The Siemens 5SY4125-8 is a SENTRON miniature circuit breaker with a D tripping characteristic, built for one pole and one modular-width unit (18 mm). It's rated to clear faults at 10 kA per EN 60898-1 and 20 kA per IEC 60947-2 at 400 V AC, with a DC breaking capacity of 15 kA at 60 V per IEC 60947-2. The D-curve means it's designed to handle the high inrush currents from motor loads, transformers, or welding gear without nuisance tripping — out here in the grease, that's the difference between a line that starts and one that drops out on every compressor kick.
Breaking capacity and standards
This MCB carries dual-standard ratings: 10 kA under IEC/EN 60898-1 and 20 kA under IEC 60947-2, both at 400 V AC. The 60947-2 figure is the one that matters for industrial panel coordination — it gives you headroom above the 10 kA utility-side fault level common in North American and European distribution. At DC, it's rated 15 kA at 60 V (IEC 60947-2). For UL 1077 / CSA C22.2 No. 235 applications, the interrupting capacity is 5 kA. The operating voltage range covers 400 V AC single-phase and 440 V AC multi-phase, with a DC max of 72 V.
D-curve and deployment context
The D characteristic (tripping between 10x and 20x In) suits motor starting, welding circuits, and transformer primaries where the inrush can hit 12–15x full-load current for a few cycles. A B- or C-curve breaker would nuisance-trip on that same surge. The 5SY4125-8 is rated for mechanical engineering and industrial environments, with a pollution degree of 3 (conductive pollution or dry non-conductive that becomes conductive) and an overvoltage category of III (fixed-installation level). It mounts in any position and snaps onto a DIN rail via the quick-assembly system — one module wide, so it fits tight panels without crowding the next breaker.
Environmental and compliance
Operating temperature range is -40 °C to 75 °C, with humidity derating: max 95% up to 55 °C, then dropping to 35% at 75 °C. The breaker is halogen-free and silicon-free, which matters for outgassing-sensitive environments like clean rooms or sealed enclosures. It's sealable (provision for a lead seal or lock) and carries IP20 protection with connected conductors. Vibration resistance per IEC 60068-2-6 covers ±1 mm at 5–25 Hz and 50 m/s² at 25–150 Hz — enough for pump skids and conveyor drives that rattle the panel.