What this MCB is and where it fits
The Siemens 5SY4125-7KK11 is a 1-pole SENTRON miniature circuit breaker rated 25 A at AC with a C tripping characteristic. It's designed for railway applications and mechanical engineering/industry panels — the -40 to 75 °C range and pollution degree 3 rating tell you it lives in a cabinet, not a climate-controlled server room. The 18 mm width (1 modular unit) snaps onto a DIN rail; any mounting position works.
Breaking capacity and what it means for your panel
Rated 10 kA per EN 60898 and 20 kA per IEC 60947-2. That dual rating matters: EN 60898 is the residential/commercial standard; IEC 60947-2 is the industrial standard. The 20 kA figure gives you headroom for high-fault industrial feeds — a panel fed from a 630 kVA transformer can push 18 kA prospective, and this breaker handles it without cascading upstream. The C-curve trips between 5 and 10 times rated current, so it's sized for moderate inrush loads like small motors or transformers.
Lifecycle and sourcing posture
Lifecycle stage is current. This part is sourced to order against an RFQ through independent distribution.
Integration and mounting details
Snap-on DIN rail mount, 18 mm wide (1 MW). Installation depth 70 mm, overall depth 76 mm. Combined terminal top and bottom — wire from either direction. Touch protection is built in. Sealable, so it can be locked in the OFF position for lockout/tagout. IP20 with connected conductors; fine inside a closed panel, not for washdown zones.
Temperature derating you need to know
The 25 A rating holds at 35 °C. At 40 °C it's 23.8 A; at 50 °C it's 22.55 A; at 60 °C it's 21.2 A. If your panel ambient runs 50 °C, size the load at 22.5 A, not 25. The maximum ambient is 75 °C, but at that point the breaker is derated to about 35% of nominal — that's a survival limit, not an operating point.