The Siemens 3RB1246-1PG21 is a solid-state overload relay from the SIRIUS family, rated 1.25 to 6.3 A at AC 110–120 V 50/60 Hz. It combines motor overload protection with integrated thermistor monitoring and internal earth-fault detection, all in one DIN-rail mount unit. The bistable output contacts — 1NO/1NC for overload/thermistor and a separate 1NO/1NC for earth fault — latch their state even if the control supply drops, so a fault condition stays visible until manually reset.
The 1.25 to 6.3 A current range covers small to medium motor loads — fractional-horsepower pumps, fans, conveyors up to about 3 kW at 400 V. The AC 110–120 V control supply is standard for North American control transformers; the relay draws its own power from that line, so no separate auxiliary supply is needed. The integrated thermistor protection (PTC sensor input) catches motor overtemperature directly from the winding, which a bimetallic overload cannot. The internal earth-fault detection measures residual current — no external core-balance CT required. All three protections feed into the bistable outputs, which hold their state without continuous power.