What This Relay Covers
The Siemens 3RB1257-0KG21 is a solid-state overload relay that packs thermistor motor protection and internal earth-fault detection into one unit. Rated for 125 to 500 A AC at 110 to 120 V 50/60 Hz, it's sized for large induction motors — think conveyors, pumps, compressors pulling that current band. The 1NO/1NC bistable outputs separate overload/thermistor from earth-fault signaling, so you can wire each alarm into a different PLC input or annunciator without external logic.
The 125 to 500 A range covers the full-load current of a 75 to 350 kW 400 V motor (roughly), but the real selection gate is the 110 to 120 V control voltage. That's the coil supply — if your panel runs 24 VDC control, this relay needs a separate transformer or power supply. The internal earth-fault detection saves an external ground-fault relay and its CT, but verify the trip level matches your system's leakage tolerance; some sensitive VFD-driven motors produce enough common-mode current to nuisance-trip a standard earth-fault element.
Mounts on DIN rail in the motor starter column alongside a contactor. The bistable outputs hold state on power loss — useful for fault memory but means a manual or remote reset circuit is required. The 50/60 Hz rating is common across the class; no derating needed for 60 Hz operation.