Integrator note — B-curve MCB for 400 V AC branch circuits
The Siemens 5SY4125-6 is a 1-pole SENTRON miniature circuit breaker with a B tripping characteristic, rated 400 V AC and 60 V DC. It breaks 10 kA per EN 60898-1 and 20 kA per IEC 60947-2, with a DC breaking capacity of 15 kA per IEC 60947-2. The B-curve trips between 3 and 5 times rated current — standard for resistive and general-purpose loads where inrush is moderate, not for motor starting or transformer primaries. Occupies a single 18 mm modular width on DIN rail, mounts in any position, and accepts supply from either end. The quick-assembly system clips onto the rail without tools. Pollution degree 3 and overvoltage category III suit it for industrial control panels and distribution boards where the environment is not clean-room but not wet.
Sourcing & lifecycle — active, no phase-out signal
Lifecycle status is current. No end-of-life notice, no last-time-buy window. The 5SY4125-6 remains in Siemens' active SENTRON catalog for new installations and replacement spares. Sourced to order against an RFQ through independent distribution; availability and current pricing confirmed at quote time.
Compliance & approvals — three-standard rated
Rated under three standards: EN 60898-1 (10 kA), IEC 60947-2 (20 kA AC, 15 kA DC), and UL 1077 / CSA C22.2 No.235 (5 kA). The UL 1077 listing means it is recognized as a supplementary protector in North American panels — not a branch-circuit rated breaker per UL 489, but accepted for internal equipment protection. Halogen-free and silicon-free construction reduces outgassing in sealed enclosures. Sealable for lockout/tagout compliance.
Panel fit — 18 mm width, 70 mm installation depth
Width of 18 mm (one modular unit) and installation depth of 70 mm from the DIN rail face. Overall depth is 76 mm including the front toggle. The combined terminal top and bottom accept supply from either end — no need to orient the breaker for line/load. IP20 with connected conductors means the terminals are finger-safe once wired.