1MRS751802-MUM
REX 521
107
Protection Relay
Technical Reference Manual, Standard Configurations
7.1.17.
3I<
The undercurrent function block is designed to be used for non-directional three-
phase undercurrent protection. Usually the undercurrent function block indicates
loss of load for pumps, fans and conveyors. Protection is important if the pump is
cooled by the liquid it pumps, or if a broken conveyor belt may cause damage for
the process. Also a broken main circuit, problems with measuring devices, and loss
of supply can be indicated.
Fig. 7.1.17.-1
Function block symbol of 3I<
1
Includes the delay of signal relay
2
Includes the delay of heavy-duty output relay
Non-directional undercurrent protection 3I<
Operation mode
Not in use
Alarm
Trip
Operation criteria
1-, 2-, 3-phase
3-phase
Start current
0.10...0.99 x In
Operate time
0.1...600.0 s
Internal blocking
Disabled
Enabled
Blocking time
0...7200 s
Measuring mode
Peak-to-peak
Fundamental frequency
Drop-off time
0.00...60.00 s
Note! The values below apply when
f/fn = 0.95...1.05:
Operation accuracies
±2.5% of set value or ±0.01 x In
Start time
Injected currents = 0.5 x start current:
internal time <112 ms
total time <120 ms
1
Reset time
40...1000 ms (depends on the minimum pulse width set
for TRIP output)
Reset ratio
Typical 1.02 (range 1.02...1.06)
Retardation time
Total retardation time when the current exceeds the
start value <80ms
2
Operate time accuracy
±2% of set value or ±25 ms
2
Frequency dependence of settings and
operate times
Measuring mode Suppression of harmonics
0 No suppression
1 -50dB at f = n x fn,
where n = 2, 3, 4, 5,...
BLOCK
GROUP
RES ET
S TART
TRIP
CBFP
3I<
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