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18
Time/current
characteristics
(modified 2002-05)
The operation of the low-set overcurrent stage
I> of the module is based on either definite
time or inverse time characteristics. The mode
of operation is selected with switches 1...3 of
switchgroup SGF1 (see page 9).
When selecting an IDMT mode of operation,
the operating time of the stage will be a func-
tion of the current; the higher the current, the
shorter the operating time.The unit comprises
of six different time/current characteristics - four
according to the BS 142 standard and two spe-
cial types called the RI and RXIDG character-
istic..
BS-type
characteristics
There are four standard curves, extremely, very,
normal and long- time inverse. The relation-
ship between current and time complies with
the standards BS 142.1966 and IEC 60255-3
and may generally be expressed as:
t [s]=
where
t = operating time in seconds
k = time multiplier
I = current value
I> = set current value
The unit includes four BS 142-specified char-
acteristics with different degrees of inversity.
The degree of inversity is determined by the
values of the constants
α
and
β
Degree of inversity
α
β
of the characteristic
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 setting current. Additionally the relay must
start at the latest when the current exceeds a
value of 1.3 times the setting, when the time/
current characteristic is normal inverse, very
inverse or extremely inverse. When the charac-
teristic is long-time inverse, the normal range
in accordance with the standard is 2...7 times
the setting and the relay is to start when the
current exceeds 1.1 times the setting.
k x
β
(I/I>)
α
-1
The following requirements with regard to operating time tolerances are specified in the standard
(E denotes accuracy in per cent, - = not specified):
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 stage of the overcurrent and earth-
fault unit SPCJ 4D24 complies with the toler-
ances of class 5 at all degrees of inversity.
The time/current characteristics specified in the
BS-standards are illustrated in Fig. 3, 4, 5 and 6.
Note.
The actual operate time of the relay, presented
in the graphs in Fig. 3…6, includes an addi-
tional filter and detection time plus the operate
time of the trip output relay. When the operate
time of the relay is calculated using the math-
ematical expression above, these additional times
of about 30 ms in total have to be added to the
time received.