What the ratings mean for fit
The 5SY4313-8CV is a Siemens 5SY series miniature circuit breaker, 3-pole, D13 tripping characteristic, rated for a 15 kA interrupting capacity at 400 VAC. The D-curve means it tolerates the inrush of motor and transformer loads without nuisance tripping — the magnetic trip operates at 10–20× rated current, so it holds through the start-up surge of a small pump or compressor and still clears a hard fault fast. The 15 kA Icu at 400 V tells you it can safely interrupt a short-circuit fault up to that level without welding contacts or cascading the failure upstream; that is the figure a panel builder checks against the prospective short-circuit current at the distribution board.