The Siemens 3RB3123-4PE0 is a SIRIUS solid-state overload relay in size S0, designed to protect motors against overload and phase failure by monitoring motor current and tripping when the current exceeds the set threshold. The trip class is adjustable across CLASS 5E, 10E, 20E, and 30E, letting you match the relay's response to the motor's starting characteristics — a 5E setting trips fast for short-start loads like pumps, while a 30E setting gives longer starting room for high-inertia fans or conveyors. The relay mounts directly onto a contactor (contactor mounting), saving DIN-rail space in the panel. Rated insulation voltage is 690 V, and the main circuit uses spring-loaded terminals accepting 2x (0.25... 1.5 mm²) solid or stranded wire, with a single 10 mm² stranded option for the main circuit.
The motor current setting is defined by upper and lower limits: the motor current must be greater than the lower current setting value, and less than the upper current setting value multiplied by 3.5. This means the relay can be adjusted to cover a wide motor current range within the S0 frame — typical for motors up to around 32 A full-load current depending on the setting range of the specific variant. The thermal current rating is 4 A for the signaling contact, which handles the trip-indication circuit. Operating temperature range is -25... +60 °C with temperature compensation across that span, so the relay maintains accurate trip timing even in a warm panel. Vibration resistance is 1-6 Hz at 15 mm amplitude and 6-500 Hz at 20 m/s² for 10 cycles, and shock resistance is 15g / 11 ms — adequate for most industrial machinery and conveyor applications. The relay includes a signaling contact rated at 24 V / 2 A, 120 V / 4 A, and 230 V / 3 A, among other voltage ratings, for remote trip indication. The note for message "tripped" is the standard SIRIUS diagnostic output — it closes when the relay has tripped, which can be wired to a PLC input or a panel lamp.