ABB Relion Technical Manual page 740

Section 21
Time inverse characteristics
734
Time
IEC05000131 V1 EN
Figure 317: 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 502 048-UUS A
Fault point
position
en05000131.vsd
Technical manual