The Siemens SENTRON 5SM3644-8 is a 4-pole residual-current circuit breaker (RCCB) — selective design, Type A, rated 40 A at 400 V AC, 50 Hz. It's the one you pick when you need fault discrimination upstream of a sub-distribution board: the selective characteristic (short-time delayed) lets a downstream RCCB trip first on a ground fault, keeping power on the main bus. Type A means it catches both sinusoidal AC and pulsating DC residual currents — common on VFD-fed motor circuits and modern electronics.
In a hot panel near a transformer or drive bank, that derating curve (–) is your real limit, not the 40 A nameplate. Surge-current resistance is 5 kA (8/20 µs), which covers most lightning-induced transients on a TN system. IP20 when installed in a distribution board with conductors connected. Four width units (72 mm) on the rail. Silicon-free construction, finger and back-of-hand safe touch protection. Overvoltage category III, so it's rated for fixed installation downstream of the main panel.