Thermal derating — the real-world current capacity
The 40 A rating holds at ambient temperatures up to 45 °C (37.2 A at both 40 °C and 45 °C). Above that, the unit derates progressively: 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. For a panel builder or site engineer, this means the RC unit's actual current-carrying capacity in a warm enclosure is lower than the nameplate 40 A — size upstream protection and cable accordingly if the ambient inside the panel exceeds 45 °C.
The 5SM9 RC unit mounts on DIN rail. It occupies four modular width units for the 4-pole variant. The electronic instantaneous design trips within standard RCD response time for Type A fault currents.