What are the differences between the temperature resistance levels of cables in the national standard, American standard and European standard?
In the design, material selection, production and sales process of wires and cables, many temperature parameters are often encountered, such as 90℃, 105℃, 125℃, 150℃, etc. The popular names for these parameters in the industry are called temperature resistance grade parameters. So where do these parameters come from? They are both materials with a temperature resistance grade of 90°C, so why do they have different aging temperatures? What are the aging temperature and temperature resistance grade? Relationship? How is the maximum long-term operating temperature of a conductor allowed by the insulation defined? What is the temperature index? What is the rated temperature of the material? Can silane cross-linked materials meet the temperature resistance level of 125°C?
To answer the above questions, we must first understand the standard system, because different standard systems have different definitions of temperature resistance levels. Our common standard systems mainly include national standards (and industry standards), UL standards, EN/IEC standards, etc.
Since the compilation of national standards and industry standards, a lot of content is based on reference to international standards, so let's first look at the regulations on temperature resistance levels in UL standards or EN/IEC standards.
1. UL standard
In the UL standard, the common temperature resistance grades are 60℃, 70℃, 80℃, 90℃, 105℃, 125℃ and 150℃. Where do these temperature resistance levels come from? Is it the long-term operating temperature of the conductor? In fact, these so-called temperature resistance levels are called rated temperatures in the UL standard. 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 formula 1.1 (see Chapter 4.3 Long-term aging of materials in UL 2556-2007). 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 112°C according to formula 1.1. The samples are placed at such test temperatures for 90 days, 120 days and 150 days to obtain the samples. The data of elongation change rate and aging days are then used to calculate the linear relationship between aging days and elongation at break through the least squares method, and then based on this linear relationship, the sample aged for 300 days at this oven temperature (112°C) is calculated. Elongation at break.
If the change rate of elongation at break is less than 50%, it is considered that the material can reach the assumed rated temperature. If the change rate of elongation at break is greater than 50%, it is considered that the rated temperature of the material cannot reach the assumed rated 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 inverse method is used, it can be considered as follows: when a material is aged for 300 days at a certain temperature A°C, its elongation change rate does not exceed 50%. Then subtract 5.463 from the temperature A, Then divide it by 1.02 to get the temperature B℃. It can be determined that this material can reach the rated temperature of temperature B℃.
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, which must be calculated at least in years. For example, in the photovoltaic cable standard EN50618, the life of the cable is designed to be 25 years, and in the UL standard The rated temperature will generally be higher than the conductor's maximum long-term operating temperature.
short term aging temperature
The short-term aging temperature of materials is the most common 7 days, 10 days, etc. in our standards. For example, for a material at 105°C, the aging condition is 136°C × 7 days. So what is the relationship between this and the rated temperature? In the UL standard, the short-term aging temperature is obtained based on the long-term use experience of the material, but some methods are also summarized to confirm. For example, the short-term aging temperature of a material is determined in Chapter 4.3.5.6 and Appendix D of the UL2556-2007 standard. First, select a rated temperature, aging temperature and aging time according to Table 1-1.
If the elongation change rate of the material tested according to the above conditions after aging is greater than 50%, it is deemed that the material can be determined according to this condition. If the elongation change rate is greater than 50%, the rated temperature and short-term aging of the material The temperature should drop one level.
In addition, a simple formula to determine the short-term aging temperature is also summarized in Chapter 14 of UL758-2010. Such as formula 1.2:
2. EN/IEC standards
In EN/IEC standards, it is rare to see the rated temperature (rating temperature) like in the UL standard. Instead, it is the conductor's long-term operating temperature (operation temperature) or temperature index. So what is the difference between these two temperatures?
In fact, in the EN/IEC standard system, the evaluation of the temperature resistance level of cables is mainly based on EN 60216 or IEC 60216. This standard mainly evaluates the thermal life of insulation materials. The evaluation method is to conduct aging tests on materials at different temperatures, and use a change rate of 50% in elongation at break as the end point of aging to obtain the number of aging days of the material at different temperatures. Then the aging days and aging temperature are linearly correlated through linear regression to obtain a linear relationship curve. Then determine the maximum operating temperature based on the life of the cable, or determine the life of the cable based on the long-term operating temperature.
The temperature index refers to the temperature corresponding to the change rate of the elongation at break of the insulation material of 50% after thermal aging for 20,000 hours. Taking the photovoltaic cable standard EN 50618:2014 as an example, the design life of the cable is 25 years, the long-term operating temperature is 90°C, and the temperature index is 120°C. The short-term aging temperature of insulation materials is also derived from the above linear relationship.
Therefore, the aging temperature of insulation 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 standard series.
From the above analysis, it is not difficult to see that the long-term operating temperature of the same conductor may have different required aging temperatures due to the different design life of the cable. Under the same long-term operating temperature, the shorter the cable design life, the lower the short-term aging temperature of the insulation material can be required.
For example, the long-term maximum operating temperature of XLPE insulation material 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 with a rated temperature of 105°C in the UL standard, but it is much different from the aging temperature of insulation in EN 50618:2014, which has the same 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.
3. National standards and industry standards
In the preparation process of my country's national standards and industry standards, many contents are based on UL standards or EN/IEC standards. However, since it is based on multiple references, some statements I believe are inaccurate. For example, in GB/T 32129-2015, JB/T 10436-2004, and JB/T 10491.1-2004, both materials and wires have temperature resistance levels of 90°C, 105°C, 125°C, and 150°C. This is obvious. It is based on UL's standard system. However, the expression for heat resistance is the maximum allowable long-term operating temperature of the conductor. This expression of heat resistance clearly refers to 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. However, there is no expression of cable life in these national and industry standards. Therefore, the statement that "the applicable maximum long-term operating temperatures of cable conductors are 90°C, 105°C, 125°C and 150°C" is open to question.
So can silane cross-linked XLPE reach the temperature resistance level of 125°C? A more rigorous answer should be that silane cross-linked XLPE can reach the rated temperature of 125°C specified in the UL standard, because the insulation and protection requirements in Chapter 40 of UL1581 In the set of general principles for materials, it has been clearly stated that the chemical composition of materials will not be specified. Whether the long-term maximum operation of XLPE conductors can reach 125°C is related to the design life of the cable and the use occasion. Currently, no relevant information has been found to systematically evaluate the life of this material. It can be inferred from short-term aging that if the design life of the cable is 25 years, the allowed long-term maximum temperature of the conductor must be greater than 90°C.
In the IEC standard, the long-term maximum operating temperature of the designed conductors of traditional power cables, building wires and even solar cables will not exceed 90°C, but this does not mean that the long-term maximum operating temperature allowed by the materials used for such cables cannot exceed 90°C. ℃. It cannot be said that radiation cross-linked materials can reach a temperature resistance level of 125°C, while silane cross-linked materials cannot reach a temperature resistance level of 125°C. This statement is unreasonable.
In short, whether a material can reach a certain temperature level cannot be answered simply yes or no, but must be considered in conjunction with the evaluation method of the material's temperature resistance level or the design life of the cable. Several standard systems cannot be mixed and used indiscriminately.
