ABB RELION REX640 Technical Manual page 954

Section 4
Protection functions
948
GUID-9260A322-2431-4453-A0F3-D703F9EE57AE V1 EN
Figure 513: Short-circuit voltage waveform, fault occurs at time 0 seconds
In a typical impedance trajectory during a short circuit, the fault impedance remains
inside the impedance circle for a longer time, in which case the underimpedance
protection provides longer tripping delay times to maintain the time selectivity.
1MRS759142 C
REX640
Technical Manual