The Siemens 5SL6120-7 is a SENTRON miniature circuit breaker (MCB) — one pole, C-curve, rated 20 A, with a 6 kA breaking capacity per both EN 60898 and IEC 60947-2. That 6 kA rating means it can safely interrupt a fault current up to 6,000 A without welding its contacts or cascading damage upstream.
Occupies a single modular width unit (18 mm) on standard DIN rail, with a depth of 76 mm and an installed depth of 70 mm. That 18 mm width is the key dimension for panel fill — one pole, one slot, no surprises. IP20 with connected conductors; fine inside a closed enclosure, not for wet locations.
Rated 20 A at 230/400 V AC, 50/60 Hz. The 6 kA breaking capacity is the same under both EN 60898 and IEC 60947-2 — no derating between the two standards for this part. Single-pole switching, so it protects one phase; for three-phase circuits you'd stack three units or move to a 3-pole variant. Power loss is 2.2 W per pole at rated current in hot operating state — negligible for thermal budgeting in a populated panel, but worth knowing if you're packing 40+ breakers into a sealed enclosure.