Siemens SIPROTEC 7UT613 series Manual page 182

2 Functions
Figure 2-80 Logic Diagram of the Inverse Overcurrent Protection for Earth Currents — example of IEC characteristic
(simplified)
Dropout
182
You can determine whether the dropout of the stage is to follow right after the thresh-
old 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-energisation. This process corresponds to the back turn of a Ferraris-
disk (explaining its denomination „disk emulation"). In case several faults occur suc-
cessively, 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 protection is combined with other devices (on electro-mechanical or
induction base) connected to the system.
7UT613/63x Manual
C53000-G1176-C160-2