The Siemens 3UG5511-2AR20 is a phase-sequence monitoring relay from the SIRIUS 3UG5 line-monitoring family. It watches for correct phase rotation on a three-phase supply from 160 to 690 V AC, 15–70 Hz, and drops its single changeover contact if the sequence is wrong or a phase is lost. That one contact — spring-loaded terminals — can kill a motor starter or sound an alarm before the line sees a reverse-spin jam.
The 160–690 V AC range covers the common low-voltage motor feeds — 208, 400, 480, and 690 V — without needing a separate control transformer or a different relay for each voltage. The 15–70 Hz window includes both 50 Hz and 60 Hz grids plus variable-frequency drives running below nominal frequency; if your line runs a VFD at 20 Hz output, this relay still sees the phase rotation correctly. No screwdriver torque check needed — push the wire in, it locks. That shaves a few seconds per termination on the panel build, and on a multi-station line those seconds add up to real cycle-time savings. Only one changeover contact (1 CO) — enough to signal a PLC digital input or drop out a contactor coil. If you need separate alarm and run-status outputs, you will need an auxiliary relay or a different variant with more contacts. Plan the I/O count before you wire.
RoHS compliant since 01-06-23, with a REACH candidate-list declaration for lead (CAS 7439-92-1 > 0.1% w/w).