Date Code 20010731 Breaker Logic 3-41
SEL-352-1, -2 Instruction Manual
Unbalance Breaker Failure (UFpu)
Range: 0–16383 cycles in 1/4-cycle steps
Use SET <ENTER> to access the UFpu setting and define the breaker failure time delay.
When you set the failure time delay, consider the disruptive effects of current unbalance that
may be caused by pole discordance (all three phases not closed simultaneously) on load or
generation equipment and system stability. There may be a limit to the amount of time the
system can withstand single phasing when the breaker carries maximum load. A recommended
setting for the example breaker is UFpu = 60 cycles.
Setting Calculations
Phase Current Unbalance Ratio (46UB)
Increasing the 46UB setting decreases the ratio the relay uses to determine whether an unbalance
exists between the three-phase current magnitudes. Lower unbalance ratio settings make the
relay more sensitive to current unbalance. An example follows.
Assume 46UB = 8 and 1 per-unit current flows in each breaker pole upon closure. If A-phase
and B-phase breaker poles close while the C-phase pole remains open, the relay makes the
following comparisons:
Ia
8
III
pu1
cba
++
>=
Ia
8
011
pu1
++
>=
Ia
8
currentunitper2
pu1
-
>=
Ib
8
currentunitper2
pu1
-
>=
Ic
8
currentunitper2
pu0
-
<=
The setting 46C asserts because C-phase current is less than the sum of phase current magnitudes
divided by 46UB. C-phase current must rise above 0.25 per unit before 46C drops out. If 46UB
equals 64, 46C drops out when C-phase current reaches 0.031 per unit. Thus, higher 46UB
settings permit greater phase current unbalance.
For the example protected breaker, 46UB = 16.
Application Guideline
You can use the current unbalance logic to detect open conductors. The sensitivity of the
detection depends on load conditions and the location of the open conductor.
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