What it is and what it does
The Siemens SENTRON 5SM9625-0KK is a 2-pole residual current unit (RC unit) from the 5SM9 design series, rated 63 A at 230/400 V AC with a 300 mA tripping residual current. It's a type AC instantaneous device — meaning it responds to sinusoidal AC fault currents only, without intentional time delay, so it trips immediately when leakage exceeds 300 mA. That 300 mA threshold puts it in the fire-protection range rather than personnel protection (which typically requires 30 mA or lower), so it's sized for branch-circuit or feeder protection where the goal is equipment and wiring insulation fault detection, not shock prevention.
Thermal derating — the real-world current capacity
The 63 A rating holds at 40 °C and 45 °C (58.59 A at both, per the spec), then starts stepping down: 56.7 A at 50 °C, 54.81 A at 55 °C, 53.55 A at 60 °C, 52.29 A at 65 °C, and 50.4 A at 70 °C. If this unit lives inside a warm panel — say next to contactors or a busbar run — the 50 °C derated figure of 56.7 A is the number to use for conductor sizing and load planning, not the nameplate 63 A. The 70 °C floor of 50.4 A tells you the thermal limit is real but not severe; the unit sheds about 20% of its rated current across a 30 °C rise.
Panel integration notes
This is a 2-pole device in the 5SM9 form factor, designed for DIN-rail mounting inside a distribution board or sub-panel. The instantaneous design means no coordination delay with downstream RCDs — it trips at the same speed as a standard residual current device. Type AC classification limits it to sinusoidal AC fault currents; if the installation has non-sinusoidal loads (VFDs, switched-mode supplies, rectifiers), a type A or type F RC unit would be the correct choice instead.