Solid-state overload relay with integrated thermistor and ground-fault monitoring
The Siemens 3RB1246-1PB00-0FM0 is a solid-state overload relay from the 3RB1 series, designed for motor protection in industrial control panels. It integrates thermistor protection (PTC sensor input) and ground-fault monitoring via an external summation current transformer, all in a single DIN-rail-mountable package. The current setting range is 1.25 to 6.3 A, making it suitable for smaller motor loads. The control supply is DC 24 V. The relay provides two sets of output contacts: one NO/NC pair for overcurrent/thermistor alarm, and a second NO/NC pair for ground-fault alarm. The output configuration is monostable. The ground-fault monitoring function requires an external summation current transformer (e.g., Kloeckner-Moeller ZWA63) to be connected. This allows the relay to detect leakage currents to earth, adding a layer of protection beyond standard thermal overload.
The relay mounts on a DIN rail (standard 35 mm) inside the motor control center or distribution panel. The DC 24 V control supply is typical for PLC-driven systems; verify the power budget on the 24 VDC bus before adding this relay. The external summation current transformer (ZWA63 or equivalent) must be wired to the relay's ground-fault input terminals. The two NO/NC contact pairs can be wired to the PLC digital inputs for alarm annunciation or to a safety circuit for trip initiation.