current does not cause protection operation. Here consideration also has to be taken to
the protection reset current, so that a short peak of overcurrent does not cause operation
of the protection even when the overcurrent has ceased. This phenomenon is described
in figure 92.
Operate current
Current (A)
The IED does not reset
Line phase current
Time (s)
Reset current
IEC05000203-en-2.vsd
IEC05000203 V4 EN-US
Figure 92: Pick up and reset current for an overcurrent protection
The lowest setting value can be written according to equation
31.
EQUATION1262 V2 EN-US (Equation 31)
where:
1.2 is a safety factor,
k is the resetting ratio of the protection, and
I
max
is the maximum load current.
The maximum load current on the line has to be estimated. From operation statistics
the load current up to the present situation can be found. The current setting must be
valid also for some years ahead. It is, in most cases, realistic that the setting values are
Section 7 1MRK 505 370-UUS A
Current protection
192 Busbar protection REB670 2.2 ANSI
Application manual
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