Siemens SIPROTEC Manual page 220

Functions
2.8 Earth Fault Protection in Earthed Systems (optional)
Inverse time overcurrent stage 3I
The logic of the stage with inverse time delay operates in the same way as the remaining stages. This stage
operates with a specially optimized digital filter that completely suppresses all harmonic components beginning
with the 2nd harmonic. Therefore it is particularly suited for a highly-sensitive earth fault detection. However,
the time delay is calculated here based on the type of the set characteristic, the intensity of the earth current
and a time multiplier 3I0p Time Dial (IEC characteristic, Figure 2-105) or a time multiplier TimeDial
TD3I0p (ANSI characteristic). A pre-selection of the available characteristics was already carried out during
the configuration of the protection functions. Furthermore, an additional fixed delay Add.T-DELAY may be se-
lected. The characteristics are shown in the Technical Data.
Fig. 2-105 shows the logic diagram. The setting addresses of the IEC characteristics are shown by way of an
example. In the setting information the different setting addresses are described in detail.
It is also possible to implement this stage equally with a definite time delay. In this case 3I0p PICKUP is the
pickup threshold and Add.T-DELAY the definite time delay. The inverse time characteristic is then effectively
bypassed.
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SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011