Solid-State Overload Relay with Ground Fault and Thermistor Protection
The Siemens 3RB1257-0KM00-0FM0 is a solid-state overload relay rated for motor currents 125 to 500 A AC, operating on a 220-240 V 50/60 Hz control supply. It integrates thermistor motor protection (PTC) and ground-fault monitoring via an external summation current transformer, with separate 1 NO + 1 NC output pairs for each function — overcurrent/thermistor and ground fault — both monostable. This means a single device covers motor overload, winding overtemperature, and ground-fault detection without stacking separate modules, saving DIN-rail space and reducing wiring complexity in motor control centers.
Application Context
The 220-240 V control voltage matches standard North American and European control transformers. The ground-fault input expects a summation CT (not included) — plan for a separate mounting point and wiring back to terminals. The monostable output configuration means the relay resets automatically when the fault clears and the coil is de-energized, which suits remotely monitored or unattended installations.
The 125-500 A AC range covers large motor loads — think 75-350 HP at 480 V — but the relay does not include a built-in current transformer; it relies on the external summation CT for ground-fault sensing. The 220-240 V control supply is non-negotiable: if your panel uses 110 V or 24 VDC control, this relay will not operate. The 1 NO + 1 NC contact arrangement per function provides both a normally-open signal (e.g., to a PLC input) and a normally-closed safety chain (e.g., to a contactor coil drop-out) for each protection channel.