The Siemens 3RA2120-4AA26-0BB4 is a SIRIUS direct online starter in size S0 — a pre-assembled combination of a contactor and a thermal overload relay, intended for switching and protecting three-phase motors. It comes supplied with a 3RT2026-1BB40 contactor and a 3RA2921-1BA00 link module, so the coordination between the two is factory-set, not something you tune on the bench. The coil is rated 24 V DC, so confirm your control voltage matches — this is not a 24 V AC or 110 V variant.
Mounting and fit
The S0 frame takes a 45 mm wide footprint on the rail, and the depth of 107 mm means it clears most standard enclosures — but check your door clearance if the starter sits near the hinge side. Clearance spaces: 50 mm upward, 20 mm forward, 20 mm to the side, 10 mm downward. Zero clearance needed behind the unit. That forward clearance is the main gotcha — if you're packing starters tight in a shallow panel, the 20 mm forward allowance for arc venting and wiring bend radius is real, not a suggestion.
RoHS compliance date is 2009-10-01, covering the substance prohibitions under the EU directive. No REACH or UL listing is explicitly stated in the spec record, but the SIRIUS family typically carries UL/CSA certification — verify the specific approval mark on the nameplate if your panel requires it.