What this 5SJ4 MCB is and where it fits
The Siemens SENTRON 5SJ4245-8HG41 is a 2-pole miniature circuit breaker in the 5SJ4 family, rated 45 A at both 30 °C and 40 °C ambient with a D tripping characteristic. That D-curve means it tolerates the high inrush of motor and transformer loads — magnetic trip kicks in between 10x and 20x rated current, so it won't nuisance-trip on a pump or compressor start. Rated for AC/DC operation, with a maximum DC voltage of 60 V. It snaps onto a standard DIN rail (35 mm) and can be mounted in any position, which simplifies panel layout.
Ratings that decide the fit
Full current rating holds at 45 A up to 40 °C ambient — no derating needed in a typical 35 °C panel. At 45 °C it drops to 44.1 A, at 50 °C to 43.2 A, at 55 °C to 42.2 A, and at 60 °C to 41.4 A. If your enclosure runs hot, check the thermal curve: a 50 °C panel knocks off about 1.8 A. Pollution degree 3 (conductive pollution) and overvoltage category III suit industrial control panels where the breaker sees transient overvoltages from the mains. Vibration resistance is rated at 50 m/s² from 25 to 150 Hz and 60 m/s² at 35 Hz — enough for machine-mounted applications near conveyors or compressors.