The Siemens SIRIUS 3RU2126-4CB0-ZX95 is a thermal overload relay — the part that sits between a contactor and a motor to protect against sustained overcurrent. The S0 frame size is the compact footprint in the SIRIUS family, sized for motor ratings up to 11 kW at 400 V.
Mounts directly onto the contactor via the contactor mounting fastening method — no DIN rail needed for the relay itself, though the contactor typically clips onto one. Screw terminals for the main circuit, sized for 2x (0.5... 1.5 mm²) or 2x (0.75... 2.5 mm²) solid or stranded wire. The auxiliary switch is integrated, with contacts rated 2 A at 24 V, 3 A at 120 V, and 2 A at 230 V — enough to signal a trip back to the PLC or annunciator panel. The note "for message 'Tripped'" on the auxiliary contact means this is the normally-open set that closes when the relay trips, giving a positive indication.