Solid-state overload relay with integrated thermistor and earth-fault protection
The Siemens 3RB1257-0KB31 is a solid-state overload relay from the SIRIUS 3RB1 series, rated for motor protection from 125 to 500 A. It integrates thermistor motor protection (PTC) and internal earth-fault detection into a single device, reducing panel wiring and component count on motor control centers handling large induction motors — pumps, compressors, conveyors in the 55–250 kW range depending on voltage and duty cycle. Control voltage is DC 24 V, so it drops into existing 24 VDC control circuits without an additional power supply. The bistable output configuration — 1NO/1NC for combined overload, thermistor, and earth-fault signaling, plus a separate 1NO/1NC for overload warning — lets the control system distinguish a trip from a pre-alarm, useful for predictive maintenance schemes.
The 125 to 500 A range covers the full-load current of large motors — a 400 V, 250 kW induction motor draws roughly 450 A at full load, so this relay sits at the top end of the SIRIUS 3RB1 family. The integrated thermistor protection (PTC) means the relay monitors motor winding temperature directly, not just current-derived thermal load, which catches cooling-fan failures or blocked ventilation that a standard overload curve would miss. Internal earth-fault detection eliminates the need for a separate ground-fault relay and core-balance CT in the motor circuit, saving panel space and wiring. The DC 24 V control supply is standard across modern PLC-based control systems; verify that the control transformer or power supply can deliver the relay's own consumption plus coil inrush for the contactor it drives.