What it is and what it does
The Siemens 3RB1246-1QG21 is a solid-state overload relay with integrated thermistor protection, covering a current range of 6.3 to 25 A on a 110 to 120 V AC control supply at 50/60 Hz. It includes internal earth-fault detection and provides bistable outputs: 1NO/1NC for overload/thermistor and 1NO/1NC for earth fault. This class of relay protects motors against overload, phase loss, and thermistor-triggered overtemperature, and adds ground-fault monitoring in a single package — reducing panel wiring and component count compared to separate relay and ground-fault modules.