The Siemens 3RB1253-0FB21 is a solid-state overload relay from the SIRUS 3RB12 series, rated for motor protection from 50 to 205 A on a DC 24 V control supply. It integrates thermistor (PTC) protection for winding temperature monitoring and internal earth-fault detection, with separate bistable output pairs: 1NO/1NC for overload/thermistor and 1NO/1NC for earth fault. The bistable outputs hold their state after a trip, so the relay remembers the fault condition even if control power is lost — useful for remote diagnostics on a conveyor or pump line.
The 50 to 205 A current range covers medium to large motor loads — think 30 kW to 110 kW three-phase induction motors at 400 V. The DC 24 V control supply is standard for modern PLC-driven panels; the relay draws its coil power from the same 24 VDC bus as the rest of the control circuit. The integrated earth-fault detection means you don't need a separate ground-fault module, saving one DIN-rail slot. The bistable outputs are mechanically latched, so they survive a control-power dropout without losing the trip indication — a detail that matters when the panel is remote and the fault log is read later.