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. These parameters are commonly referred to as temperature resistance level parameters in the industry. So, how did these parameters come about? Why do materials with a temperature resistance level of 90 ℃ have different aging temperatures? What is the relationship between aging temperature and temperature resistance level? What is the definition of the maximum long-term operating temperature allowed for conductors with insulation? What is temperature index? What is the rated temperature of the material? Can silane crosslinking material meet the temperature resistance level of 125 ℃?
1, UL standard
In UL standards, the common temperature resistance levels are 60 ℃, 70 ℃, 80 ℃, 90 ℃, 105 ℃, 125 ℃, and 150 ℃. How did these temperature resistance levels come about? Is it the long-term operating temperature of the conductor? In fact, these so-called temperature resistance levels are referred to as rated temperature in UL standards. It is not the long-term operating temperature of the conductor.
(1) Rated operating temperature
The confirmation of rated temperature in UL standards 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 ℃, and then calculate the test temperature of the oven at 112 ℃ according to formula 1.1. The samples are placed at these test temperatures for 90 days, 120 days, and 150 days, respectively, to obtain data on the elongation change rate and aging days of the samples. Then, the linear relationship between the aging days and the elongation at break is calculated using the least squares method. Based on this linear relationship, the elongation at break of the samples aged for 300 days at this oven temperature (112 ℃) is calculated. If the rate of change in elongation at break is less than 50%, it is considered that the material can reach this assumed rated temperature. If the rate of change in elongation at break is greater than 50%, it is considered that the rated temperature of the material cannot reach the assumed rated temperature and needs to be re assumed. Continue the above experiment at a rated temperature.
In the UL standard system, if the reverse calculation method is used, it can be considered as follows: a material is aged for 300 days at a certain temperature A ℃, and its elongation change rate does not exceed 50%. Then, the temperature A is subtracted by 5.463, and then divided by 1.02 to obtain the temperature B ℃. This indicates that the material can reach the rated temperature of temperature B ℃. This rated temperature is by no means the long-term maximum operating temperature allowed by the insulation layer for the conductor. Because the "long-term" in the long-term maximum working temperature should actually refer to the lifespan of the cable at this working temperature, calculated at least in years. For example, in the photovoltaic cable standard EN50618, the lifespan of the cable is designed to be 25 years, and the rated temperature in UL standards is generally higher than the long-term maximum working temperature of the conductor.
(2) Short term aging temperature
The short-term aging temperature of the material, which is the most common 7 days, 10 days, etc. in the standard, such as 105 ℃ material, the aging condition is 136 ℃ x 7 days. What is the relationship between this and the rated temperature? In UL standards, the temperature for short-term aging is obtained based on the long-term usage experience of the material, but some methods have also been summarized to confirm it. Firstly, select a rated temperature, aging temperature, and aging time. If the elongation change rate of the material tested under the above conditions after aging is greater than 50%, it is considered that the aging temperature of this material can be determined according to this condition. If the elongation change rate is greater than 50%, the rated temperature and short-term aging temperature of the material will decrease by one level.
2, EN/IEC standards
In EN/IEC standards, it is rare to see the rated temperature as in UL standards. Instead, the conductor's long-term operating temperature or temperature index is used. So what is the difference between these two temperatures?
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 the material at different temperatures, with a change rate of 50% in elongation at break as the endpoint of aging, to obtain the number of aging days of the material at different temperatures. Then, through linear regression, the aging days and aging temperature are linearly correlated to obtain a linear relationship curve. Then determine the maximum operating temperature based on the lifespan of the cable, or determine the lifespan of the cable based on long-term operating temperature. The temperature index refers to the temperature at which the change rate of the elongation at break of the insulation material after thermal aging for 20000 hours is 50%. Taking the photovoltaic cable standard EN 50618:2014 as an example, the design life of the cable is 25 years, with a long-term operating temperature of 90 ℃ and a temperature index of 120 ℃. The short-term aging temperature of insulation materials is also derived from the above linear relationship. So, the aging temperature of insulation materials in EN 50618:2014 is 150 ℃. This aging temperature is very close to the aging temperature of 158 ℃ for materials with a rated temperature of 125 ℃ in the UL standard series.
The long-term operating temperature of the same conductor may require different aging temperatures due to the different design lifetimes of the cables. At the same long-term operating temperature, the shorter the design life of the cable, the lower the short-term aging temperature required for the insulation material.
3, National and industry standards
In the process of formulating national and industry standards in our country, many contents are based on and borrowed from UL standards or EN/IEC standards. For example, in GB/T 32129-2015 and JB/T 10491.1-2004, both materials and wires have temperature resistance levels of 90 ℃, 105 ℃, 125 ℃, and 150 ℃, which clearly draws on UL's standard system. However, the description of heat resistance is the maximum allowable long-term operating temperature for conductors. The expression of heat resistance clearly refers to the IEC standard system.
