What it is and what it does
The Siemens 3UG4851-1AA40 is a SIRIUS RPM monitoring relay with digital setting — it watches a rotating shaft or motor via a sensor input and trips an output if the speed falls outside a programmed window or below a threshold. It runs on a 24 V AC/DC supply (rated 24... 24 V) and draws up to 4 VA at 24 V maximum, so it fits into a standard 24 V control circuit without a separate power module. 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 — these are the switching limits for the alarm or trip signal, not a motor-load contactor.
Configurable response and reset behavior
Start-up delay is adjustable from 0 to 999.9 s, which lets you suppress nuisance trips during motor acceleration or conveyor start sequences. The relay supports both automatic reset (auto-RESET = Yes) and external reset via a remote pushbutton or PLC contact, giving the commissioning engineer flexibility for attended vs. unattended restart. An error memory (Error Memory = Yes) latches the fault state so the last trip condition is retained even if the monitored speed returns to normal — useful for post-mortem diagnostics on a line-down event.
Mounting and environmental fit
The housing is 22.5 mm wide, 91 mm high, and 102 mm deep — a standard single-module DIN-rail footprint that snaps onto the rail (screw and snap-on mounting) and leaves zero clearance needed on any side (0 mm upwards, forwards, backwards, downwards, and at the side). Terminals are rated IP20 (finger-safe), the enclosure is IP40, and the unit operates from -25 to +60 °C with storage/transport down to -40 °C — fine for most control cabinets and unheated warehouses. Pollution degree 2 (normal industrial environment) and galvanic isolation between input and output mean you can share a 24 V supply with the PLC without worrying about ground loops corrupting the speed signal.