The Strt I2>(n) signal is fed into a timer to produce the Trip I2>(n) signal.
The Negative Sequence overcurrent trip signal can be blocked by:
• The Second Harmonic blocking function which is for all three phases. The blocking is activated
by setting the I2> (n) 2H Blocking cell to ‘Enabled’, where (n) is the number of the stage.
• User defined blocking logic using ‘IO Mask’ functionality.
2.4 Earth Fault Protection
Earth faults are simply overcurrent faults where the fault current flows to earth (as opposed to between
phases). They are the most common type of fault. There are a few different kinds of earth fault, but the
most common is the single phase-to-earth fault. Consequently, this is the first and foremost type of
fault that protection devices must cover.
Typical settings for earth fault IEDs are around 30-40% of the full load current. If greater sensitivity is
required, then Sensitive Earth Fault should be used.
Earth faults can be measured directly from the system by means of:
• A separate CT located in a power system earth connection
• A separate Core Balance CT (CBCT)
• A residual connection of the three line CTs, whereby the Earth faults can be derived
mathematically by summing the three measured phase currents.
Depending on the device model, it will provide one or more of the above means for Earth fault
protection.
2.4.1 Earth Fault Protection Elements
Earth fault protection is implemented in the columns EARTH FAULT 1 and EARTH FAULT 2 of the
relevant settings group.
Each column contains an identical set of elements, whereby the EARTH FAULT 1 (IN1) column is
used for earth fault current that is measured directly from the system, whilst the EARTH FAULT 2
(IN2) column contains cells, which operate from a residual current quantity that is derived internally
from the summation of the three-phase currents.
The product provides three stages of Earth Fault protection with independent time delay
characteristics, for each EARTH FAULT column.
Stages 1, 2 and 3 provide a choice of operate and reset characteristics, where you can select
between:
• A range of standard IDMT (Inverse Definite Minimum Time) curves
• DT (Definite Time)
For the IN1 column, this is achieved using the cells:
• IN1>(n) Function for the overcurrent operate characteristics
• IN1>(n) Reset Char for the overcurrent reset characteristic
For the IN2 column, this is achieved using the cells:
• IN2>(n) Function for the overcurrent operate characteristics
• IN2>(n) Reset Char for the overcurrent reset characteristic
where (n) is the number of the stage.
Stages 1, 2 and 3 provide a Timer Hold facility. This is configured using the cells IN1>(n) tRESET for
Earth Fault 1 and IN2>(n) tRESET for Earth Fault 2.
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