Path of the short-circuit current from the transformer to the short-circuit Main Distribution Board (MDB) Sub-Distribution Board (SDB) Step 2: Step 3: Determining the rated short-time withstand current I CW of the main distribution board (MDB) Determining the rated short-time withstand current I CW of the sub-distribution board (SDB) Determining the lowest rated short-time withstand current I CW of the device installed in the main distribution board. Determining the lowest rated short-time withstand current I CW of the device installed in the in the sub-distribution board. Lowest value of the devices: I CC / I CU = 50kA Lowest value of the busbars:
I CW = 15kA I CW (MDB) = 15kA I CW (MDB) ≥ I K " 15kA ≥ 9.025kA * see table 2 Lowest value of the devices: I CC / I CU = 50kA Lowest value of the busbars: I CW = 15kA it follows: I CW (SDB) = 15kA I CW (SDB) ≥ I K " 15kA ≥ 9.025kA * see table 2 Determining the rated short-time withstand current I CW MDB Determining the rated short-time withstand current I CW SDB The rated short-time withstand current I CW of the MDB must be equal to or greater than the short-circuit current I K " of the transformer: I cp is the prospective short-circuit current at the installation site of the assembly at the incoming terminals. It (I cp ) is calculated from transformer and cable data (length, cross section).
Here, the cable attenuation due to distance and associated cable length between the transformer and sub-distribution board (SDB) is considered. The cable attenuation reduces the I K " of the transformer. I cw (MDB) ≥ I K " (transformer) In this analysis, the cable attenuation between the transformer and MDB is not considered. The cable attenuation can mean a reduction of the short-circuit current I K ". The prospective short- circuit current I cp at the installation site of the MDB is smaller because of the cable attenuation than I K " of the transformer. If a calculation is not possible, I cp = I K " can be assumed.
The rated short-time withstand current of the assembly results from the rated short-time withstand current of the installed equipment and busbars. The original manufacturer, such as HENSEL, specifi es these values in the technical data. The rated short-time withstand current (I CW ) must satisfy the following requirements: I CW (SDB) ≥ I cp (SDB) The rated short-time withstand current (I CW ) of the sub-distribution board is determined the same way as for the main distribution board. The respectively lowest value of the devices also determines the maximum rated short-circuit withstand current I CW of the sub-distribution board.
The panel builder must specify this value in the documentation of the assembly! The respective lowest value determines the maximum rated short-time withstand current I CW of the main distribution board. The panel builder must specify this value in the documentation of the assembly! 21 I cp 400A I CW I cp 250A I CW M MDB SDB MDB installed devices I or I CW CU Circuit breaker 400 A I = 50kA * CU Busbars 400 A I = 15kA / 1s * cw MCCB 250 A I = I = 8 kA / 690 V a.c. cs cu I = I = 36 kA / 415 V a.c.* cs cu SDB installed devices I CW Circuit breaker 250 A I = 50kA * CU Busbar 250 A I = 15kA / 1s * cw MCCB 160 A I = I = 8 kA / 690 V a.c. cs cu I = I = 36 kA / 415 V a.c.* cs cu