This is an inverse time over-current alarm.
The alarm response is based on the highest phase current true RMS
values, based on IEC 60255 part 151, as measured by the controller.
The alarm response time depends on an approximated integral of the
current measurement over time. The integral is only updated when the
measurement is above the activation threshold. See the description below
for more details.
Note: The diagram on the right is a simplified representation of this alarm.
The diagram does not show the integral over time.
1.1 = Threshold default setting.
Inverse time over-current calculation method
The controller uses this equation from IEC 60255 to calculate the time that the current measurement may be over the set point
before the inverse time over-current alarm is activated:
where:
t(G)
Theoretical operating time constant value of G, in seconds
k, c and α Constants for the selected curve (k and c in seconds, α (alpha) has no unit)
G Measured value, that is, I
phase
G
S
Alarm set point (G
S
= I
nom
* LIM / 100 %)
TMS Time multiplier setting
Parameters
Configure the parameters under Configure > Parameters > [Source] > Current protections > Inverse time over-current.
Table 3.39
Three-phase inverse time over-current alarm default parameters
Parameter Range Inverse time over-current
Curve See the table below IEC inverse
Limit (the set point, also known as LIM) 2 to 200 % of nominal current 110 %
Time multiplier setting (TMS) 0.01 to 100.0 1.0
Threshold 1.0 to 1.3 1.1
k (only used if custom curve is selected) 0.001 s to 2 min 0.14 s
c (only used if custom curve is selected) 0.000 s to 1 min 0 s
alpha (α, or a) (only used if custom curve is selected) 0.001 to 1 min 0.02
Enable Not enabled, Enabled Not enabled
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