During the design, material selection, production, and sales of wires and cables, we often encounter numerous temperature parameters, such as 90°C, 105°C, 125°C, and 150°C. These parameters are commonly referred to in the industry as temperature ratings. So, how are these parameters derived? Why do materials with the same 90°C temperature rating have different aging temperatures? What is the relationship between aging temperature and temperature rating? How is the maximum permissible long-term operating temperature of an insulation conductor defined? What is a temperature index? What is the rated temperature of a material? Can silane-crosslinked materials meet a 125°C temperature rating?
To answer these questions, we must first understand the standard system, as different standards define temperature ratings differently. Common standard systems include national standards (and industry standards), UL standards, and EN/IEC standards.
Since many aspects of national and industry standards are referenced and borrowed from international standards, let's first examine the temperature ratings defined in UL and EN/IEC standards.
1
UL Standards
Common temperature ratings in the UL standard are 60°C, 70°C, 80°C, 90°C, 105°C, 125°C, and 150°C. How are these temperature ratings determined? Are they the long-term operating temperatures of the conductor? In fact, these so-called temperature ratings are called rated temperatures in the UL standard. They are not the long-term operating temperatures of the conductor.
▎Rated Operating Temperature
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 rating for the material, such as 105°C. Then, using formula 1.1, calculate the oven test temperature to 112°C. Samples are then placed at this test temperature for 90, 120, and 150 days, respectively. Data on the sample's elongation change and aging days are obtained. The least squares method is then used to derive a linear relationship between aging days and elongation at break. Based on this linear relationship, the elongation at break of the sample after 300 days of aging at this oven temperature (112°C) is then calculated.
If the change in elongation at break is less than 50%, the material is considered to meet the assumed rated temperature. If the change in elongation at break is greater than 50%, the material's rated temperature is considered unattainable, and a new rated temperature must be assumed before continuing the test.
Therefore, using the reverse calculation method within the UL standard system, we can assume that if a material undergoes aging at a certain temperature, A°C, for 300 days and its elongation does not change by more than 50%, then subtract 5.463 from temperature A and divide by 1.02 to obtain temperature B°C, the material can be considered to meet the rated temperature of B°C.
This rated temperature is by no means the maximum long-term operating temperature of the conductor permitted by the insulation layer. The "long-term" in the long-term maximum operating temperature refers to the cable's lifespan at this operating temperature, which should be measured in years at least. For example, the photovoltaic cable standard EN50618 specifies a 25-year cable lifespan. The rated temperature in UL standards is generally higher than the conductor's long-term maximum operating temperature.
Short-term Aging Temperature
The short-term aging temperature of a material is the most common aging temperature in standards, typically 7 days or 10 days. For example, for a material at 105°C, the aging condition is 136°C for 7 days. What is the relationship between this and the rated temperature? In UL standards, the short-term aging temperature is determined based on long-term material usage experience, but several methods have been developed to confirm this. For example, in Chapter 4.3.5.6 and Appendix D of the UL2556-2007 standard, a material's short-term aging temperature is determined as follows. First, select a rated temperature, aging temperature, and aging time according to Table 1-1.
If the elongation change after aging for a material tested under these conditions is greater than 50%, the material is considered eligible for aging temperature determination under these conditions. If the elongation change is greater than 50%, the material's rated temperature and short-term aging temperature are reduced by one level.
Cable Temperature Resistance Rating
In addition, Chapter 14 of UL758-2010 also summarizes a simple formula for determining the short-term aging temperature. For example, Equation 1.2:
Cable Temperature Resistance Rating
2
EN/IEC Standards
Unlike UL standards, EN/IEC standards rarely list rated temperatures. Instead, they list the conductor's long-term operating temperature or temperature index. So what's the difference between these two temperatures?
In fact, within the EN/IEC standard system, the evaluation of cable temperature resistance is primarily based on EN 60216 or IEC 60216. This standard primarily assesses the thermal life of insulation materials. The evaluation method involves subjecting the material to aging tests at different temperatures, using a 50% change in elongation at break as the aging endpoint. The number of days the material ages at each temperature is then calculated. A linear regression is then used to correlate the aging days and aging temperature, yielding a linear relationship curve. The maximum operating temperature is then determined based on the cable's lifespan, or the cable's lifespan is determined based on the long-term operating temperature.
The temperature index refers to the temperature at which the insulation material's elongation at break changes by 50% after 20,000 hours of thermal aging. For example, the photovoltaic cable standard EN 50618:2014 specifies a 25-year design life, a long-term operating temperature of 90°C, and a temperature index of 120°C. The short-term aging temperature of the insulation material is also derived using this linear relationship.
Therefore, the insulation material aging temperature 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 clear that the same conductor's long-term operating temperature may require different aging temperatures due to different cable design lives. At the same long-term operating temperature, the shorter the cable's design life, the lower the required short-term aging temperature for the insulation material.
For example, IEC 60502-1:2004 requires a maximum long-term operating temperature of 90°C for XLPE insulation, while the aging temperature for this material is 135°C. This 135°C is very close to the aging temperature of 136°C for materials rated at 105°C in the UL standard, but significantly different from the insulation aging temperature in EN 50618:2014, which also has a maximum long-term operating temperature of 90°C. Although the design life of the cable is not specified in 60502-1:2004, the design lives of the two cables are certainly different.
3
National and Industry Standards
During the development of my country's national and industry standards, much of the content was referenced and borrowed from UL and EN/IEC standards. However, due to this multifaceted reference, some of the terms 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 are listed with temperature resistance levels of 90°C, 105°C, 125°C, and 150°C, clearly borrowing from the UL standard system. However, the term "heat resistance" refers to the maximum permissible long-term operating temperature of the conductor. This heat resistance statement clearly draws on the IEC standard system.
In the IEC standard system, the maximum long-term operating temperature of the conductor is supposed to be correlated with the cable's design lifespan. However, these national and industry standards do not mention cable lifespan at all. Therefore, the statement that "the maximum permissible long-term operating temperatures for applicable cable conductors are 90°C, 105°C, 125°C, and 150°C" is debatable.
So, can silane-crosslinked XLPE achieve a temperature rating of 125°C? A more rigorous answer is that silane-crosslinked XLPE can meet the 125°C rated temperature specified in the UL standard, as UL 1581, Chapter 40, General Rules for Insulation and Sheathing Materials, explicitly states that the chemical composition of the material is not specified. Whether the long-term maximum operating temperature of XLPE conductors can reach 125°C depends on the cable's design life and application. Currently, no relevant data has been found to systematically evaluate the lifespan of this material. Based on short-term aging, if the cable has a design life of 25 years, the permissible long-term maximum temperature of the conductor will definitely be greater than 90°C.
IEC standards specify that the long-term maximum operating temperature of the conductors of traditional power cables, building cables, and even solar cables will not exceed 90°C. However, this does not mean that the materials used in these cables cannot have a long-term maximum operating temperature greater than 90°C. Nor can we say that radiation-crosslinked materials can achieve a 125°C temperature rating, while silane-crosslinked materials cannot. Such a statement is illogical.
In short, whether a material can meet a certain temperature rating cannot be answered with a simple yes or no. Instead, it must be considered in conjunction with the material's temperature rating evaluation method or the cable's design life. Multiple standards should not be mixed and applied indiscriminately.
