The Siemens 3UG4511-1AP20 is a SIRIUS phase monitoring relay that watches for phase loss and phase sequence on three-phase AC supplies. It's a single-function unit — one relay, one job: monitor the line and drop the output if a phase goes missing or the rotation reverses. Rated for 320 to 500 V AC at both 50 and 60 Hz, it covers the common 400 V industrial distribution bus. The output is a single changeover contact rated 3 A at 250 V AC or 400 V AC (50/60 Hz), and 1 A at 24 V DC, 0.2 A at 125 V DC, 0.1 A at 250 V DC — enough to signal a PLC input or drop out a contactor coil. Housed in a 22.5 mm wide enclosure, IP20 rated, with snap-on DIN rail mounting.
If you're freezing a design or filling a production run, this part is a safe choice for the foreseeable future.
The 22.5 mm width means it takes up one standard modular device slot — no extra space needed for side clearance (0 mm clearance specified upwards, downwards, forwards, backwards, and at the side). Strip length and screw torque follow standard Siemens SIRIUS practice for the 3UG4 family.