18
When IDMT characteristic has been selected,
the operate time of the stage will be a function
of the current; the higher the current, the shorter
the operate time.The stage includes six time/
current curve sets - four according to the BS 142
and IEC 255 standards and two special curve
sets, named RI type and RXIDG type, accord-
ing to ABB standards.
Four standard curves named extremely inverse,
very inverse, normal inverse and long- time
inverse are available. The relationship between
current and time complies with the BS 142.1966
and IEC 255-4 standards and can be expressed
as follows:
t [s] =
where
t = operate time in seconds
k = time multiplier
I = measured current value
I> = set start current value
The relay includes four time/current curve sets
according to BS 142.1966 and IEC 255-4.
The slope of the time/current curve sets is deter-
mined by the constants
α
and
β
as follows:
Slope of the time/
α
β
current curve set
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
According to the standard BS 142.1966 the
normal current range is defined as 2...20 times
the set start current. Additionally the relay must
start at the latest when the current exceeds 1.3
times the set start current, when the time/
current characteristic is normal inverse, very
inverse or extremely inverse. At long-time in-
verse characteristic, the normal range is 2...7
times the set start current and the relay must
start when the current exceeds 1.1 times the
setting.
The following requirements with regard to op-
erate time tolerances are specified in the stand-
ard (E denotes accuracy in per cent, - = not
specified):
k x
β
I
α
I>
- 1
( )
I/I>
Normal inv.
Very inv.
Extremely inv.
Long-time inv.
2
2.22 E
2.34 E
2.44 E
2.34 E
5
1.13 E
1.26 E
1.48 E
1.26 E
7
-
-
-
1.00 E
10
1.01 E
1.01 E
1.02 E
-
20
1.00 E
1.00 E
1.00 E
-
In the defined normal current ranges, the in-
verse-time stages of the overcurrent and earth-
fault unit SPCJ 4D29 comply with the toler-
ances of class 5 for all time/current curves.
The time/current curves specified in the BS-
standards are illustrated in Fig. 3, 4, 5 and 6.
Time/current
characteristic
The operation of the low-set overcurrent stage
I> and the low-set earth-fault stage I
0
> is based
on definite time or inverse time characteristic, as
selected by the user. The operation characteris-
tic is selected with switches 1...3 of switchgroup
SGF1 for the overcurrent stage I> and with
switches SGF1/6...8 for the earth-fault stage I
0
>
(see chapter "Selector switches", page 7).
IDMT
characteristic
