What it is and what it does
The Siemens 5SY8440-7 is a SENTRON 4-pole miniature circuit breaker rated 40 A with a C tripping characteristic. It breaks up to 25 kA at 440 V AC per IEC 60947-2, and 15 kA at DC per the same standard. Under UL 1077 and CSA C22.2 No. 235, it's listed at 5 kA. That 25 kA interrupting rating means it can clear a bolted fault on a 400 V industrial distribution board without upstream fuses needing to open — useful for selectivity in a sub-panel feeding motor control centers or machine tools.
Where it goes and how it fits
Snaps onto a DIN rail via the quick assembly system — 4 modular width units (72 mm wide). Mounts in any position, which simplifies layout in a crowded enclosure. Installation depth is 70 mm, overall depth 76 mm. Pollution degree 3 and overvoltage category III mean it's rated for the conductive-dust and transient environment inside an industrial panel, not just a clean residential load center.
Thermal derating — the real-world current
Rated 40 A at 45 °C ambient, but drops to 39 A at 50 °C and 37.96 A at 55 °C. If this breaker sits in a non-ventilated enclosure near other heat sources, the 55 °C figure is the one that governs continuous load. The C-curve (5-10x In magnetic trip) is standard for motor branch circuits and general industrial loads where inrush isn't extreme.