What the ratings mean for your panel
The Siemens 5SY4125-7CV is a 1-pole miniature circuit breaker with C-curve trip characteristic, rated 25 A at 35 °C and 230/400 V AC. The 15 kA breaking capacity per IEC 60947-2 means it can safely interrupt fault currents up to that level without upstream damage — important for high-fault installations like industrial distribution boards or sub-distribution panels. That 15 kA rating at 400 V AC is the headline number for selectivity studies; if your SCCR requirement is lower, you still have headroom.
Temperature derating — don't ignore it
At 35 °C ambient, the breaker carries its full 25 A. At 40 °C it derates to 22.22 A; at 45 °C to 20.7 A; at 50 °C to 19.1 A; at 55 °C to 17.27 A. If your panel runs hot — say 50 °C inside the enclosure — you're effectively buying a 19 A breaker, not 25 A. The humidity limits are also specified: max 95% RH up to 55 °C, then dropping to 55% at 70 °C and 35% at 75 °C. This matters for washdown or outdoor cabinets.
Physical fit and mounting
Single modular width (18 mm) on DIN rail, 76 mm depth, 90 mm height. The quick-assembly fastening system snaps onto standard EN rail. Mount in any position. Terminals accept combined top/bottom wiring — no special orientation needed. IP20 with conductors connected; pollution degree 3 (conductive pollution, typical for industrial panels). Installation depth 70 mm leaves clearance behind the rail.