What this MCB is and what the ratings mean for fit
The Siemens 5SJ6240-7FC is a SENTRON miniature circuit breaker (MCB) — a 2-pole, C-curve device rated 40 A at 400 V AC, with a 6 kA breaking capacity per EN 60898. That C-curve means it trips at 5 to 10 times rated current, so it handles moderate inrush from motor starters or small transformers without nuisance tripping, while still clearing a hard short. The 6 kA SCCR at 400 V tells you it safely interrupts faults up to that level on a residential or light-commercial panel — adequate for most branch circuits, but check coordination if you're feeding a sub-panel with a larger upstream breaker. Rated 72 V DC on each pole, so it handles DC control circuits or small battery-backed loads. The 40 A rating at 30 °C derates to 37.57 A at 40 °C and 36.8 A at 45 °C — expect to downrate by roughly 3 A per 10 °C above 30 °C if the panel runs warm. Pollution degree 2 (typical for clean, non-conductive environments) and overvoltage category III suit it for fixed-installation branch circuits in residential or infrastructure panels. Halogen-free and silicon-free construction matters if this breaker sits in a sealed enclosure near sensitive electronics — no corrosive off-gassing during a fault. IP20 with connected conductors means it's protected against finger contact but not moisture; keep it inside a dry panel.
Lifecycle and sourcing — active, not obsolete
The SENTRON series is Siemens' standard MCB line for residential and infrastructure duty. The 2-pole, 2-module-width (36 mm) format is a common footprint that drops into any standard DIN-rail enclosure. No special adapter or rewiring needed if replacing an equivalent 2-pole C-curve 40 A MCB.
Panel fit and integration
Mounts on standard 35 mm DIN rail. Two modular width units (36 mm wide) — a tight fit for a 2-pole 40 A breaker; check fill factor if your panel is dense. Depth of 76 mm and installation depth of 70 mm leave clearance for wiring and gland plates. Mounting position any, so vertical or horizontal rail orientation works. Sealable (can be locked in the OFF position with a padlock) — useful for lockout/tagout on a downstream load. Touch protection not fitted as standard; if the panel is open to live busbars, add a finger-safe terminal shield separately.