Current-production 2-pole B-curve MCB, 20 A rated, railway-rated construction
The Siemens SENTRON 5SY4220-6KK11 is a 2-pole miniature circuit breaker with B-curve tripping characteristic, rated 20 A at 35 °C (18.78 A at 40 °C, 16.08 A at 60 °C per the derating curve). Breaking capacity is 10 kA per EN 60898 and 20 kA per IEC 60947-2 — the higher figure governs industrial panel coordination where fault levels can exceed 10 kA. The design is explicitly for railway applications, so it carries vibration and temperature cycling margins beyond a standard commercial MCB.
Derating and thermal budget — the 40 °C and 60 °C numbers matter more than the 35 °C sticker
The 20 A rating at 35 °C is the catalogue headline, but the real-world current you can pull depends on panel ambient. At 40 °C the breaker is good for 18.78 A; at 50 °C, 17.48 A; at 60 °C, 16.08 A. If your panel runs at 50 °C and you're loading the circuit to 18 A, this breaker will nuisance-trip on thermal overload — size up to the next rating or improve ventilation. The -40 to 75 °C storage range means it survives cold warehouses and hot shipping containers without damage.