What the ratings mean for your panel
The Siemens 5SY8304-8 is a SENTRON miniature circuit breaker with a D tripping characteristic, rated 4 A and built to protect circuits with high inrush currents — think motor starters, small transformers, or solenoid banks that pull several times their running current on startup. The D curve means it tolerates short-duration surges up to 10–20 times the rated current before tripping, which keeps nuisance trips off the floor on a line full of inductive loads. Its 50 kA breaking capacity at 400 V AC (per IEC 60947-2) tells you it can safely clear a fault up to that level without welding its contacts or blowing apart — important when the available fault current at the panel is high. That rating is the one that governs selectivity with upstream gear; you match the kA rating to the supply transformer's short-circuit capability. Temperature derating is baked in: at 45 °C it holds the full 4 A, at 50 °C it drops to 3.9 A, and at 55 °C to 3.8 A. If your panel runs hot — near a drive cabinet or in a non-climate-controlled enclosure — factor that derating into the load calculation so you don't get unexpected trips on a warm afternoon.
Mounting and integration
Snaps onto standard DIN rail via the quick-assembly system, 3 modular-width units wide (54 mm). Mounting position is any orientation, which helps in tight enclosures. Depth is 76 mm with an installation depth of 70 mm — factor that into door clearance if you're using a cover or handle extension. IP20 with connected conductors — fine inside a closed panel, not for washdown zones. Pollution degree 3 and overvoltage category III mean it's built for industrial environments where conductive dust or condensation may be present.