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12
The operation of the low-set stage of the over-
current relay can be based on definite time
characteristic or inverse time characteristic.
When the definite time characteristic has been
selected the operate time of the relay is inde-
pendent of the magnitude of the fault current.
At inverse time characteristic, on the contrary,
the operate time is a function of the fault current
level: the greater the fault current, the shorter
the operate time. Therefore, the relay operate
time is short at close-by faults.
Due to the inverse time characteristic short
overloads, e.g. inrush currents, do not cause
spurious operations. If fuses with a high rated
current are used in the network, inverse time
characteristic has advantages over definite time
characteristic, when time selectivity is concerned.
The low-set stage of the overcurrent relay has
four available inverse time characteristics. The
desired characteristic is selected with the SG1
switches.
In order to obtain selectivity in a network pro-
tected by fuses the characteristic "Extremely
inverse" is recommended. This characteristic is
also recommended to be used when, in every
switching configuration, the short circuit cur-
rent is several times greater than the rated cur-
rent of the feeder. When employing an extreme
Switch SG1/SPCJ 3C3
1
0
}
extremely inverse
2
0
3
1 IDMT characteristic 0 not in use
4
0 no self-holding
5
0 no I>> doubling
6
0 I>> = 2.5...20 x I
7
0
}
t>> = 0.04...1 s
8
0
∑
4
With above switch settings the output relays of
SPAJ 131 C have the following functions:
Output relay (contact)
A (65-66)
B (68-69)
C (80-81)
D (77-78)
E (70-71-72)
inverse characteristic, the relay allows a tempo-
rary overload in the feeder, for instance, during
the run-up of a large motor.
In networks with large fault current variations a
normal inverse characteristic is recommended.
In such a case the protection relay trips the
circuit breaker relatively quickly, even though
the short circuit current exceeds the rated cur-
rent of the feeder only slightly. A normal inverse
characteristic does not permit very heavy over-
loads.
The very inverse characteristic is an intermedi-
ate form between normal inverse and extremely
inverse. In a short circuit situation the operate
time is rather short, even though the short
circuit current varies according to the switching
configuration. On the other hand, the "very
inverse" characteristic, too, allows temporary
overloading of the feeder.
The accuracy limit factor should be considered
when current transformers are selected, because
the use of instantaneous tripping, in particular,
requires current transformers with good capa-
bilities of reproducing high fault currents.
The selector switches of the phase overcurrent
relay SPAJ 131 C can be set as follows:
SGB/SPCJ 3C3
0 not in use
0 not in use
0 no blocking to t>
0 no blocking to t>>
0 not in use
n
0 not in use
0 not in use
Function
CB open (I>, I>>)
Signal on final trip (I>>)
Signal on final trip (I>)
Start of I> stage
Self-supervision alarm
SGR
0 no blocking signal
0 no I>> start to output relay D
1 I> start to output relay D
0 no I> start to output relay D
0 no I>> trip to output relay D
1 I>> trip to output relay A
1 I> trip to output relay C
0 no I> trip to output relay B
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