What the ratings mean for the panel
The Siemens 5SY4305-7CV is a 3-pole miniature circuit breaker with a C-trip curve and a 0.5 A rated current. The Icu (ultimate breaking capacity) of 15 kA at 400 V AC is the fault current it can safely interrupt without damage to itself or upstream gear — that's the figure you coordinate against for selective tripping in a distribution board. On a 400 V three-phase supply, the 15 kA rating covers most industrial sub-distribution scenarios short of a transformer secondary. The C-curve (5-10x In magnetic trip) suits it for moderate inrush loads like small contactor coils or control transformers where you don't want nuisance trips from the magnetizing surge.