Step 2: Design of an assembly and design verification Determining the rated short-time withstand current (I cw ) of a circuit of an assembly A switchgear assembly must be designed such that it withstands the thermal and dynamic stresses resulting from the short-circuit current. The maximum short circuit current at the connection point of an assembly must be determined on site. Example: I K " U N = 400 VAC Trans- former The panel builder must specify the rated short-time withstand current I cw of the connection point in his documentation, e.g. in the circuit diagram or technical document. The original manufacturer of the switchgear system, e.g.
HENSEL, is responsible for the verifi cation of the short circuit withstand capacity of the system components, e.g. the I cw value of the busbars. Step 1: Determining the transformer power and determining the value I K " The I K " can be determined by reading table 1. Rated short-circuit withstand current is determined by the values I K ", I cw , I cp , I cu . Alternatively, the I K is calculated using the formula: I K in kA S r in kVA U N in V u K in % HV = Main Distribution board UV = Sub-distribution board 3 ∙ U N ∙ u K I K " = S r ∙ 100 Table 2: Rated short-circuit withstand current of installation device in HENSEL distribution boards Table 1:
Excerpt from HENSEL main catalogue 20 Transformer S = 250 kVA see identifi er plate r U = 400 VAC see identifi er plate N I = 360 A see table 1 N I " = 9.025kA K see table 1 Rated power of the trans- former Rated cur- rent at rated voltage U =400 V a.c.
n Initial short- circuit cur- rent at u = 4% k Initial short- circuit cur- rent at u = 6% k S r in kVA I N in A I " K in kA I " K in kA 100 144 3.610 2.406 160 230 5.776 3.850 250 360 9.025 6.015 315 455 11.375 7.583 400 578 14.450 9.630 Installation device in HENSEL distribution boards Short-circuit withstand capacity Busbar 250 A / 400 A I =15kA / 1s CW NH fuse switch disconnector 250 A I = 50kA CC Circuit breaker 250 A / 400 A I = 50kA CU Switch disconnector 160 A I = 50kA CC I = I = 8 kA / 690 V a.c. MCCB 160 A / 250 A CS CU I = I = 36 kA / 415 V a.c. CS CU Other values c an be obtained from the device manufacturers or in the HENSEL main catalogue!