What this part is and what it does
The Siemens 5SM3321-4 is a 2-pole residual current circuit breaker (RCCB) from the SENTRON series, designed for 230 V AC supply at 50/60 Hz. It is rated at 16 A with a 30 mA residual current sensitivity and is type B, meaning it detects AC, pulsating DC, and smooth DC fault currents — required where variable-speed drives, UPS systems, or photovoltaic inverters are on the circuit. The short-time delayed design (selective type) provides discrimination: in a fault event, the downstream RCCB trips first, leaving this unit and upstream devices undisturbed. This suits sub-distribution boards feeding multiple socket-outlet circuits or mixed loads.
Mounting and integration
Snaps onto DIN rail (REG) per EN 60715. Occupies 4 modular width units (72 mm). Depth is 70 mm — verify cabinet door clearance; some shallow enclosures need the 70 mm plus wiring space. Mounting position is unrestricted, but the IP20 rating is conditional on the distribution board being installed and conductors connected.