CH-ES 30-32.20 E
ABB Switzerland Ltd
3/3
1.
I
I
PE PB
PE Pr
n n'
N
ct. of N
´
+
+
´£
n : rated overcurrent factor (ALF = accuracy limit factor)
n' : necessary effective overcurrent factor, as a function of
fault current IK, ( at nominal frequency and time constant of
the network)
PB : connected burden at rated current
PE : ct losses of secondary windings
Pr : rated ct burden
I
N
: nominal current related to the protected object
and 2.
the dependence of the curves of fig 1, where:
for fault currents £ 3 * I
N
the CT's should not saturate, respectively
the exact boundary is at b*Ö3 (b = setting value of the characteristic).
0
1
2
3
4
5
6
7
8
9
10
03468101214
with 50% remanence without remanence
IK/ IN
n'
Figure 1: Overcurrent factors
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