The Siemens SIRIUS 3RB2123-4QD0 is a solid-state overload relay designed for contactor mounting, protecting motor circuits against overload and phase failure. It offers adjustable trip classes 5E, 10E, 20E, and 30E, letting you match the thermal curve to the motor's starting profile — a 5E setting for fast-starting pumps, 30E for high-inertia loads like centrifuges.
The solid-state design means no heating elements to drift — the trip current stays consistent over the -25 to +60 °C operating range, and the relay draws negligible power (0.05 W per pole at rated current). Mounts directly onto a contactor (contactor mounting), saving DIN-rail space in a crowded panel. The front face is IP20 — finger-safe for the enclosure interior — and the housing is 45 mm wide, 73 mm tall, 75 mm deep, which fits standard 45 mm modular spacing. Main circuit terminals accept screw-type connections for 2x (0.25... 1.5 mm²) solid or 2x (14... 10 AWG) stranded — sized for control wiring, not power tails. The auxiliary contact block is not included (0 auxiliary contacts), so plan for a separate contactor auxiliary if you need a status signal back to the PLC.
This class of solid-state overload relay is used in motor control centers and individual starter enclosures to protect three-phase induction motors from sustained overcurrent. The ATEX certification (PTB 06 ATEX 3001 Ex II (2) GD) means it can be installed in the safe area feeding motors in Zone 1 or 2 gas or dust hazardous locations — a common requirement in chemical, oil & gas, and grain-handling lines.