What it is and what the ratings mean
The Siemens 5SY6303-8CC is a SENTRON miniature circuit breaker (MCB) with a D tripping characteristic, rated 3 A at AC, 50/60 Hz. The D-curve means it tolerates high inrush currents — typically 10 to 20 times the rated current for short durations — making it the right choice for motor, transformer, or lighting ballast circuits where the start-up surge would nuisance-trip a B or C curve. The 6 kA breaking capacity per EN 60898 (5 kA per UL 1077 / CSA C22.2 No.235) tells you it can safely interrupt a fault current up to that level without welding contacts or cascading damage upstream. Overvoltage category III and pollution degree 3 mean it's rated for fixed-installation distribution panels, not just clean indoor environments.
Derating and thermal environment
The 3 A rating holds at 40 °C ambient. Above that, the breaker derates linearly: 2.82 A at 40 °C, 2.73 A at 45 °C, 2.63 A at 50 °C, and 2.53 A at 55 °C. If your panel runs hot — say inside a non-ventilated enclosure near a drive — size the load or the breaker accordingly. The operating range spans -40 °C to 75 °C, with humidity limits that step down above 55 °C (95% RH up to 55 °C, 55% up to 70 °C, 35% up to 75 °C). That covers most industrial environments short of direct washdown.
Panel integration and mounting
Snaps onto DIN rail via the quick assembly system — no tools needed. Occupies 3 modular width units (54 mm wide). Mounting position is any orientation, which simplifies layout in crowded enclosures. Installation depth is 70 mm; overall depth is 76 mm. Combined terminals top and bottom accept the same conductor range, so you don't need separate feed-through lugs. IP20 with connected conductors — fine for enclosed distribution boards, not for exposed outdoor use without an outer cabinet. Sealable, halogen-free, silicon-free — relevant for food, pharma, or clean-room panels where outgassing or contamination is a concern.