The Siemens 5SM3646-6MJ is a SENTRON residual current circuit breaker (RCCB) — Type A, 4-pole (3+N), rated 63 A with 300 mA earth leakage sensitivity and a 10 kA breaking capacity at 400 V AC, 50 Hz. This is the part that sits downstream of the main breaker, protecting a three-phase plus neutral circuit against ground faults. The 300 mA threshold means it is sized for equipment protection rather than personnel shock protection (30 mA is the typical human-safety threshold); you spec this one where the leakage budget from drives, heaters, or long cable runs would nuisance-trip a finer device.
Panel integration
The 3+N pole arrangement means the neutral is switched and monitored internally; no separate neutral bar needed downstream of this device. Rated for 50 Hz only — not a 50/60 Hz universal part, so confirm your line frequency before committing. The 400 V AC rated voltage covers 230/400 V three-phase networks common in European and APAC distribution.