What it is and what the ratings mean
The Siemens SENTRON 5SM3446-8LA is a 4-pole residual current operated circuit breaker (RCCB) rated 63 A at 400 V AC, with a 100 mA trip threshold and Type A fault current sensitivity. The 100 mA rating means it's selective — it will not trip on a ground fault that a downstream 30 mA RCCB should clear, so only the faulted branch is isolated and the rest of the distribution board stays live. Type A detection covers sinusoidal AC and pulsating DC fault currents, which is the standard requirement for most commercial and light-industrial panels with single-phase electronics. The 10 kA short-circuit current rating (SCCR) at 400 V means this device can safely interrupt a fault current up to that level without welding its contacts or rupturing — critical for coordinating with an upstream MCB or fuse in the distribution board. It includes thermal overload protection, so it also guards against sustained overcurrents, not just leakage — a feature that saves an extra protective device in the same DIN-rail slot.
Mounting and panel integration
Snaps onto standard DIN rail (REG) and occupies 4 width units (about 72 mm at 18 mm per unit). Installation depth is 70 mm, so it fits flush in most 80 mm deep enclosures. Mounting position is any — vertical, horizontal, upside-down — no derating needed for orientation. Terminals accept solid or stranded conductors from 1.5 to 25 mm²; tighten to 2.5–3 N·m. Rated IP20 with conductors connected and the distribution board installed — standard for enclosed panel mounting, not for wet or outdoor locations. Overvoltage category III means it's rated for fixed-installation distribution level, not for downstream equipment.