What this SIRIUS direct starter delivers
The 3RA2125-4AH26-0FB4 is a Siemens SIRIUS direct-on-line starter — a pre-assembled combination of a contactor, thermal overload relay, and circuit-breaker in one S0 frame, designed to snap onto 60 mm busbar systems. The thermal overload is bimetallic, Trip Class 10 — it protects motors with normal acceleration times (up to 10 seconds at locked-rotor current) without nuisance tripping on startup.
Mounting and wiring — busbar snap-on with spring terminals
Mounts by snapping onto 60 mm busbar systems in a vertical orientation — no DIN rail required, which saves panel width and simplifies busbar tap-offs in distribution-style builds. Main power and auxiliary/control circuits both use spring-loaded terminals — no screw torque checking, faster wiring, and vibration-resistant terminations. Dimensions are 165 mm deep, 45 mm wide, 260 mm tall — the 45 mm width (roughly 2.5 standard modular units) fits tightly spaced busbar layouts. Clearance requirements: 50 mm upwards, 20 mm forwards, 10 mm downwards, 20 mm at the side — plan these into the enclosure layout for airflow and access.
Supplied sub-assemblies — what comes in the package
The starter ships as a complete unit with the contactor (3RT2026-2FB40), link module (3RA2921-2AA00), circuit-breaker (3RV2021-4AA25), and busbar adapter (8US1251-5NT11) already assembled and tested. The circuit-breaker provides magnetic short-circuit protection coordinated with the contactor and overload — no separate fuse or MCCB needed upstream for the motor branch. An auxiliary switch can be added via the product extension slot — the frame accepts one without additional wiring modifications.
Environmental limits and operating conditions
Storage and transport range is -50 °C to +80 °C — wide enough for unheated warehouses or container shipping in most climates. Rated for 50/60 Hz supply and relative humidity 10 to 95 % during operation — suitable for indoor industrial environments without condensation control.