[logik-umz-abh-erdstrom-iec-kennlinie-121102-st, 1, en_GB]
Figure 2-82
Logic Diagram of the Inverse Overcurrent Protection for Earth Currents — example of IEC char-
acteristic (simplified)
Dropout
You can determine whether the dropout of the stage is to follow right after the threshold undershot or
whether it is evoked by disk emulation. “Right after” means that the pickup drops out when approx. 95 % of
the set pickup value is undershot. For a new pickup the time counter starts at zero.
The disk emulation evokes a dropout process (time counter is decrementing) which begins after de-energisa-
tion. This process corresponds to the back turn of a Ferraris-disk (explaining its denomination “disk emula-
tion”). In case several faults occur successively, it is ensured that due to the inertia of the Ferraris disk the
“history” is taken into consideration and the time behaviour is adapted. Reset begins as soon as 90 % of the
setting value is undershot, in accordance with the dropout curve of the selected characteristic. In the range
between the dropout value (95 % of the pickup value) and 90 % of the setting value, the incrementing and the
decrementing process is in idle state. If 5 % of the setting value is undershot, the dropout process is
completed, i.e. when a new pickup occurs, the timer starts again at zero.
The disk emulation offers its advantages when the grading coordination chart of the time overcurrent protec-
tion is combined with other devices (on electro-mechanical or induction base) connected to the system.
Functions
2.5 Time Overcurrent Protection for Earth Current
164 SIPROTEC 4, 7UT6x, Manual
C53000-G1176-C230-5, Edition 09.2016
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