The Siemens 5SY4150-8CV is a single-pole miniature circuit breaker (MCB) with a D50 trip characteristic and a rated ultimate breaking capacity (Icu) of 15 kA at both 230 V and 400 V AC.
What the rating means in the panel
A breaker with 15 kA Icu gives you headroom for most sub-distribution boards downstream of a larger main breaker. The D50 curve (50 A rated current, D-trip magnetic release at 10–20× In) means it'll hold through the magnetizing inrush of a 15–20 kW motor without popping, but still clear a bolted fault fast enough to keep let-through energy within cable limits.