Siemens SENTRON 5SM3644-6KK01 is a 4-pole residual current operated circuit breaker (RCCB), type A, short-time delayed, rated 40 A at 400 V AC with a 300 mA trip threshold. The short-time delay means it withstands surge currents up to 3 kA without nuisance tripping — useful on lines with motor starts or capacitor banks that produce brief leakage pulses.
Type A detection catches pulsating DC fault currents on top of sinusoidal AC — the standard for circuits with single-phase rectifiers, switched-mode supplies, or variable-speed drives. The 300 mA trip threshold is a fire-protection level, not personnel protection (30 mA is the touch-safe boundary); this RCCB goes upstream of sub-distribution boards or feeds circuits where equipment leakage is expected but ground-fault energy must still be limited. The 40 A continuous rating and 400 V AC system voltage match a three-phase 4-wire (TN or TT) supply feeding a moderate load centre. Short-time delayed design (3 kA surge withstand) lets it ride through transient earth faults caused by switching surges, so it won't drop a feeder on a momentary event. Terminals accept 1.5 to 25 mm² solid or stranded copper, tightened to 2.5–3 N·m. That covers standard panel wiring from a 4 mm² sub-feed up to a 16 mm² main tail. Overall dimensions are 72 mm wide × 90 mm high × 77 mm deep, with 70 mm installation depth into the enclosure. Four width units (4 × 18 mm) on a DIN rail — plan for that slot count in your panel layout.
This RCCB is a DIN-rail device for distribution boards (IP20 with conductors connected and the board installed). It sits in a sub-distribution panel feeding general-purpose sockets, lighting circuits, or small machinery where the supply is 400 V three-phase and the design calls for a 300 mA delayed RCCB for fire protection. The SENTRON family is Siemens' standard modular protection range; the 5SM3 design is the short-time delayed variant. Mounting position is any, and supply can enter top or bottom — no orientation restrictions.