The Siemens 5SM3444-8 is a 4-pole SENTRON residual current circuit breaker (RCCB) rated 40 A at 400 V AC, 50 Hz, with a 100 mA Type A trip characteristic. It's a selective (time-delayed) design — meaning it coordinates with downstream RCCBs so only the faulted circuit trips, keeping the rest of the distribution board live.
Thermal derating — the real-world current rating
Above that, it derates linearly: 37.6 A at 50 °C, 35.2 A at 55 °C, 32.8 A at 60 °C, 30.4 A at 65 °C, and 28 A at 70 °C.
Type A fault current and selective coordination
Type A detects sinusoidal AC residual currents and pulsating DC residual currents — the kind produced by single-phase rectifiers, switching power supplies, and variable-speed drives. The 100 mA trip threshold suits sub-distribution boards feeding socket outlets or mixed loads where a 30 mA device would nuisance-trip. The selective (S-type) design withstands a 5 kA surge current without tripping, so lightning or capacitor inrush won't drop the board.