The Siemens 3RB1257-0KG10 is an overload relay for full motor protection, rated 125...500 A and sized for S10 or S12 contactor frames. It provides Class 5...30 trip protection, meaning you can set the trip curve to match motor starting characteristics — Class 5 for short starts, Class 30 for high-inertia loads like fans or centrifuges. The main circuit connects via busbar, and the auxiliary circuit uses screw terminals. Control voltage is AC 110...120 V, 50/60 Hz.
Deployment context
This relay mounts directly to S10 or S12 contactors, or individually on a panel. The busbar connection for the main circuit means it integrates into a busbar distribution system rather than individual cable runs — common in motor control centers. The screw terminals on the auxiliary circuit accept standard wiring for the overload warning and reset functions.