What this RCCB does and why it matters
The Siemens 5SM3342-6KK03 is a 4-pole residual current circuit breaker (RCCB) from the SENTRON 5SM3 series, rated 25 A at 400 V AC with a 30 mA tripping threshold. The Type A designation means it detects pulsating DC fault currents superimposed on AC, which covers modern electronics and rectified loads (switch-mode power supplies, variable-speed drives) that a plain AC-sensitive RCCB would miss.
Ratings that govern fit
The instantaneous design (no intentional delay) trips within a cycle on a residual fault, which is the right choice for circuits where nuisance tripping from transient leakage is not a concern. The 25 A current rating is continuous; the thermal derating curve shows full rating up to 45 °C, then steps down to 17.5 A at 70 °C — so if the panel ambient runs hot, you need to account for the reduced capacity.
Mounts on DIN rail (REG profile) and occupies 4 width units (72 mm). The IP20 rating applies when installed in a distribution board with conductors connected; the enclosure itself provides the ingress protection. The top or bottom power supply position means no forced wiring order.