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ABB Substation Automation
Products and Systems
1MRS750940-MUM
Technical Reference Manual
β
×
k
t[s] =
α
I
−
1
>
I
where
t = operate time
k = time multiplier
I = phase current value
I> = set current value.
Table 4.1.3.2-1 The values of constants α and β.
Time/current curve group
Normal inverse
Very inverse
Extremely inverse
Long-time inverse
The standard BS 142.1966 defines the normal current range to be 2...20 times the
setting value. In addition, the relay has to start at the latest when the current exceeds
the setting value 1.3 times if the time/current characteristic is normal inverse, very
inverse or extremely inverse. For the long-time inverse characteristic the normal
current range is specified to be 2...7 times the setting, and the relay is to start when
the current exceeds the setting value 1.1 times.
Table 4.1.3.2-2 The operate time tolerances specified by the standard
I/I>
Normal
2
2,22E
5
1,13E
7
-
10
1,01E
20
1,00E
E = accuracy in per cent; - = not specified
In the normal current range, specified above, the inverse-time stage of the overcurrent
relay fulfils the tolerance requirements of class 5 at all degrees of inversity.
The time/current characteristics according to the IEC and BS standards are illustrated
in figures 4.1.3.2-1...4.1.3.2-4.
α
0.02
1.0
2.0
1.0
Very
Extremely
2,34E
2,44E
1,26E
1,48E
-
-
1,01E
1,02E
1,00E
1,00E
REJ 523
β
0.14
13.5
80.0
120
Long time
2,34E
1,26E
1,00E
-
-
15
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