Siemens 3RB1257-0KG01 Solid State Overload Relay, 125-500 A,

The Siemens 3RB1257-0KG01 is a solid state overload relay rated for full motor protection in the 125...500 A range, designed for Size S10/S12 contactor assemblies. It offers adjustable trip class from 5 to 30, so you can match the thermal curve to the motor start profile — class 10 for standard starts, class 20 for high-inertia loads, class 30 for very long acceleration times. The control voltage is AC 110...120 V, 50/60 Hz, which is common across European and North American control panels. This relay supports manual, automatic, and remote reset modes, giving the maintenance team flexibility to reset from a central HMI or locally at the panel. Bistable outputs mean the relay holds its state after a trip even if control power is lost — useful for fault diagnostics on unattended lines. Ground fault signaling is built in, so you get a separate signal path for earth leakage events without an additional module.