Siemens SENTRON 5SL6140-7CC — Type C 40 A MCB, 6 kA
The 40 A rating (at 30 °C) is the headline figure, but the derating curve is what governs real-world panel fit: at 40 °C ambient the breaker is rated 36.88 A, at 45 °C it drops to 35.22 A, and at 50 °C to 33.48 A. If your enclosure runs warm — common in a crowded DIN-rail panel — size the upstream protection and load to the derated value, not the 30 °C number. The C-curve means magnetic trip occurs between 5 and 10 times rated current, so a 40 A breaker clears short circuits between 200 A and 400 A magnetically; below that threshold the thermal bimetal handles overloads. For industrial panels with higher fault levels, a higher-rated MCB or backup fuse would be needed.