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14.7.2.5.3 Operating time calculation
The operating time depends on the measured current and the selected current-time characteristic.
The formulas for the trip time according to British Standard (BS 142) and IEC 60255-3 are the
following:
where:
t:
Time to trip
k:
Time multiplier to vary time delay (BS 142, 0.05 < K < 1.5)or time value (IEC 60255-3, see table)
á:
Constant according to the list below
â:
Constant according to the list below (BS 142)
I/Ieb:
Fault current factor
I = G:
Actual measured current
I
= G
:
Base current setting value
EB
S
The following table shows the two constants α and ß for the different current-time characteristics.
Current-time characteristic
Normal inverse
Very inverse
Extremely inverse
Long time inverse
The formula is implemented in accordance with BS 142 and the k-factor ranges from of 0.05 to
1.50. When the time multiplier k in the "parameters" dialogue window is set to one (k=1) the IDMT
protections operate in accordance with IEC 60255-3.
The tripping characteristics of the four different IDMT-curves are shown in the following figures.
According to the Standard, the characteristic is only defined for G/Gs or I/I
If the values of the above-mentioned ratio G/G
remains constant as the operation time at the border value of 20.
The IDMT curves are time-dependent according to IEC up to 20 times Ieb when the Ieb value is set
within the permissible range of In, the Nominal Primary Current. If Ieb is set to a value higher than In,
for example 1250A, then the time to trip will be calculated according to IEC curves up to 20 times
1250A=25kA and will then remain constant.
BS142
α α α α α
0.02
1.0
2.0
1.0
S
EC60255-3
ß (BS142) [s]
0.14
13.5
80.0
120.0
EB
or I/I
is higher than 20, the operating time
EB
k (IEC 255-3) [s]
0.14
13.5
80.0
120.0
in the range up to 20.
Note
73
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