Siemens 7SC80 Manual page 75

When configuring one of the inverse-time characteristics, the definite-time elements 50-3, 50-2,and 50-1 are
also active (see "Section Definite-time High-set Current Elements 50-3, 50-2, 50N-3, 50N-2" and "Definite-time
Overcurrent Elements 50-1, 50N-1").
A voltage dependency can optionally be set for 51 (see Section "Inverse Time Overcurrent Protection (voltage-
controlled / voltage-dependent)").
Pickup Behavior
For each element, an individual pickup value 51 PICKUP or 51N PICKUP is set. Apart from Fundamental,
the True RMS can also be measured. Each phase and ground current is separately compared with the setting
value 51 PICKUP or 51N PICKUP per element. If a current exceeds 1.1 times the setting value, the corre-
sponding element picks up and is signaled individually. If the inrush restraint function is used, either the
normal pickup signals or the corresponding inrush signals are issued as long as inrush current is detected. If
the 51 element picks up, the tripping time is calculated from the actual fault current flowing, using an inte-
grating method of measurement. The calculated tripping time depends on the selected tripping curve. Once
this time has elapsed, a trip signal is issued provided that no inrush current is detected or inrush restraint is
disabled. If the inrush restraint function is enabled and an inrush condition exists, no tripping takes place but a
message is issued indicating when the overcurrent element time delay elapses.
These elements can be blocked by the automatic reclosing feature (79 AR).
For ground current element 51N, the characteristic may be selected independently of the characteristic used
for phase currents.
Pickup values of elements 51 (phase currents) and 51N (ground current) and the relevant time multiplicators
may be set individually.
The following two figures show the logic diagrams for the inverse time overcurrent protection.
SIPROTEC Compact, 7SC80, Manual
E50417-G1140-C486-A8, Edition 07.2017
2.2 Overcurrent Protection 50, 51, 50N, 51N
Functions
75