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I was going through an IEC standard - 61439-1, table 6.

For aluminum, the standard specifies the value as 55K which is a temperature rise.

IEC 60085 specifies the class B insulator absolute value as 130°C.

Why is the absolute temperature mentioned and not the temperature rise in the latter case? Is there any specific reason?

Table-6 IEC 61439-1

Manoj Vignesh T
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  • 55 K does not imply a rise and neither does it imply an absolute value. It's the context in which it's used that is important and, unfortunately your question doesn't contain the context. – Andy aka Apr 19 '23 at 08:57
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    can you tell us roughly what 61439-1 table 6 is for those of us who do not have IEC subscriptions? It could be that this is talking about the temperature generated by the current through a cable, which is naturally a rise, while the other is talking about the temperature where an insulator melts or burns, which is naturally absolute. The difference is then made up by presuming something about the ambient temperature (e.g. it is not higher than 65 degrees C) – user253751 Apr 19 '23 at 09:57
  • @user253751 For your reference I have added the image of Table-6 of IEC 61439-1 – Manoj Vignesh T Apr 19 '23 at 10:27

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There are different reasons for limits shown in the standards, which they may or may not choose to justify. Table 6 ("Temperature-rise Limits") in IEC 61439-1 (concerned with switchgear) may be on account of something like bus bar fasteners loosening from cycling. They do mention annealing of copper and it is implied that things that get touched and are of metal (a thermally conductive material) should not be more than 15K hotter than ambient (25K if made of insulating material). Switchgear operates under certain typical conditions- you may note that they've specified the maximum ambient as 40°C and average over 24 hrs should not exceed 35°C (without additional considerations). That also places constraints on the maximum absolute temperature.

Class B is a general class of insulation that will function up to 130°C. It's up to the engineer to ensure that the sum of the ambient (which may be far from typical outdoor temperature) and temperature rise does not exceed that temperature. That places a constraint on the temperature rise.

Whether one or the other choice is made by the drafters of a standard would depend on which they feel is more appropriate in a given situation. Historical factors and de facto industry practice may also come into play.

Spehro Pefhany
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  • Your statement, "Switchgear operates under certain typical conditions- you may note that they've specified the maximum ambient as 40°C and average over 24 hrs should not exceed 35°C (without additional considerations)." implies that temperature rise is constrained due to the ambient temperature. In 61439, ambient is 35/40 (avg/peak), and hence 105K temperature rise for copper (considering annealing post 105K TR). But I have seen OEMs declaring certificates with 55C ambient and retaining temperature rise as per Table 6. Is that correct ? – DaSnipeKid Apr 19 '23 at 13:33
  • @DaSnipeKid tbh, electrical distribution switchgear is well outside of my normal work envelope. You could ask the OEM about any conditions on their compliance. – Spehro Pefhany Apr 19 '23 at 13:54