What it is and what the ratings mean
The Siemens 5SL3320-7MB is a 3-pole SENTRON miniature circuit breaker (MCB) with a C-curve trip characteristic, rated 20 A at 400 V AC. The C-curve means it trips between 5 and 10 times rated current — standard for general-purpose loads like lighting, socket outlets, and small motor circuits where moderate inrush is expected. The 4.5 kA breaking capacity per EN 60898 tells you it can safely interrupt a fault current up to that level at 400 V, which covers most residential and light commercial distribution boards. Overvoltage category III confirms it's rated for fixed-installation panels, not just downstream equipment. The IP20 rating (with connected conductors) means it's protected against finger contact but not against moisture — standard for dry indoor enclosures. It's sealable, halogen-free, and silicon-free, which matters if the panel goes into a clean environment or one with sensitive electronics nearby.
Mounting and integration
Occupies 3 modular width units (54 mm) on a DIN rail. Depth is 76 mm, installation depth 70 mm. Mounting position is any — no derating needed for horizontal or upside-down orientation. Accepts supplementary devices (auxiliary switches, shunt trips, alarm contacts) for remote status or emergency shutdown. Rated for pollution degree 2, so it's fine in a typical control cabinet with non-conductive dust. The mechanical service life is 10,000 operating cycles — not a wear item in normal distribution use.