What it is and what it does
The Siemens 3RT1916-2LD21 is a SIRIUS auxiliary switch — a clip-on accessory for contactors and reversing starters that adds a timed OFF-delay function to the control circuit. Its OFF-delay is adjustable from 0.5 to 10 s, meaning you set the interval between the main coil dropping out and the auxiliary contacts changing state — useful for sequenced shutdowns or post-run cooling on a conveyor drive. Rated control voltage is 200 to 240 V at both 50 and 60 Hz, so it works across the common European and North American control-power standards without a separate transformer tap.
Integration and mounting
Mounting is clip-on — it snaps directly onto the front of a SIRIUS contactor or reversing starter without tools, adding 45 mm width and 38 mm height to the assembly. Depth is 75 mm from the mounting surface, which fits standard 80 mm deep enclosures with room for wiring. Terminals are rated IP20, so the switch is protected against finger contact but not washdown — keep it inside the panel.