What it is and what it does
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 S0 frame size (45 mm wide) fits the standard SIRIUS compact footprint; mounting position is any, so it adapts to tight enclosures or horizontal DIN layouts without derating.
Ratings and what they mean for fit
Rated insulation voltage is 690 V. 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. Operating range is -40 to +70 °C; storage range -55 to +80 °C.
Lifecycle and sourcing reality
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.