Determination of Temperature Rise – Deviation of ± 5K – IEC 61439-1

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,

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)

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. But 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 is correct from the above statement ? Additionally, I have attached the snap from IEC 61439-1 for your reference.

It is encouraging to see that IEC 61439-1 has a written requirement on the determination of temperature rise with a specified deviation of ±5K for all panels and enclosures. This is a very important measure of control for proper design and to ensure the safe operation of electrical equipment in any facility. Materials selection and the quality of workmanship when constructing enclosures is very important in achieving the desired results. Without accurate temperature rise measurements, the manufacturer and the facility could experience problems down the line.

In most cases, refractive index depends mostly on density. Density usually decreases as temperature increases. However, there are exceptions.

In semiconductors, the density of carrier electrons increases with temperature. The refractive index increases as carrier density increases.

There is no simple, one size fits all, rule.

 

In most cases, refractive index depends mostly on density