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.
What the ratings mean for fit
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.
Lifecycle and sourcing
It is sourced to order against an RFQ through the standard Siemens distribution channel.
Mounting and environment
Mounts in any position on a DIN rail; the enclosure is IP40, the terminals are IP20. Operating temperature range is -25 to +60 °C; storage and transport range is -40 to +80 °C. Degree of pollution 2 — suitable for normal industrial environments with non-conductive contamination.