The Siemens 5SM9442-0KK is a SENTRON residual current unit — an RC unit electronic, 4-pole, type AC, rated 40 A at 400 V AC with a 100 mA trip threshold. It's an instantaneous design, meaning no intentional time delay; it trips as soon as the residual current exceeds 100 mA. That makes it a fit for general-purpose fault protection where you want fast clearing, not selectivity with downstream RCDs.
Thermal derating curve
The 40 A rating holds at 40 °C. Above that, the unit derates: 37.2 A at 45 °C, 36 A at 50 °C, 34.8 A at 55 °C, 34 A at 60 °C, 33.2 A at 65 °C, and 32 A at 70 °C. If this lives in a crowded DIN-rail enclosure with other heat sources, size the upstream protection for the derated value at your ambient — not the 40 A label.