What it is and what it does
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.
Key ratings and what they mean for fit
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.
Where it fits in the panel
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.