What it is and what it does
The Siemens 3RT1916-2FJ21 is a SIRIUS auxiliary switch — a compact add-on block that clips onto a contactor or starter to provide additional signal contacts for the control circuit. It's the part you add when the main contactor's built-in auxiliary contacts are already used up, or when you need a normally-open / normally-closed combination the base unit doesn't offer. Rated for a maximum switching current of 3 A, it handles 1 A at 24 V DC, 0.2 A at 125 V DC, and 0.1 A at 250 V DC — these are the numbers that govern whether it reliably breaks a PLC input or a small relay coil without welding contacts. The adjustable time-delay range of 0.5 to 10 s (with a 150 ms recovery time and a 200 ms minimum ON period) means this variant includes an onboard timer for sequenced start or delayed dropout — not every auxiliary switch in the SIRIUS family carries that.
Mounting and integration
Clip-on fastening means it snaps directly onto the front or side of a SIRIUS contactor without tools — no DIN-rail footprint consumed beyond the base unit itself. Mounting position is any orientation, which simplifies panel layout when space is tight. Dimensions are 45 mm wide, 38 mm high, and 75 mm deep — the depth is the dimension that projects forward from the contactor face, so verify clearance to the enclosure door or adjacent devices. Wire termination accepts solid or stranded conductors from 18 to 14 AWG (2x 20...14 AWG solid), so it fits standard control wiring without pigtails or ferrule adapters.
Environmental and lifecycle
Rated for operation from -25 to +60 °C, storage and transport from -40 to +85 °C — the storage range is wider, so the part can sit in a cold warehouse or hot truck without damage, but the operating limit is the one that governs in-panel performance.