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 \$I_1\$ at the maximum permissible temperature-rise \$\Delta T_1\$, the current rating may be calculated from the actual test current I2 if the measured temperature-rise \$\Delta T_2\$ of the current carrying parts (e.g. busbars and terminals) deviates from the permissible value by not more than \$\pm 5\text{K}\$, using the following formula: given in equation \eqref{1}
Let's examine this scenario by an example.
DATA:
- Copper-copper joint Temperature rise limit \$\Delta T_1 = 105\text{K}\$
- Tested temperature \$\Delta T_2 = 102\text{K}\$
- Test current \$I_2 = 973\text{A}\$
TO FIND:
- Maximum current rating
SOL:
The maximum current rating can be determined as followed: $$\frac{I_1}{I_2}={\left[\frac{ΔT_1}{ΔT_2}\right]}^{0.61}\label{1}\tag{1}$$ Substituting above values in the equation, we get, $$I_1 = 990\text{A}\label{2}\tag{2}$$ 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. Now, 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, $$ \Delta T_1-\Delta T_2 > 5\text{K} $$ then the equation \eqref{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.
