Selectivity is assured if the time difference between the curves is larger than a
critical time difference.
Time-current curves
Fault Current
10
10
0.01
10000
en05000204.ai
Str
tfunc1
n
n
tfunc2
Trip time
n
IEC05000204 V2 EN-US
Figure 46: Fault time with maintained selectivity
The operation time can be set individually for each overcurrent protection.
To assure selectivity between different protection functions in the radial network,
there has to be a minimum time difference Dt between the time delays of two
protections. To determine the shortest possible time difference, the operation time
of the protection, the breaker opening time and the protection resetting time must
be known. These time delays can vary significantly between different protective
equipment. The following time delays can be estimated:
Protection operation
time:
15-60 ms
Protection resetting time: 15-60 ms
Breaker opening time: 20-120 ms
1MRK 504 169-UEN A Section 7
Current protection
Transformer protection RET650 2.2 IEC 115
Application manual
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