Siemens 5SY4120-8CC11, 1-pole miniature circuit breaker, D-curve, 20 A, 230/400 V, 10 kA breaking capacity, railway applications.
The 5SY4120-8CC11: The D-curve (10–20× In) handles high inrush loads such as traction motor auxiliaries, transformer magnetising currents, or DC-link precharge circuits common in rolling stock and wayside power distribution.
Both are 1-pole D-curve 20 A MCBs with 10 kA breaking capacity. The 5SY4120-8CC11 is designated for railway applications; the 5SY4120-8CC10 is the standard industrial variant without the railway-specific rating.
It fits a standard DIN rail enclosure, but the D-curve is designed for high-inrush loads — in a residential panel it would not provide adequate protection for general lighting or socket circuits, which typically require B- or C-curve breakers.
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