The Siemens 5SY5225-6KK11 is a SENTRON miniature circuit breaker rated 25 A at AC (35 °C ambient) with a B tripping curve, meaning it trips between 3 and 5 times rated current — good for resistive and general-purpose loads where you want fast clearing on short circuits but tolerance for mild inrush. The 440 V DC rating is the standout here: this 2-pole unit handles DC circuits at that voltage, which puts it in railway-signal and industrial-DC bus territory. Breaking capacity sits at 10 kA per EN 60898 (AC) and 10 kA per EN 60898-2 (DC) — enough for most sub-distribution panels behind a larger upstream device. Designed for railway applications per the spec, but don't read that as a restriction — it's a standard 2-module (36 mm wide) DIN-rail mount that drops into any panel. The quick-assembly fastening and any-position mounting mean it's a straightforward swap on a service call.
Thermal Derating — the Real-World Current
The 25 A rating holds at 35 °C. Above that, the thermal-magnetic trip starts pulling back: 23.48 A at 40 °C, 22.68 A at 45 °C, 21.85 A at 50 °C, 21 A at 55 °C, and 20.1 A at 60 °C. The B-curve characteristic stays the same across the range; only the thermal threshold shifts.
Panel and Compliance Notes
IP20 with connected conductors — protected against finger contact once wired, but not against water ingress. Overvoltage category III covers fixed-installation distribution panels. Sealable, halogen-free, silicon-free — useful for food and pharma lines where contamination from outgassing matters.