The Siemens 3RB1257-0KG11 is a solid-state overload relay from the SIRIUS 3RB1 series, sized for full motor protection in the 125 to 500 A range. It mounts in S10 or S12 frames and accepts busbar connections on the main circuit, with screw terminals for the auxiliary circuit. The control voltage is 110-120 VAC, 50/60 Hz. Trip class is adjustable from 5 to 30. Bistable outputs hold the trip state after a fault. Reset options include manual, automatic, and remote, which gives flexibility for both attended and unattended installations. An overload warning output is standard, useful for predictive maintenance before a hard trip.
Verify compatibility with any replacement family against your panel layout.
Busbar connection on the main circuit means you need busbar adapters or pre-drilled busbars in the panel — not a standard cable lug fit. The auxiliary circuit uses screw terminals, which is straightforward for control wiring. The 110-120 VAC control voltage is common in North American panels. Verify coil draw against your control transformer capacity.