The Siemens 3RB1246-1QM21 is a solid-state overload relay from the SIRIUS 3RB12 family, designed to protect motors against overload, thermistor overtemperature, and earth faults in a single package. It covers a current range of 6.3 to 25 A on a 220 to 240 VAC 50/60 Hz control supply. The unit integrates three protection functions: overload monitoring via the solid-state trip curve, thermistor motor protection (PTC sensor input), and internal earth-fault detection. It provides bistable outputs — 1NO/1NC for the overload/thermistor channel and a separate 1NO/1NC for the earth-fault channel — meaning the relay latches its state after a trip and requires a manual or remote reset, which is standard for safety-critical motor circuits.
Mounts on a DIN rail in the motor starter assembly, typically paired with a SIRIUS 3RT contactor. The solid-state design eliminates the mechanical wear of bimetal relays, making it suitable for frequent switching or vibration-prone installations like conveyor lines or compressor skids. The 220-240 VAC control supply is drawn from the motor circuit, so verify the control transformer taps before wiring.