue the above test.
It can be seen that in the UL standard system, if the reverse push method is adopted, it can be considered that a certain material is aged at a certain temperature A °C for 300 days, the rate of change of the elongation is not more than 50%, and then the temperature A is decreased by 5.463. Then divide by 1.02 to get the temperature B °C, it can be determined that the material can reach the temperature of B ° C rated temperature.
This rated temperature is by no means the long-term maximum operating temperature of the conductor allowed by the insulation. 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, such as the photovoltaic cable standard EN50618, the cable life is designed for 25 years, in the UL standard The rated temperature 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 the standard, such as 105 ° C material, the aging conditions are 136 ° C × 7 days. So what does this have to do with the rated temperature? In the UL standard, the temperature of short-term aging is obtained by long-term experience of materials, but some methods are also summarized to confirm. The short-term aging temperature of a material is determined as in UL.22.5-2007 Standard 18.104.22.168 and Appendix D. First select a rated temperature, aging temperature and aging time according to Table 1-1.
If the rate of change of elongation after aging according to the above conditions is greater than 50%, it is determined that the aging temperature can be determined according to this condition. If the rate of change of elongation is greater than 50%, the rated temperature and short-term aging of the material The temperature has to drop by one level.
In addition to this, a simple formula is also summarized in Chapter 14 of UL 758-2010 to determine the short-term aging temperature.
Second, EN / IEC standards
In the EN/IEC standard, the rating temperature is rarely seen as in the UL standard, and the conductor's long-term operating temperature or temperature index is replaced. So what is the difference between these two temperatures?
In fact, in the EN/IEC standard system, the evaluation of the temperature rating of the cable is mainly evaluated in accordance with EN 60216 or IEC 60216. This standard is mainly to evaluate the thermal life of insulating materials. The evaluation method is to carry out the aging test of the material at different temperatures, and the rate of change of the elongation at break is 50% as the end point of aging, and the aging days of the materials at different temperatures are obtained. Then linearly correlate the aging days and the aging temperature by linear regression to obtain a linear relationship curve. The maximum operating temperature is then determined based on the life of the cable, or the life of the cable is determined based on the long-term operating temperature.
The temperature index refers to the temperature corresponding to the insulation material after the heat aging of 20000H, the rate of change of elongation at break is 50%. Taking the photovoltaic cable standard EN 50618:2014 as an example, the cable has a design life of 25 years, a long-term operating temperature of 90 ° C, and a temperature index of 120 ° C. The short-term aging temperature of the insulating material is also derived from the linear relationship described above.
Therefore, the aging temperature of the insulating material 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 standard series.
It is not difficult to see from the above analysis that the long-term operating temperature of the same conductor may not require the same aging temperature due to the different design life of the cable. At the same long-term operating temperature, the shorter the cable design life, the lower the short-term aging temperature of the insulating material can be required.
For example, the long-term maximum operating temperature of XLPE insulation required in IEC 60502-1:2004 is 90 ° C, and the aging temperature of this material is 135 ° C. The 135 ° C here is very close to the aging temperature of 136 ° C, which is rated at 105 ° C in the UL standard, but it is much different from the aging temperature of EN 50618:2014 which is also the long-term maximum operating temperature of 90 ° C. Although the design life of the cable was not found in 60502-1:2004, the design life of the two cables is definitely different.
Third, the national standard and industry standards
In the process of compiling China's national standards and industry standards, many of the contents are referenced and borrowed from UL standards or EN/IEC standards. However, because it is a multi-party reference, some of the authors believe that it is inaccurate. For example, in GB/T 32129-2015, JB/T 10436-2004, JB/T 10491.1-2004, the temperature grades of materials and wires are 90 ° C, 105 ° C, 125 ° C and 150 ° C, which is obvious. It is based on the standard system of UL. However, the expression for heat resistance is the maximum long-term operating temperature of the conductor allowed. This expression of heat resistance is clearly based on the IEC standard system.
In the IEC standard system, the long-term maximum operating temperature of the conductor should be related to the design life of the cable. In these national standards and standards, there is no representation of cable life at all. Therefore, the expression "the applicable cable conductors allow the maximum operating temperatures of 90 ° C, 105 ° C, 125 ° C and 150 ° C for a long time" is open to question.
So can the silane crosslinked XLPE reach the temperature resistance level of 125 °C? A more rigorous answer should be that the silane crosslinked XLPE can meet the 125 ° C rated temperature specified in the UL standard, as it is clearly stated in the general insulation and sheath material generals of UL1581 Chapter 40 that the chemical composition of the material is not specified. Whether the long-term maximum work of XLPE conductor can reach 125 °C, which is related to the design life and use of the cable. At present, no relevant information system has been found to evaluate the life of the material. It is speculated by short-term aging that if the design life of the cable is 25 years, the allowable long-term maximum temperature of the conductor must be greater than 90 °C.
In the IEC standard, the long-term maximum operating temperature of conventional power cables, building cables and even solar cables does not exceed 90 °C, but it does not mean that the long-term maximum operating temperature allowed for materials used in such cables cannot exceed 90. °C. It can not be said that the radiation cross-linking material can reach the temperature resistance level of 125 ° C, and the silane cross-linking material can not reach the temperature resistance level of 125 ° C. This expression is unreasonable.
In short, whether a material can reach a certain temperature level, can not simply answer yes or no, but should be considered in combination with the evaluation method of the temperature resistance level of the material or the design life of the cable, and it is not possible to mix several standard systems indiscriminately.