IEC10000273 V1 EN
Figure 94: Fault time with maintained selectivity
To assure selectivity between different protections, in the radial network, there have to be
a minimum time difference ∆t 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 operate time: 15-60 ms
• Protection resetting time: 15-60 ms
• Breaker opening time: 20-120 ms
7.4.3.3 Example
Assume two substations A and B directly connected to each other via one line, as shown
in the figure
95. Consider a fault located at another line from the station B. The fault
current to the overcurrent protection of IED B1 has a magnitude so that the protection will
1MRK 506 334-UUS A Section 7
Current protection
197
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