The Siemens 3RB1257-0KM40 is an overload relay with an analog output, covering a setting range of 125 to 500 A and a control supply of 220 to 240 V at 50/60 Hz. This is the component that sits between the contactor and the motor circuit, providing thermal overload protection and a continuous analog signal (likely 4-20 mA or 0-10 V, depending on the specific output variant) representing the motor load current. The analog output feeds directly into a PLC analog input card or a drive controller, enabling real-time load monitoring without a separate current transformer and transmitter.
The 125 to 500 A setting range covers a wide swath of medium-to-large induction motors — think 55 kW up to 250 kW on a 400 V supply, depending on the motor's full-load current. The relay's analog output tracks the actual motor current, not just a trip flag, so a panel builder can trend load changes over a shift or trigger a pre-alarm before the thermal element trips. On a 220-240 V control supply, the relay's internal electronics power the analog output and the trip circuit; verify the control transformer secondary is rated for the relay's burden, which is typically a few VA for the electronics plus the output loop power.
The relay mounts directly onto a Siemens SIRIUS 3RT contactor or onto a DIN rail via a separate adapter base. When used with a contactor, the power connections are made through the contactor's load terminals, and the relay's current-sensing elements are in series with the motor leads. The analog output terminals are typically screw-type or spring-cage, sized for 1.5 mm² to 2.5 mm² control wiring. Ensure the analog output loop is wired as a two-wire (loop-powered) or three-wire (separate supply) configuration per the connection diagram — mixing them will damage the output stage.