The Siemens 3RU2126-1EC0 is a SIRIUS thermal overload relay — a CLASS 10 device meaning it trips within 10 seconds at 7.2× the set current, fast enough to protect standard induction motors from locked-rotor heating without nuisance tripping on normal start-up. It mounts directly to a contactor (contactor mounting) and uses spring-loaded terminals for the main circuit, so no screwdriver needed for the power connections — just strip and push.
The AC-3e maximum operating voltage is 690 V. Power ratings at various voltages tell you the motor size it can protect: 1.5 kW at 400 V, 2.2 kW at 500 V, 3 kW at 690 V. That's a 2 to 4 HP range in practice — sized for small pumps, fans, conveyors. Auxiliary contact ratings span 24 V (2 A) through 400 V (1 A) — enough to switch a PLC input or a contactor coil directly. The integrated auxiliary switch means no add-on block for the basic trip-signal feedback. Temperature compensation is active from -40 to +60 °C, so the trip curve stays accurate in unheated enclosures or near hot ductwork.
No official successor has been issued because none is needed — the 3RU2126-1EC0 is the active variant for its rating window. If you see a date-code mismatch between this and another S0 relay in the same panel, verify only that the trip current range matches; the mounting and wiring are identical across the S0 family.
Installation and wiring notes
The auxiliary contact is integrated, so no separate block to order for the basic trip-indication circuit. For message 'Tripped' — that's the contact state when the relay has operated; wire it to the PLC or HMI input for fault annunciation. Power dissipation per pole is 1.9 W. In a dense panel with multiple S0 relays side by side, account for that heat in the enclosure thermal budget — not a concern for a single unit, but relevant in a multi-motor group.