The Siemens 3RB1257-0KM31 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 125 to 500 A AC on a 220-240V 50/60Hz control supply. The outputs are bistable: one set of 1NO/1NC handles the combined overload/thermistor/earth-fault trip, and a separate 1NO/1NC set gives an overload warning signal before the trip threshold is reached.
Where it lands in a panel
This relay goes into a motor control center or distribution panel, sized for the 125-500 A range. The integrated thermistor input means you don't need a separate motor protection relay for PTC sensors — it's one device doing three jobs (overload, thermistor, earth fault). The bistable outputs hold their state on power loss, which matters for fault indication in unattended sites.