What this MCB is and what the ratings mean for your panel
The Siemens 5SY4604-8CV is a 3-pole-plus-neutral miniature circuit breaker with a D tripping curve, rated for 4 A and a short-circuit breaking capacity (Icu) of 15 kA at 400 V AC. The D curve means it's designed for loads with high inrush currents — think motor starters, transformers, or discharge lighting — where the magnetic trip won't nuisance-trip on startup but still clears a hard fault fast. The 15 kA Icu tells you this breaker can safely interrupt a fault current up to that level without welding its contacts or rupturing the case, which is critical for coordination downstream of a transformer or a high-capacity bus.