Phase monitoring relay for AC line supervision
The Siemens 3UG4511-1BN20 is a SIRIUS phase monitoring relay designed to supervise AC supply lines for undervoltage, overvoltage, and phase loss. The relay's output contacts are rated at 3 A at 250 V or 400 V AC (50/60 Hz), which is sufficient to signal a PLC input or drop out a contactor coil. This unit does not include asymmetry detection, overvoltage detection, or undervoltage detection as separate functions — it is a single-function relay that triggers on phase loss or voltage deviation outside the set window. A green LED on the front confirms the output state, and the relay offers a delayed switching time of 2 seconds to ride through brief dips. The 22.5 mm width fits standard panel spacing.
Wiring and integration notes
Terminals are screw-type, and the unit has two auxiliary contact sets (changeover). The auto-reset function is enabled by default, meaning the output re-energizes once the monitored voltage returns to the acceptable window after the delay period. If a latched fault indication is needed, an external reset circuit must be added. Mounting position is unrestricted, and the relay requires zero clearance above, below, or to the sides (0 mm clearance specified in all directions). This simplifies panel layout when space is tight.