What it is and what the ratings mean
The Siemens 5SY8263-8BB08 is a SENTRON miniature circuit breaker (MCB) with magnetic-only tripping — no thermal bimetal, so it responds exclusively to short-circuit current, not overload. That makes it a straight short-circuit protector for circuits where overload protection is handled upstream or by the load itself. Rated 63 A at 400 V AC, with a D tripping characteristic that allows inrush currents up to 10–14× rated current before the magnetic trip fires. That D-curve is sized for motor starting, transformer energisation, or capacitive loads where the inrush spike would nuisance-trip a B or C curve breaker. Breaking capacity is 20 kA per IEC 60947-2 at 400 V AC — enough for most industrial distribution boards with a fault level under that threshold. For UL 1077 / CSA C22.2 No. 235 applications, the interrupting rating is 5 kA at 400 V AC, so verify your available fault current if the panel carries a UL/CSA sticker. On DC circuits, it is rated 72 V maximum, so it works in 48 V or 60 V DC control buses but not on a 110 V DC battery bank.
Where it goes and how it mounts
Two-pole (2P) design in a 36 mm wide housing — that is 2 modular width units on a DIN rail. Snaps onto the rail via the quick-assembly system; mounting position is any, so it fits vertical or horizontal busbars without derating. Installation depth is 70 mm; overall depth is 76 mm including the front face. IP20 with connected conductors — standard for enclosed distribution boards. Rated for pollution degree 3 and overvoltage category III, meaning it is suited for the fixed installation inside an industrial panel, not for outdoor exposed locations. Sealable with a lead seal or lockout tag, halogen-free and silicon-free construction. Mechanical service life is 10 000 switching cycles typical.
Lifecycle and compliance
Rated for use in mechanical engineering and industrial environments per the manufacturer's suitability statement. The product function code per DIN EN 61346-2 is F (protective device).