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2.1.3
Phase Undercurrent Protection
The phase undercurrent protection provides is used to detect a decrease in current caused by a loss
of load, typically motor load. Two stage undercurrent protection UC1 and UC2 are available.
The undercurrent element operates for current falling through the threshold level. But the
operation is blocked when the current falls below 4 % of CT secondary rating to discriminate the
loss of load from the feeder tripping by other protection. Figure 2.4.50 shows the undercurrent
element characteristic.
Each phase has two independent undercurrent elements for tripping and alarming. The elements
are programmable for instantaneous or definite time delayed operation.
The undercurrent element operates on per phase basis, although tripping and alarming is three-
phase only.
Scheme Logic
Figure 2.1.51 shows the scheme logic of the phase undercurrent protection.
The undercurrent elements UC1 and UC2 output UC1 TRIP and UC2 ALARM through delayed
pick-up timers TUC1 and TUC2.
This protection can be disabled by the scheme switch [UC1EN] and [UC2EN] or PLC logic signal
UC1 BLOCK and UC2 BLOCK.
Setting value
Operating zone
0.04×In
0
In: rated current
Figure 2.1.50
Undercurrent Element Characteristic
⎯
⎯
57
|I| ≤ UC1 setting
|I| ≤ UC2 setting
I
|I| ≥ 0.04×In
6 F 2 S 0 8 4 2
&
UC1
UC2
&
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