ABB REB670 Applications Manual page 187

1MRK 505 337-UUS -
Application manual
The operate times of the phase overcurrent protection has to be chosen so that the fault
time is so short so that equipment will not be destroyed due to thermal overload, at the
same time as selectivity is assured. For overcurrent protection, in a radial fed network, the
time setting can be chosen in a graphical way. This is mostly used in the case of inverse
time overcurrent protection. In the figure below is shown how the time-versus-current
curves are plotted in a diagram. The time setting is chosen to get the shortest fault time
with maintained selectivity. Selectivity is assured if the time difference between the
curves is larger than a critical time difference.
IEC05000204 V1 EN
Figure 83: Fault time with maintained selectivity
The operation time can be set individually for each overcurrent protection. To assure
selectivity between different protective 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 between we must have knowledge about operation time of protections,
breaker opening time and protection resetting time. These time delays can vary
significantly between different pieces of equipment. The following time delays can be
estimated:
Section 7
Current protection
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