The Siemens 5SY4308-6 is a 3-pole miniature circuit breaker from the SENTRON family, rated for AC circuits at 400/440 V and 50/60 Hz.
The B-curve is the key selection parameter here — it's not for motor or transformer starting (that's C or D curve territory). For lighting, heating, control transformers, and general distribution where the load doesn't slam the breaker on startup, this is the right call. The 10 kA SCCR at 400 V AC per EN 60898 covers the vast majority of commercial and light industrial panelboards; the 20 kA per IEC 60947-2 gives headroom if you're coordinating downstream of a larger transformer. DC rating is 15 kA at up to 72 V DC per IEC 60947-2, which matters if this lands in a DC distribution board.
Snap-on DIN rail mount, 2 modular width units (about 36 mm), 70 mm installation depth. Mounting position any — vertical, horizontal, upside down, doesn't matter. Halogen-free and silicon-free construction, which is a plus for environments where outgassing matters (clean rooms, certain chemical plants).