The Siemens 5SY4240-8FA is a 2-pole miniature circuit breaker from the SENTRON series, rated 40 A with a D tripping characteristic. The 10 kA breaking capacity at 400 V AC (per EN 60898) tells you it can safely interrupt a fault up to that level without welding the contacts or cascading upstream. Mounts on DIN rail, occupies 2 width units (36 mm), and accepts conductors up to 35 mm² solid or stranded.
The D-curve's magnetic trip threshold (10–20× In) is what keeps it from dropping out on motor start — a 7.5 kW motor drawing 120 A locked-rotor for 200 ms stays below the instantaneous trip point, while a hard short still clears in under 10 ms. The 10 kA SCCR at 400 V means the breaker itself is rated to interrupt a 10 kA prospective fault current; if your panel's available fault current exceeds that, you need an upstream current-limiting device to coordinate. The 2-pole configuration switches both phase conductors simultaneously — no neutral switching (confirms neutral conductor switching is not a function of this unit).
Depth is 76 mm, installation depth 70 mm — leaves 6 mm clearance behind the rail for wiring. Height 90 mm, width 36 mm (2 width units). Terminal torque spec is 3.5 N·m; accepts 0.75 to 35 mm² solid or stranded. Ambient operating range -25 to +55 °C; storage range -40 to +75 °C. Halogen-free and silicon-free construction. Sealable for tamper-proofing in metering or rental applications.