What this MCB is and what the ratings mean
The Siemens 5SJ3140-6RC is a 1-pole miniature circuit breaker from the Retail line, rated 40 A with a B tripping characteristic and a 4.5 kA breaking capacity. The B curve means it trips between 3 and 5 times rated current — suited for resistive and general-purpose loads where inrush is moderate, not for motor circuits that draw higher starting currents. The 4.5 kA breaking capacity tells you the maximum fault current it can safely interrupt at the rated voltage; if the prospective short-circuit current at the panel exceeds that, you need an upstream limiting device or a higher-rated MCB.