The Siemens SENTRON 5SL1120-6MB is a 1-pole miniature circuit breaker with B-curve tripping characteristic, rated 20 A at 230/400 V AC. The single-module (18 mm) width and 76 mm depth fit standard DIN-rail enclosures without crowding adjacent ways.
B-curve (per EN 60898) means the magnetic trip activates at 3 to 5 times rated current — so 60 to 100 A instantaneous. That suits resistive and lighting loads with modest inrush; it avoids nuisance tripping on cable charging but clears a hard short fast. In a residential board fed by a 6 kA or 10 kA upstream device, this breaker is coordinated downstream — the upstream MCB or fuse clears faults above 3 kA before this unit sees them.
The 18 mm width (1 modular unit) and 70 mm installation depth leave room for cable bending and adjacent devices. Halogen-free and silicon-free construction reduces toxic smoke and contact corrosion in a fire event.