What it is and what it does
The 5SM3644-0LB: This RCCB mounts on a DIN rail (REG) and occupies 4 width units (about 72 mm) with a 70 mm installation depth, fitting standard distribution boards. The IP20 rating applies when installed with conductors connected — it's protected against solid objects larger than 12 mm, but not against water ingress, so it belongs inside an enclosed panel.
Key ratings and what they mean for fit
The 40 A rated current at AC tells you this RCCB can carry 40 A continuously on each pole without overheating — sized for a 3-phase load up to roughly 27 kVA at 400 V line-to-line. The 300 mA residual tripping threshold is higher than the typical 30 mA personal-protection level; it's intended for equipment protection or fire-prevention on circuits where nuisance tripping from normal leakage (long cable runs, industrial loads) would be a problem. The 4-pole design switches all three phases plus neutral, common for 3-phase 400 V distribution where the neutral must be disconnected on a fault. Terminal cross-section accepts solid or stranded conductors from 1.5 to 25 mm², with a tightening torque range of 2.5 to 3 N·m — standard for screw-type terminals in this class. Rated surge current resistance of 1 kA (8/20 µs waveform) means it can withstand typical lightning-induced surges without damage, provided the upstream overcurrent device limits let-through energy. Ambient temperature range is -5 to +45 °C for normal operation, with storage from -40 to +75 °C — fine for unheated electrical rooms in temperate climates.
Where it's used
This RCCB is a panel-mounted residual current protection device for 3-phase 400 V AC distribution in commercial and industrial buildings — think lighting panels, small machine feeds, HVAC equipment, or general sub-distribution boards. The type AC designation means it detects sinusoidal AC residual currents only; it's not suitable for circuits with pulsed DC or smooth DC fault currents (e.g., from variable-speed drives, UPS systems, or EV chargers). For those, you'd need a type A or type B RCCB. The finger and back-of-hand safe protection against electrical shock (per the evidence) confirms the live parts are shielded when the device is installed — a basic safety requirement for accessible panel components.