A Brief Analysis of the Temperature Resistance Grades of Wires and Cables

In the design, material selection, production and sales of wires and cables, many temperature parameters are often encountered, such as 90℃, 105℃, 125℃, 150℃, etc. These parameters are commonly known in the industry as temperature resistance grade parameters. But where do these parameters come from? Why are the aging temperatures of materials with the same temperature resistance grade of 90℃ different? What is the relationship between aging temperature and temperature resistance grade? How is the long-term maximum operating temperature of a conductor allowed for insulation defined? What is the temperature index? What is the rated temperature of a material? Can silane cross-linked materials meet the temperature resistance grade of 125℃?

To answer the above questions, it is first necessary to understand the standard system, as different standard systems have different definitions of temperature resistance grades. The common standard systems we often see mainly include UL standards, EN/IEC standards, national standards, industry standards, etc.

I. UL Standards

In UL standards, common temperature resistance grades are 60℃, 70℃, 80℃, 90℃, 105℃, 125℃ and 150℃. How are these temperature resistance grades derived? Is it the long-term operating temperature of a conductor? In fact, these so-called temperature resistance grades are called rated temperature in UL standards. It is not the long-term operating temperature of the conductor.

(1) Rated operating temperature

The confirmation of the rated temperature in UL standards is determined according to Formula 1.1 (refer to Chapter 4.3 on 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℃, and then calculate the test temperature of the oven at 112℃ according to formula 1.1. The samples are placed at such test temperatures for 90 days, 120 days, and 150 days respectively to obtain the elongation change rate and aging days data of the samples. Then, the linear relationship between the aging days and the elongation at break is calculated through the least square method. Based on this linear relationship, the elongation at break of the sample after aging for 300 days at this oven temperature (112℃) is calculated. If the change rate of the elongation at break is less than 50%, it is considered that this material can reach this assumed rated temperature. If the rate of change of elongation at break is greater than 50%, it is considered that the rated temperature of this material cannot reach the assumed rated temperature, and a new rated temperature needs to be assumed to continue the above test.

From this, it can be seen that in the UL standard system, if the reverse deduction method is adopted, it can be considered as follows: If A certain material is aged at A certain temperature A℃ for 300 days and its elongation change rate does not exceed 50%, then subtract 5.463 from temperature A and then divide by 1.02 to obtain 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 refer to the lifespan of the cable at this operating temperature, which should be calculated in years at least. For instance, in the photovoltaic cable standard EN50618, the lifespan of the cable is designed to be 25 years, and the rated temperature in the UL standard is generally higher than the long-term maximum operating temperature of the conductor.

(2) Short-term aging temperature

The short-term aging temperature of materials, which is the most common ones we usually see in standards such as 7 days and 10 days, for example, for materials at 105℃, the aging condition is 136℃×7 days. So what is the relationship between this and the rated temperature? In UL standards, the short-term aging temperature is obtained based on the long-term usage experience of the material, but some methods have also been summarized to confirm it. For example, first 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 determined that the aging temperature of this material can be determined according to these conditions. If the elongation change rate is greater than 50%, the rated temperature and short-term aging temperature of the material should be reduced by one grade.

Ii. EN/IEC Standards

In EN/IEC standards, rated temperature (rating temperature) is rarely seen as in UL standards. Instead, the long-term operating temperature (operation temperature) or temperature index of the conductor is used. So, what's the difference between these two temperatures?

In fact, in the EN/IEC standard system, the evaluation of the temperature resistance grade of cables is mainly based on EN 60216 or IEC 60216. This standard is mainly used to evaluate the thermal life of insulating materials. The evaluation method is to conduct aging tests on the material at different temperatures, taking the change rate of elongation at break of 50% as the end point of aging, and obtaining 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, the maximum operating temperature is determined based on the cable's lifespan, or the lifespan of the cable is determined based on the long-term operating temperature. The temperature index refers to the temperature corresponding to the change rate of elongation at break of insulating materials being 50% after 20,000 hours of thermal aging. Take the photovoltaic cable standard EN 50618:2014 as an example. The design life of its cable is 25 years, the long-term operating temperature is 90℃, and the temperature index is 120℃. 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℃ for materials rated at 125℃ in the UL standard series.

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

Iii. National Standards and Industry Standards

In the process of formulating national and industry standards in our country, many contents refer to and draw on UL standards or EN/IEC standards. However, since it was based on multiple references, some of the expressions, in the author's opinion, are inaccurate. For instance, in GB/T 32129-2015 and JB/T 10491.1-2004, both materials and wires have temperature resistance grades of 90℃, 105℃, 125℃ and 150℃. This is obviously drawing on UL's standard system. However, the term "heat resistance" refers to the maximum long-term operating temperature of a conductor that is allowed. This description of heat resistance clearly refers to the IEC standard system. In the IEC standard system, the long-term maximum operating temperature of conductors should be associated with the design life of cables. However, in these national and industry standards, there is no description of cable life at all. Therefore, the statement that "the long-term allowable maximum operating temperatures of the applicable cable conductors are 90℃, 105℃, 125℃ and 150℃" is questionable.

Then, can the silane cross-linked XLPE achieve a temperature resistance grade of 125℃? A more rigorous answer should be that silane cross-linked XLPE can reach the rated temperature of 125℃ as stipulated in UL standards, because in Chapter 40 of UL1581, the general provisions on insulating and sheathing materials, it has been clearly stated that no regulations are made on the chemical composition of the materials. Whether the long-term maximum operating temperature of XLPE conductors can reach 125℃ is related to the design life of the cable and the application scenarios. Currently, no relevant data has been found to systematically evaluate the lifespan of this material. Based on short-term aging, it can be inferred that if the design life of the cable is 25 years, the long-term maximum allowable temperature of the conductor must be greater than 90℃. In IEC standards, the long-term maximum operating temperature of the designed conductors for traditional power cables, building wires and even solar cables does not exceed 90℃, but this does not mean that the long-term maximum operating temperature allowed for the materials used in such cables cannot be greater than 90℃. It cannot be said that irradiation cross-linked materials can achieve a temperature resistance level of 125℃, while silane cross-linked materials cannot. Such a statement is unreasonable.

In conclusion, whether a material can reach a certain temperature grade cannot be simply answered as yes or no. Instead, it should be considered in combination with the evaluation method of the material's temperature resistance grade or the design life of the cable. Several standard systems should not be mixed and used randomly.