The Siemens 3RB1257-0KG40 is an overload relay with an analog output, designed for motor protection in industrial control panels. Its setting range covers 125 to 500V at a control supply of 110 to 120V 50/60Hz, making it suitable for monitoring voltage conditions on the line side of a motor starter.
The 125 to 500V setting range means the relay's analog output scales proportionally to the monitored line voltage within that window. The 110 to 120V 50/60Hz control supply powers the internal electronics and the analog output loop — verify your control transformer matches this voltage before wiring, as a mismatch will prevent the relay from powering up.
Deployment Context
This relay mounts on a DIN rail inside the motor control center enclosure. The analog output (typically 0-10V or 4-20 mA, depending on the variant) feeds a PLC analog input card for remote voltage monitoring or process feedback — useful for tracking line-side sag or phase imbalance that could affect motor torque on critical loads like pumps or conveyors.