The Siemens 3RB1257-0KM00 is a solid state overload relay from the SIRIUS 3RB1 series, designed for full motor protection in the 125 to 500 A range. It mounts in S10 or S12 frame sizes and accepts busbar connections on the main circuit, with screw terminals on the auxiliary circuit. The control voltage is AC 220 to 240 V at 50/60 Hz. The trip class is adjustable from 5 to 30, so you can match the relay's thermal response to the starting characteristics of the motor load — class 10 for standard starts, class 20 or 30 for high-inertia loads like fans or centrifuges. Ground fault signaling is built in, and the reset mode can be set to manual, automatic, or remote.
The 125-500 A range and S10/S12 frame are the key fit parameters — verify the busbar configuration and control voltage against your panel design.
The 125 to 500 A current range covers a wide band of motor FLA ratings, but the S10/S12 frame size determines the physical footprint and busbar spacing. If your panel was laid out for a different frame, the relay will not bolt in without mechanical modification. The AC 220-240 V control coil matches standard European control voltages; verify your control transformer taps before wiring. The adjustable trip class (5 to 30) means this single part can serve motors with short or extended acceleration times. Set it to class 10 for a standard pump or conveyor, class 20 for a loaded crusher start, class 30 for a high-inertia fan. The ground fault signaling output can be wired into a PLC or annunciator for early warning of insulation degradation.