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3-6
Protection Functions
Overcurrent Stage I>
(50/51)
857-UM001A-EN-P – July 2009
Current protection function dependencies
The current based protection functions are relative to I
is dependent of the application mode. In the motor protection
mode all of the current based functions are relative to I
the feeder protection mode to I
I
> (46), I
>> (47), I
> (48), N> (66) are always dependent on
2
2
st
I
and they are only available when application mode is in the
mot
motor protection.
Overcurrent protection is used against short circuit faults and heavy
overloads.
The overcurrent function measures the fundamental frequency
component of the phase currents. The protection is sensitive for the
highest of the three phase currents. Whenever this value exceeds the
user's pick-up setting of a particular stage, this stage picks up and a
start signal is issued. If the fault situation remains on longer than the
user's operation delay setting, a trip signal is issued.
Three Independent Stages
There are three separately adjustable overcurrent stages: I>, I>> and
I>>>. The first stage I> can be configured for definite time (DT) or
inverse time operation characteristic (IDMT). The stages I>> and
I>>> have definite time operation characteristic. By using the
definite delay type and setting the delay to its minimum, an
instantaneous (ANSI 50) operation is obtained.
Figure 3.9 shows a functional block diagram of the I> overcurrent
stage with definite time and inverse time operation time. Figure 3.10
shows a functional block diagram of the I>> and I>>> overcurrent
stages with definite time operation delay.
Inverse Operation Time
Inverse delay means that the operation time depends on the amount
the measured current exceeds the pick-up setting. The bigger the
fault current is the faster will be the operation. Accomplished
inverse delays are available for the I> stage. The inverse delay types
are described in "Inverse Time Operation". The device will show the
currently used inverse delay curve graph on the local panel display.
mode
with following exceptions.
n
, which
and in
mot
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