The Siemens SIRIUS 3UG3501-1AL20 is a 22.5 mm monitoring relay built for level detection — think pump-tank high/low, sump alarms, or any conductive liquid-level boundary where you need a simple on/off signal. It runs on AC control supply voltage in the 196–253 V range at 50 or 60 Hz, so it drops straight into a standard 230 V panel without a separate power supply.
The adjustable sensitivity spans 5 to 100 kΩ — that covers most conductive liquids (water, weak acids, coolants) but won't trigger on high-resistance foams or dry contacts. Set the threshold with the front potentiometer; no programming tool needed. The single changeover contact (1 CO) handles the load directly for small contactors or PLC inputs, but if you're switching a motor starter, interpose a proper contactor — the relay itself isn't rated for heavy inductive inrush. The screw-type terminals accept solid or stranded wire from 20 to 14 AWG, and finely stranded with ferrule from 0.5 to 2.5 mm². Two conductors per terminal are allowed, which helps when daisy-chaining supply or looping probes. Strip length follows standard practice — about 8 mm — though the spec doesn't call it out explicitly.
Where you'll use it
This class of relay lives in control panels for water/wastewater, process tanks, or any vessel where you need to detect a liquid level and trigger a pump or alarm. The 22.5 mm housing clips onto DIN rail (screw and snap-on mounting,) and fits standard enclosure fill patterns. Depth is 100 mm, height 78 mm — leaves room for wiring ducts in a 200 mm deep cabinet.