The Siemens 3UG4851-2AA40 is a SIRIUS RPM monitoring relay with digital setting — a DIN-rail mounted speed monitor that watches a rotating shaft or motor and trips an output if the RPM falls outside a set window. Rated supply is 24 V, and the output relay is rated 1 A at 24 V, 0.2 A at 110/125 V, and 0.1 A at 230/250 V — enough to signal a PLC input or drop out a contactor coil. The 22.5 mm width and screw-and-snap-on mounting let it slot into any standard panel without extra drilling.
The 24 V rated supply is the coil voltage — not the signal input from a sensor. You feed 24 V to the relay terminals, and it powers the internal electronics that compare the incoming pulse train against the setpoint. Start-up delay is adjustable 0 to 999.9 s, which means you can suppress nuisance trips during motor acceleration or conveyor start-up. Auto-reset and error memory are both selectable — set it to auto-reset for a simple over-speed alarm, or latch the error memory for a safety-related stop that needs a manual clear. The full-scale value is 1.25 and the initial value is 0.75 — these set the window boundaries for the RPM threshold.
Degree of pollution 2 — suitable for normal industrial environments with non-conductive contamination.