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16
T i m e / c u r r e n t
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 7).
When selecting an IDMT mode of operation, the operating time of the stage will be a function of the current;
the higher the current, the shorter the operating time.The unit comprises six different time/current characte-
ristics - four according to the BS 142 standard and two special types called the RI and RXIDG characteristic..
BS-type characteristics
There are four standard curves, extremely, very, normal and long- time inverse. The relationship between
current and time complies with the standards BS 142.1966 and IEC 255-4 and may generally be expressed
as:
k x β
t [s]=
α
(I/I>)
-1
The unit includes four BS 142-specified characteristics with different degrees of inversity.
The degree of inversity is determined by the values of the constants α and β
Degree of inversity
of the characteristic
Normal inverse
Very inverse
Extremely inverse
Long-time inverse
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 characteristic
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.
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.
2
2.22 E
5
1.13 E
7
-
10
1.01 E
20
1.00 E
In the defined normal current ranges, the inverse-time stage of the overcurrent and earth-fault unit
SPCJ 4D24 complies with the tolerances of class 5 at all degrees of inversity.
The time/current characteristics specified in the BS-standards are illustrated in Fig. 3 and 4.
PAINOS
PAINOS
c h a r a c t e r i s t i c s
where
t = operating time in seconds
k = time multiplier
I = current value
I>= set current value
Extremely inv.
2.34 E
2.44 E
1.26 E
1.48 E
-
-
1.01 E
1.02 E
1.00 E
1.00 E
α
β
0.02
0.14
1.0
13.5
2.0
80.0
1.0
120.0
Long-time inv.
2.34 E
1.26 E
1.00 E
-
-
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