ABB REG670 Technical Manual page 1252

Section 25
Inverse time characteristics
1246
Time
IEC05000131 V1 EN
Figure 576: Inverse time overcurrent characteristics with inst. function
The inverse time characteristic makes it possible to minimize the fault clearance time
and still assure the selectivity between protections.
To assure selectivity between protections there must be a time margin between the
operation time of the protections. This required time margin is dependent of following
factors, in a simple case with two protections in series:
• Difference between pickup time of the protections to be co-ordinated
• Opening time of the breaker closest to the studied fault
• Reset times of the protections
• Margin dependent of the time delay inaccuracy of the protections
Assume we have the following network case.
1MRK502052-UEN B
Fault point
position
en05000131.vsd
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