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 they come from?

1、 UL standard

In UL standards, 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.

Rated operating temperature

The confirmation of rated temperature in UL standards is determined according to formula 1.1 (refer to Chapter 4.3 of UL 2556-2007 for long-term aging of materials). 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 assume a new rated temperature and continue the above test.

From this, it can be seen that in the UL standard system, if the reverse inference method is used, it can be considered that a material aged at a certain temperature A ℃ for 300 days has an elongation change rate of no more than 50%. Then, the temperature A is subtracted by 5.463, and then divided by 1.02 to obtain the temperature B ℃. This can be considered that the material can reach the rated temperature of 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 lifespan of the cable at this operating temperature, at least calculated 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 the UL standard 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, which is the most common 7 days, 10 days, etc. in the standard, such as 105 ℃ material, with an aging condition of 136 ℃ × 7 days. What is the relationship between this and the rated temperature? In UL standards, the temperature of short-term aging is obtained based on the long-term use experience of the material, but some methods are also summarized to confirm. As determined in Chapter 4.3.5.6 and Appendix D of UL2556-2007 standard, the short-term aging temperature of a material is determined. Firstly, select a rated temperature, aging temperature, and aging time according to Table 1-1.

If the aging elongation change rate of the material tested according to the above conditions is greater than 50%, it is deemed that the aging temperature of 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 temperature of the material will be reduced by one level.

2、 EN/IEC standards

In the EN/IEC standard, rated temperature is rarely seen as in the UL standard, and is replaced by the long-term operating temperature or temperature index of the conductor. 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 is mainly used to evaluate 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, and determine the 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 elongation at break of an insulating material changes by 50% after thermal aging for 20000 hours. Taking the photovoltaic cable standard EN 50618:2014 as an example, the design life of the cable is 25 years, the long-term working temperature is 90 ℃, and the temperature index is 120 ℃. 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 ℃. 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.

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

3、 National and industry standards

In the process of formulating national and industry standards in China, many of them refer to and draw on UL standards or EN/IEC standards. However, due to 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 ℃, 105 ℃, 125 ℃, and 150 ℃, which is clearly based on the UL standard system. However, the expression of heat resistance is the maximum allowable long-term operating temperature of the conductor. The expression 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 related to the design life of cables, but there is no expression of cable life in these national and industry standards. Therefore, the statement that the maximum allowable long-term operating temperatures for suitable cable conductors are 90 ℃, 105 ℃, 125 ℃, and 150 ℃ remains to be discussed.

Can silane crosslinked XLPE achieve a temperature resistance level of 125 ℃? The more rigorous answer should be that silane crosslinked XLPE can reach the rated temperature of 125 ℃ specified in the UL standard, because in Chapter 40 of UL1581, the general principles of insulation and sheath materials have clearly stated that the chemical composition of the material should not be specified. Whether the long-term maximum working temperature of XLPE conductor can reach 125 ℃ is related to the design life and usage situation of the cable. Currently, there is no relevant data system to 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 allowable long-term maximum temperature of the conductor can definitely be greater than 90 ℃.

In IEC standards, traditional power cables, building wires, and even solar cables are designed with conductors that do not exceed a long-term maximum operating temperature of 90 ℃. However, this does not mean that the material used for such cables cannot exceed a long-term maximum operating temperature of 90 ℃. It cannot be said that irradiated cross-linked materials can reach a temperature resistance level of 125 ℃, while silane cross-linked materials cannot reach a temperature resistance level of 125 ℃. This statement is unreasonable.

In short, whether a material can reach a certain temperature level cannot be simply answered as yes or no. It should be considered based on the evaluation method of the material's temperature resistance level or the design life of the cable. Multiple standard systems should not be mixed and used indiscriminately.

Article source: Changjiang Nonferrous Metals Network

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