Siemens SIPROTEC 4 7VK61 Manual page 104

Functions
2.5 Under and over-voltage protection (optional)
[logikdia-unterspgschutz-phasenspg-wlk-310702, 1, en_GB]
Figure 2-29
Phase-to-phase undervoltage
Basically, the phase-to-phase undervoltage protection operates like the phase-to-earth protection except that
it detects phase-to-phase voltages. Accordingly, both phases are indicated during pickup of an undervoltage
stage the value fell below one of the stage thresholds Uph-ph< (address 3762) or Uph-ph<< (address 3764).
Beyond this, Figure 2-29
It is sufficient for the current criterion that current flow is detected in one of the involved phases.
104
Logic diagram of the undervoltage protection for phase voltages
applies in principle.
SIPROTEC 4, 7VK61, Manual
C53000-G1176-C159-5, Edition 05.2018