The Siemens 3RB1253-0FG21 is a solid-state overload relay that combines motor overload protection with integrated thermistor (PTC) monitoring and internal earth-fault detection in one package. It's rated for motor currents from 50 to 205 A AC on a 110 to 120 VAC 50/60 Hz control supply. The relay provides separate 1NO/1NC output pairs for overload/thermistor and earth-fault conditions, all with bistable (latching) outputs so the trip state holds until manually reset — no power-loss dropout to mask a fault.
The 50-to-205 A adjustment range covers a wide swath of standard IEC motor sizes. That 110–120 VAC control supply is the coil voltage for the internal electronics — make sure your control transformer taps match. The integrated thermistor protection means you can run a PTC sensor lead from the motor windings straight back to this relay, catching overtemp before the overload curve alone would trip. The internal earth-fault detection saves an external ground-fault module; it monitors residual current and trips the dedicated 1NO/1NC output pair. Bistable outputs are key for safety circuits — they latch on trip and don't reset on a control-power blip, so the fault stays visible until someone physically resets the relay.