The Siemens 3RA2912-0 is a SIRIUS function module that serves as the basic building block for a star-delta (wye-delta) motor starter, designed to mount directly onto 3RT2 contactors. This module handles the interlocking and timing logic that a star-delta transition requires, so you don't need to wire separate timers and auxiliary contactors for the changeover.
Integration with the contactor set
The module snaps onto the contactors and physically bridges the power connections between the line, star, and delta contactors in a 3RT2-based starter assembly. Because it's a dedicated function module rather than a handful of loose timers and relays, the panel wiring is cleaner and the transition timing is fixed by the module's internal circuitry — no field adjustment of timing relays.