ABB REL650 Applications Manual page 201

1MRK 506 334-UUS A
Application manual
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.
Current I
Pickup current
Reset current
The IED does not reset
ANSI10000274 V1 EN
Figure 93: Pickup and reset current for an overcurrent protection
The lowest setting value can be written according to equation 106.
Im ax
³ ×
Ipu 1.2
k
EQUATION1262 V2 EN
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
Section 7
Current protection
Line phase current
Time t
ANSI10000274-1-en.vsd
(Equation 111)
195