What it is and what it does
The Siemens 5SM3644-4BG is a 4-pole residual current circuit breaker (RCCB) with Type B tripping characteristic, rated 40 A continuous current and 300 mA residual fault sensitivity at 400 V AC. Type B means it detects AC pulsating DC and smooth DC fault currents — the full spectrum you get from variable-speed drives, UPS systems, and rectifier-fed loads. The short-time delayed (S-type) selectivity rating means it coordinates with downstream RCCBs so only the faulted branch opens, not the whole sub-distribution board. Snaps onto standard DIN rail in a panel or enclosure.