What it is and what it does
The Siemens 5SN6440-6CN is a 4-pole SENTRON miniature circuit breaker with a B tripping characteristic, rated 40 A at 400 V AC, 50/60 Hz. Its 6 kA breaking capacity per EN 60898 covers standard residential and commercial branch circuit protection. The B curve means it trips between 3 and 5 times rated current — suited for resistive and general-purpose loads where inrush is low, not for motor starting or transformer primaries.
Fit and footprint
At 72 mm wide (4 modular width units), it occupies four standard 18 mm DIN-rail slots. Depth is 76 mm, installation depth 70 mm. Mounting position is any, so it works in horizontal or vertical panel layouts. The IP20 rating (with connected conductors) is typical for enclosed distribution boards — not for wet or outdoor locations without a secondary enclosure.
What the ratings mean for your panel
The 6 kA rated short-circuit capacity per EN 60898 at 400 V AC means it safely interrupts fault currents up to that level. Overvoltage category III and pollution degree 2 suit it for fixed-installation distribution boards. Energy limitation class 3 means it lets through less let-through energy than class 1 or 2 breakers — better for protecting downstream equipment. The 3.4 W power loss per pole at rated current in hot state matters for thermal budgeting in a crowded enclosure.