ABB RET650 Technical Manual page 632

Section 21
Time inverse characteristics
626
Time
IEC05000131 V1 EN
Figure 269: 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 pick-up time of the protections to be co-ordinated
• Opening time of the breaker closest to the studied fault
• Reset time of the protection
• Margin dependent of the time-delay inaccuracy of the protections
Assume we have the following network case.
1MRK 504 135-UEN A
Fault point
position
en05000131.vsd
Technical manual