The pickup current setting inverse time protection or the lowest current step constant
inverse time protection must be given a current setting so that the highest possible load
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
93.
Pickup current
Current I
The IED does not reset
Line phase current
Time t
Reset current
ANSI10000274-1-en.vsd
ANSI10000274 V1 EN
Figure 93: Pickup and reset current for an overcurrent protection
The lowest setting value can be written according to equation 106.
EQUATION1262 V2 EN (Equation 111)
where:
• 1.2 is a safety factor,
• k is the resetting ratio of the protection, and
• Imax is the maximum load current.
The maximum load current on the line has to be estimated. There is also a demand that all
faults, within the zone that the protection shall cover, must be detected by the phase
1MRK 506 334-UUS A Section 7
Current protection
195
Application manual
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