The Siemens 5SM1642-0 is a SENTRON 5SM1-series residual current circuit breaker (RCCB) — 4-pole, type AC, rated 25 A with a 300 mA trip threshold at 400 V AC.
The 25 A rated current holds across the ambient range from -5 °C to 45 °C — no derating needed in most indoor panel environments. Above 45 °C, the current capacity steps down: 24 A at 50 °C, 23 A at 55 °C, down to 20 A at 70 °C. If your panel runs hot, factor that curve in. The 300 mA residual trip is a standard choice for general-purpose protection where personnel protection (30 mA) isn't required — often used on lighting, distribution subfeeds, or equipment where nuisance tripping from leakage is a concern. That's fine for standard resistive and inductive AC loads, but if your circuit includes VFDs, UPS systems, or rectified supplies, you'd want a type A or B RCCB instead. The 4-pole version (3 phases + neutral) suits three-phase distribution; the neutral pole is switched and monitored.
Snaps onto DIN rail (EN 60715) in any orientation — top or bottom feed allowed. Occupies 4 width units (72 mm at 18 mm per unit). Installation depth is 55 mm.