EQUATION1265 V1 EN-US (Equation 28)
where:
1.2 is a safety factor
k
t
is a factor that takes care of the transient overreach due to the DC component of the fault current
and can be considered to be less than 1.05
Iscmax is the largest fault current at a fault at the most remote point of the primary protection zone.
The operate time of the phase overcurrent protection has to be chosen so that the fault time
is short enough that the protected equipment will not be destroyed due to thermal overload
while, at the same time, 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. Figure
89 shows 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.
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 89: 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
Section 7 1MRK 505 370-UEN D
Current protection
150 Busbar protection REB670
Application manual
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