The Siemens 5SL6120-6 is a 1-pole SENTRON miniature circuit breaker rated 20 A with a B tripping characteristic, breaking 6 kA at 230/400 V AC per both EN 60898 and IEC 60947-2.
This is an active, current-production part — no end-of-life notice, no last-time-buy window to track. The 5SL6120-6 is a standard catalogue item in the SENTRON family, so it's routinely available through independent distribution channels. If you're holding spares for a panel that uses these, you can still order fresh units without worrying about a manufacturer cut-off.
That's fast enough to protect 1.5 mm² and 2.5 mm² copper wiring in final circuits, but it won't nuisance-trip on the modest inrush of a tungsten lighting bank or a small power supply. If you're feeding motor loads or capacitive drivers, step up to a C-curve sibling — but for general-purpose distribution, B is the default. In a densely populated board with multiple MCBs, that heat adds up — account for it in your thermal derating if the enclosure is sealed or stacked without ventilation gaps. The 10 000 mechanical operating cycles typical service life means it's not a daily-switch duty device; it's a protective device that you operate occasionally for isolation or testing. Sealable construction (yes) and halogen-free / silicon-free materials matter for installations that require low-smoke, zero-halogen cable management or where silicone outgassing could contaminate sensitive contacts.