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.
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.