What this MCB is and where it fits
The Siemens 5SJ4103-8HG40 is a 1-pole miniature circuit breaker from the SENTRON 5SJ4 series, rated 3 A with a D tripping characteristic and a 10 kA breaking capacity per EN 60898. It snaps onto standard DIN rail and occupies one modular width unit (18 mm). The D curve means it tolerates high inrush currents — typically motor or transformer loads — without nuisance tripping on startup.
Ratings that govern the real-world fit
At 30 °C and 40 °C ambient the breaker holds a full 3 A continuous; derate to 2.9 A at 45 °C, 2.8 A at 50 °C, 2.7 A at 55 °C, and 2.6 A at 60 °C. If the panel runs hot — say a sealed enclosure near a motor — use the 55 °C or 60 °C row to size the load. The 10 kA breaking capacity per EN 60898 covers most commercial and light industrial fault levels; for higher SCCR environments you'd step to an industrial-class MCB or fuse.
Mounting and environment notes
Mounts on standard DIN rail in any position. Installation depth matches the body depth at 70 mm. The enclosure rating is IP20 with connected conductors, IP40 in the handle area — fine for a dry indoor panel; not for washdown or outdoor exposure. Halogen-free and silicon-free construction matters if the panel serves sensitive electronics or clean-room zones.