What this relay does and how it fits
The Siemens 3RU2126-4BC1 is a SIRIUS thermal overload relay in the size S0 footprint, designed to protect motors against overload and phase failure. It uses a thermal bimetallic release and integrates the auxiliary switch directly into the housing. Rated at 690 V with a maximum AC-3e switching voltage of 690 V, it handles motor circuits up to 7.5 kW at 400 V, 11 kW at 500 V, and 15 kW at 690 V. The auxiliary contact ratings span the control-voltage range you are likely to find in a panel: 2 A at 24 V, 3 A at 120 V, 2 A at 230 V, 1 A at 400 V, and 0.75 A at 690 V. That covers most PLC output cards and contactor coils without a separate interposing relay. Mounting position is any, and the relay is designed for stand-alone installation — it clips directly onto the contactor or can be mounted separately on a DIN rail. The size S0 is the most common frame for motor starters up to about 15 kW; it mates with the corresponding S0 contactor (3RT10 series) without adapters. Spring-loaded terminals accept 2 × (0.5 to 2.5 mm²) solid or stranded wire, which speeds up wiring compared to screw terminals and holds better under vibration.
What the ratings mean for your panel
The physical dimensions — 114 mm height, 45 mm width, 95 mm depth — fit the standard S0 slot. If you are replacing a failed relay in an existing panel, measure the depth clearance: 95 mm is shallower than some older European designs, so it should drop in without hitting the backplane or gland plate. The spring-loaded terminals require a screwdriver with a 3 mm shaft diameter and a 3.0 × 0.5 mm tip — a standard slotted driver, nothing special. Temperature compensation is active from -40 to +60 °C. The MTTF under high-demand operation is 2,280 years.
Compliance and documentation
The relay carries a substance prohibition date of 10/01/2009, which aligns with the original RoHS directive (RoHS 1). Current production complies with RoHS 2/3 and REACH. The SIRIUS family is designed to IEC 60947-4-1 and carries UL/CSA approvals for the North American market. A full declaration of conformity and the technical datasheet are available from Siemens Industry Online Support.