The Siemens 5SM3646-5BG is a 4-pole residual current operated circuit breaker (RCCB), type B, selective, rated 63 A with a 300 mA earth-leakage trip threshold on a 400 V AC supply. Type B detection means it catches AC, pulsating DC, and smooth DC fault currents — essential on circuits with VFDs, UPS feeds, or EV chargers that can generate DC leakage a standard AC/type A device would miss. The selective (S-type) characteristic introduces a deliberate time delay so the downstream RCCB trips first on a fault, keeping this unit as the back-up and avoiding a whole-panel blackout on a branch fault.
This is a DIN-rail mounted unit, 4-pole, so it occupies four modular spaces in the distribution board. The 63 A rating suits it as a main incoming RCCB on a sub-distribution board or as a feeder protecting downstream circuits. The 300 mA threshold is typical for fire protection or selective coordination — not for personal shock protection (30 mA is the standard for that).