Determination of Temperature Rise - Deviation of ± 5K

[DaSnipeKid]

I was referring IEC 61439-1 standard for the deriving the temperature rise of an assembly through comparison. We can derive the current rating of an assembly (low voltage switchboard) at any point (contacts such as busbar joints, SCPD joint, or any other contact) by using the formula of Copper Development Association, Publication No. 22:1996 formula no. 8. As per the standard clause 10.10.2.3.1,

[tt]To reduce the testing required to determine the rated current of a circuit I1 at the maximum permissible temperature-rise ΔT1, the current rating may be calculated from the actual test current I2 if the measured temperature-rise ΔT2 of the current carrying parts (e.g. busbars and terminals) deviates from the permissible value by not more than ±5 K, using the following formula: given in equation (1)[/tt]

Let's examine this scenario by an example.

DATA:
Copper-copper joint Temperature rise limit ΔT1 = 105K
Tested temperature ΔT2 = 102K
Test current I2 = 973A

TO FIND:
Maximum current rating

SOL:
The maximum current rating can be determined as followed:
I1/I2=[ΔT1/ΔT2]^0.61.....(1)
Substituting above values in the equation, we get,
I1=990A.....(2)
Hence, we can deduce that the maximum current rating of the specified circuit is 990A which can be carried safely at 105K temperature rise which is also the limit. Here's the catch. This formula can be used if the tested temperature is less than ±5K than the permissible limit. If my tested temperature is 98K where,
ΔT1−ΔT2>5K
then the equation (1) is not applicable. Now the question is, whether my inference correct from the above statement ? Additionally, I have attached the snap from IEC 61439-1 for your reference.

 

[che12345]

Mr DaSnipeKid (Electrical)(OP)15 May 23 08:01
".... we can deduce that the maximum current rating ..... 105K temperature rise which is also the limit. This formula can be used if the tested temperature is less than ±5K than the permissible limit. If my tested temperature is 98K where, ΔT1−ΔT2>5K, then the equation (1) is not applicable. Now the question is, whether my inference correct from the above statement? "
I am of the opinion as following for your consideration.
1. The formular is applicable up to (the temperature deviation from the [permissible value] by not more than +- 5K).
The key word is [permissible value]. That is, it is applicable for any value lower than the [permissible value]. This is to set the limitation or prevent/abuse of the formular.
2. In your case, the test was conducted at 98K; which is < 105K the [permissible value].
3. Conclusion: No problem, the formula is applicable.
Che Kuan Yau (Singapore)

 

[DaSnipeKid]

Hello @che12345
Thank you for your response.
Please find below response to your answer.
1. The formular is applicable up to (the temperature deviation from the [permissible value] by not more than +- 5K).
The key word is [permissible value]. That is, it is applicable for any value lower than the [permissible value]. This is to set the limitation or prevent/abuse of the formular.
2. In your case, the test was conducted at 98K; which is < 105K the [permissible value]. - Firstly, the tested temperature rise was 98K. This happened because my system was overkilled for the given rating. Secondly, the difference between permissible value and tested result is greater than +5K. Sometimes the tested result can go beyond the permissible value (this happens when sufficient cooling is not provided, or the cross section given is lesser resulting to the greater resistance and ultimately the temperature rise.) In that case, difference between tested result and permissible value is lesser than -5K. Please note the word less than -5K. Hence the standard uses the word deviates from +/- 5K. It means on the scale of integer, and for the given example, the tested temperature should not be less than 100K and greater than 110K.
3. Conclusion: No problem, the formula is applicable. - Please see response to the pointer 2. Please confirm if the above explanation is correct.
Che Kuan Yau (Singapore)

 

[DaSnipeKid]

-@che12345. FYI, I found the formula in Copper Development Association Publication No. 22, Ed. 14th - 1996 and it's been referred in IEC 61439-1. However, the CDA, does not mentions the condition of +/-5K. IEC 61439-1 does mentions the condition, as mentioned by you, to stop abuse of the formula.

 

[che12345]

Mr DaSnipeKid (Electrical)(OP)17 May 23 14:38 #1.".... I found the formula in CDA Publication No. 22, Ed. 14th - 1996 and it's been referred in IEC 61439-1. However, the CDA, does not mentions the condition of +/-5K. IEC 61439-1 does mentions the condition, as mentioned by you, to stop abuse of the formula".
a)The CDA formula 8) appears in CDA Pub 22 in 1984 and earlier prints, which are in my collection.
b)The CDA formula 8) does NOT appears in (now superseded IEC 60439-1 edition 4.1 2004-04
c) It is clear that IEC "adopted/accepted" the CDA formula 8) with qualification/limitation.
#2. "....Sometimes the tested result can go beyond the permissible value....In that case, difference between tested result and permissible value is lesser than -5K. Please note the word less than -5K. Hence the standard uses the word deviates from +/- 5K. It means on the scale of integer, and for the given example, the tested temperature should not be less than 100K and greater than 110K".
I am of the opinion that: the formular is valid for any temperature < 100K and Within 100K to 110K ; but NOT >110K
Che Kuan Yau (Singapore)

 

[DaSnipeKid]

@che12345 Apologies for the late response. However, do we have any citation, to prove otherwise (+/- 5K violation is permissible). I really need it because upon checking on Stack Exchange, I got a different response.

https://electronics.stackexchange.c...nation-of-temperature-rise-deviation-of-±-5-k