What it is and where it fits
The Siemens 5SJ4108-8HG40 is a SENTRON miniature circuit breaker (MCB) from the 5SJ4 family, designed for 1-pole protection of branch circuits at a rated 8 A AC. The D tripping characteristic means it tolerates high inrush currents — typically used for motor, transformer, or lighting loads where the starting surge would nuisance-trip a B or C curve. It snaps onto standard DIN rail (35 mm) and occupies a single modular width unit (18 mm), so it fits tight panel layouts without crowding adjacent breakers.
Thermal derating and real-world current
At 30 °C and 40 °C the breaker holds its full 8 A rating. At 45 °C it derates to 7.8 A, at 50 °C to 7.6 A, at 55 °C to 7.4 A, and at 60 °C to 7.2 A. If your panel ambient runs hot — say, a sealed enclosure near a process line — size the load to the derated value, not the nameplate 8 A. The operating range spans -40 °C to 75 °C, which covers most industrial environments from cold storage to near-oven zones.