What this 0.3 A D-curve MCB is for
The Siemens 5SL4614-8 is a 4-pole miniature circuit breaker from the SENTRON 5SL4 family, wired 3P+N. It's a D-curve device rated at 0.3 A — that's the trip threshold for short-circuit and overload, not the continuous load you'd run through it. The 10 kA breaking capacity (per both EN 60898 and IEC 60947-2) tells you it can safely interrupt a fault up to that level without welding the contacts or cascading upstream — that's the standard for residential and light commercial distribution boards.
This MCB snaps onto DIN rail — 4 width units (72 mm wide), 90 mm tall, 76 mm deep including the terminals. The terminals accept 0.75 to 25 mm² solid or stranded copper, torqued to 2.5–3 N·m. Mounting position is any orientation; ambient range is -25 to +55 °C with up to 95% humidity.