ABB REB670 Applications Manual page 178

Section 7
Current protection
172
IEC05000204 V1 EN
Figure 80: Fault time with maintained selectivity
The operation time can be set individually for each overcurrent protection.
To assure selectivity between different protections, in the radial network, there have to be
a minimum time difference Dt between the time delays of two protections. The minimum
time difference can be determined for different cases. To determine the shortest possible
time difference, the operation time of protections, breaker opening time and 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 15-60 ms
time:
Protection resetting time: 15-60 ms
Breaker opening time: 20-120 ms
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Application manual