The Siemens SIRIUS 3RT1955-5QF31 is a changeover operating mechanism designed specifically for the 3RT105 and 3RT145 contactor frames. It is an electronic magnet coil that accepts a control supply voltage of 96 to 127 V, operating on either AC (50/60 Hz) or DC. The coil integrates AS-Interface communication and an RLT signal, making it a drop-in component for contactor assemblies that require remote status feedback and bus-level control.
The coil is rated for 127 V at both 50 Hz and 60 Hz, and for 96 V and 127 V on DC. This dual-frequency, dual-voltage AC/DC capability means the same coil works across 50 Hz and 60 Hz line supplies without a change in part number, and also accepts DC control circuits — useful in battery-backed or DC-distributed panels. The 96 V DC rating covers the lower end of common nominal DC bus voltages, while the 127 V DC rating matches the standard 110-125 V DC control voltage range used in many industrial installations. The electronic design with AS-Interface and RLT signal means this is not a simple solenoid coil — it contains onboard electronics for bus communication and status feedback. The RLT (Remote Local Trip) signal provides a discrete output that can be wired back to a PLC input or annunciator panel, giving the maintenance team a direct indication of the contactor's state without needing to open the cabinet door.
This mechanism mounts onto the 3RT105 or 3RT145 contactor frame. Those are large-frame contactors (size S6 and up) typically used for motor loads above 100 A or for heavy-duty switching. The changeover mechanism replaces the standard coil assembly, so the panel layout and wiring to the contactor's main power terminals remain unchanged. The AS-Interface connection requires a bus drop cable run to the nearest AS-i segment; the RLT signal is a two-wire dry contact output.