What the ratings mean for fit
The 5SY4403-7CV is a Siemens miniature circuit breaker (MCB) with a 15 kA rated ultimate breaking capacity (Icu) at 400 V AC, 4 poles, and a C-trip curve at 3 A. The 15 kA Icu means it can safely interrupt fault currents up to that level without welding its contacts or failing catastrophically — critical for installations where the prospective short-circuit current at the panel is high. The C-curve (magnetic trip between 5× and 10× In) is sized for moderate inrush loads like small motors, transformers, or banks of LED drivers; it avoids nuisance trips on start-up that a B-curve would catch. The 4-pole configuration handles three-phase plus neutral, common in European-style distribution boards feeding mixed single- and three-phase loads.