18
Time/current
characteristics
ANSI-type
characteristic
The operation of the overload stage I
b
> is based
on ANSI and IEC inverse time characteristic.
I/Ib>
t [s]
standard durations [s]
1.15
1620
1800
IEC 871-1
1.20
270
300
IEC 871-1
1.30
54
60
ANSI 18-1980, IEC 871-1
1.40
13.5
15
ANSI 18-1980
1.70
0.9
1
ANSI 18-1980
2.00
0.27
0.3
ANSI 18-1980
2.20
0.10
0.12
ANSI 18-1980
Minimum operating time is 100 ms.
The graph of the characteristic is shown in Fig.5.
Phase unbalance-
type characteristic
(modified 99-10)
The operation of the phase unbalance stage
∆
I
2
>
is based on inverse time characteristic near to
normal inverse. The characteristic is based on
the following mathematical expression:
t[s] = k x 101.2/[(100 x
∆
I/
∆
I
2
>)-97]+0.02
where
t
= operating time in seconds
k
= time multiplier
∆
I
= current value
∆
I
2
> = set current value
Note!
It is possible to increase the inverse time char-
acteristic by a factor 10 by setting switch SGF/6
to position 1.
The normal current range is defined as 1...2
times the setting current. Additionally the relay
must start at the latest when the current exceeds
a value of 1.05 times the setting current.
Minimum operating time is 100 ms. For cur-
rents above two times the rated current
∆
I the
relay levels out to definite time operation.
The graph of the characteristic is shown in Fig.6.