What this SIRIUS starter packs
The Siemens 3RA2120-4DA27-0FB4 is a SIRIUS direct online starter — a factory-assembled combination of a contactor, thermal overload relay, and circuit breaker in one S0-size package. The built-in overload is thermal bimetallic (Class 10 trip), and the short-circuit protection is magnetic — the supplied circuit breaker is a 3RV2021-4DA10. That means it coordinates the motor branch without needing separate components.
Mounting and panel fit
Mounts on 35 mm DIN rail with a snap-on foot, or directly with screws — the S0 footprint is 45 mm wide, 193 mm tall, and 107 mm deep. The vertical mounting position is specified, and clearance around the unit matters: 50 mm above, 20 mm in front, 20 mm to the side, and 10 mm below. No clearance needed behind the rail. Both the main and control circuits use screw-type terminals — no special tooling needed, just a screwdriver. The control coil is 24 V DC, drawing 5.9 W holding power.