The Siemens 3RV2321-4AC20-0BA0 is a SIRIUS S0 frame circuit breaker built for starter combinations — the kind of box that sits between a contactor and a motor in a crusher drive or conveyor panel. It screws or snaps onto a 35 mm DIN rail per DIN EN 60715, and the spring-loaded terminals on the main circuit accept 2x 1–10 mm² solid or stranded copper. That terminal pitch and the 45 mm width mean it rows up cleanly with a SIRIUS contactor on the same rail without adapters.
Rated breaking capacity — the number that matters for fault coordination
At 400 V AC this breaker clears 55 kA — that is the SCCR the panel label will carry, and it is enough to sit behind a 630 kVA transformer feeding a mill motor center. At 240 V AC it holds 100 kA, at 500 V AC 10 kA, and at 690 V AC 4 kA. The backup fuse limits are tighter: a gG 63 A fuse at 400 V, gG 50 A at 500 V, gG 40 A at 690 V. If you are coordinating with upstream fuses, those are the numbers the selectivity study will use, not the breaker's own interrupting rating.
Clearances and panel layout
The body is 97 mm deep, 45 mm wide, 119 mm tall. That zero-forward spec is unusual for an S0 frame; most need some breathing room for arc chute venting. This one does not, which simplifies layout in a tight enclosure.