What it is and what it does
The Siemens SENTRON 5SM9420-0KK is a 2-pole residual current unit (RC unit) rated for 25 A operational current at 230/400 V AC, 50 Hz. It trips at 100 mA residual current and is designed as an instantaneous (non-time-delayed) type AC device — meaning it detects sinusoidal AC fault currents only, not pulsating DC or smooth DC. The type AC designation suits it for general-purpose circuits where the load is purely resistive or inductive with no rectified components.
Thermal derating and real-world current
The 25 A rating holds at 40 °C and 45 °C (23.25 A each), then steps down through the temperature range: 22.5 A at 50 °C, 21.75 A at 55 °C, 21.25 A at 60 °C, 20.75 A at 65 °C, and 20 A at 70 °C. If the RC unit sits in a warm enclosure — common in a crowded DIN-rail panel — use the 50 °C or 60 °C column to size the load, not the 25 A headline.
Panel integration
As a 2-pole RC unit in the SENTRON family, it snaps onto a standard DIN rail inside a distribution board or sub-panel. The instantaneous design means it coordinates with upstream overcurrent devices but does not provide selectivity against transient leakage — use a time-delayed (S-type) RC unit if you need coordination with downstream RCDs.