What this SIRIUS direct starter is
The Siemens 3RA2125-4NH27-0FB4 is a SIRIUS direct-on-line starter — a fully assembled combination motor starter that integrates a contactor, a thermal overload relay, and a circuit-breaker in one package, sized S0. The thermal (bimetallic) overload release is set to CLASS 10 trip characteristics, which means it protects standard squirrel-cage motors during start-up without nuisance tripping on normal acceleration. A magnetic short-circuit trip handles the fault side — the supplied circuit-breaker is a 3RV2021-4NA25, coordinated with the contactor for Type 2 coordination per IEC 60947-4-1.
Mounting and integration — built for 60 mm busbars
This starter is designed specifically for snapping onto 60 mm busbar systems — not for DIN rail. The supplied busbar adapter (8US1251-5NT11) locks it onto the busbar, so plan the panel layout around vertical busbar mounting, not a standard top-hat rail. Mounting position is vertical only. The physical footprint: 45 mm wide, 260 mm tall, 165 mm deep — the width is tight for a size S0, but the depth pushes into the enclosure, so allow clearance behind the panel. Clearance distances are specified: 50 mm upwards, 20 mm forwards, 20 mm at the sides, 10 mm downwards, and 0 mm backwards — the back is flush against the busbar, no extra space needed behind. Both the main power circuit and the auxiliary/control circuit terminate with spring-loaded terminals — push-in style for solid or ferruled stranded wire. No screw torque to check; just strip, push, and pull-test. The auxiliary switch extension is supported.
Supplied components — what's in the box
The 3RA2125-4NH27-0FB4 ships as a pre-assembled unit containing a 3RT2027-2FB40 contactor, a 3RV2021-4NA25 circuit-breaker, a 3RA2921-2AA00 link module, and the 8US1251-5NT11 busbar adapter — all matched for coordination. Operating ambient temperature range is -20 to +60 °C with a relative humidity of 10 to 95 % during operation — suitable for most indoor industrial environments without condensation control.