22
The relay module incorporates four interna-
tionally standardized time/current curve groups
named "extremely inverse", "very inverse", "nor-
mal inverse" and "long-time inverse". The rela-
tionship between time and current is in accord-
ance with the standards BS 142 and IEC 255-4,
and can be expressed as follows:
t [s] =
where t = operate time
k = time multiplier
I = phase current value
I> = set current value
k x
β
I
α
I>
Time/current
characteristics
The overcurrent stage I> and the low-set re-
sidual current stage I
0
> can be given definite
time or an inverse definite time operation char-
acteristic. The settings of the switches SGF1/
1...3 determine the mode of operation of stage
I> and the switches SGF1/6…8 that of the stage
I
0
>. See section "Setting switches".
At the IDMT characteristic, the operate time of
the stage will be a function of the current: the
higher the current, the shorter is the operate
time. Six time/current curve groups are avail-
able. Four of these comply with the BS 142 and
IEC 255 standards and two curve groups, the RI
and the RXIDG curve groups are special type
curve groups according to ABB praxis.
The values of the constants
α
and
β
determine
the slope as follows:
Time/current
α
β
curve group
Normal inverse
0.02
0.14
Very inverse
1.0
13.5
Extremely inverse
2.0
80.0
Long-time inverse
1.0
120.0
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
by 1.3 times, if the time/current characteristic is
normal inverse, very inverse or extremely in-
verse. 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 by 1.1
times.
The operate time tolerances specified by the
standard are as follows (E denotes accuracy in
per cent, - = not specified):
-
1
I/I>
Normal
Very
Extremely
Long time
2
2,22E
2,34E
2,44E
2,34E
5
1,13E
1,26E
1,48E
1,26E
7
-
-
-
1,00E
10
1,01E
1,01E
1,02E
-
20
1,00E
1,00E
1,00E
-
In the normal current ranges specified above the
inverse time stages of the overcurrent and earth-
fault relay module SPCJ 4D28 fulfil the toler-
ance requirements of class 5 at all degrees of
inversity.
The time/current characteristics according to
the IEC and BS standards are illustrated in Fig.
8 – 11.
Characteristics
according to
IEC 255 and
BS 142
(
)
Summary of Contents for SPAA 341 C /E351
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