Siemens 5SY7225-8 SENTRON MCB — 2-Pole D-Curve, 25 A, 400 V AC
The Siemens SENTRON 5SY7225-8 is a 2-pole miniature circuit breaker with a D tripping characteristic, rated 25 A at 35 °C and 400 V AC. It breaks fault currents up to 15 kA per EN 60898 and 25 kA per IEC 60947-2, giving you selectivity headroom for motor-starting and high-inrush loads where a C-curve would nuisance-trip. The D-curve (10–20× In) is the right call for transformer or motor branch circuits — it rides through the inrush without opening, then clears a hard fault decisively.
Ratings and what they mean for fit
Rated 25 A at 35 °C ambient, the 5SY7225-8 derates to 23.8 A at 40 °C, 23.18 A at 45 °C, 22.55 A at 50 °C, 21.88 A at 55 °C, and 21.2 A at 60 °C. If your panel runs hot — say 50 °C inside the enclosure — you lose roughly 10 % of the current capacity. Size the load accordingly; do not count on the full 25 A in a tight cabinet. The 15 kA breaking capacity per EN 60898 covers most commercial and light industrial installations; the 25 kA per IEC 60947-2 gives you the higher fault-current rating needed when the prospective short-circuit current exceeds 15 kA, common on larger transformer-fed panels. Pollution degree 3 and overvoltage category III mean this breaker is rated for the industrial environment — conductive dust, condensation, and transient overvoltages from the mains distribution side. It is not a residential-grade device. Mount it in any position; the quick-assembly system clips onto standard DIN rail. At 2 modular width units (36 mm wide, 76 mm deep, 90 mm tall), it occupies the same footprint as any other 2-pole SENTRON MCB. The combined terminals at top and bottom accept both busbar and wire connections without re-termination. The 5SY7225-8 is halogen-free and silicon-free, which matters for installations in clean rooms, semiconductor fabs, or areas where outgassing from a fault could contaminate optics or sensitive surfaces. Sealable with a lead or tag for lockout/tagout compliance. Touch protection is built in — finger-safe terminals with conductors connected. IP20 with conductors installed; no washdown rating, so keep it inside the panel, not on the machine exterior.
Integration — DIN-rail panel, 2-module width
Snaps onto standard 35 mm DIN rail. The 36 mm width (2 modular units) matches the standard SENTRON grid; you can busbar it directly with the matching SENTRON busbar system. Combined terminals top and bottom accept up to 25 mm² conductor (typical for a 25 A circuit). No special tools beyond a screwdriver and a DIN-rail stop. The 70 mm installation depth leaves clearance for wiring behind the rail in a standard 120 mm deep enclosure.