The Siemens 3RB2123-4SB0 is a SIRIUS solid-state overload relay sized for S0 contactors, protecting three-phase motor circuits up to 690 V AC-3 with adjustable trip classes 5E, 10E, 20E, or 30E — so you match the trip curve to the motor start profile (fast for submersible pumps, slow for high-inertia fans). It mounts directly onto the contactor (no separate DIN-rail footprint needed), saving panel width — the body is 45 mm wide and 75 mm deep, which keeps the contactor/overload pair within a single S0 slot. The front face carries IP20 touch protection, and the unit is ATEX certified Ex II (2) GD for use in potentially explosive gas and dust atmospheres.
The auxiliary contact block gives you one NO and one NC contact — enough for a basic run-status feedback and a fault signal back to the PLC, without needing a separate auxiliary contactor. Switching capacity varies with control voltage: 2 A at 24 V, 4 A at 120 V, 3 A at 230 V — these are the make/break ratings for the aux contacts, so size your PLC input or relay coil draw accordingly. Rated surge voltage withstand is 6 kV, and ESD protection meets IEC 61000-4-2 at 6 kV contact / 8 kV air discharge — the relay can live in a panel with drives or contactors without extra suppression on the aux circuit. Operating temperature range is -25 to +60 °C, storage/transport -40 to +80 °C, with 100 % relative humidity allowed — fine for unheated plant rooms or washdown areas as long as the enclosure keeps direct spray off the IP20 front. Shock rated at 15 g for 11 ms; mounting position is any orientation — install it vertically, horizontally, or upside-down without derating.