ABB REL650 Applications Manual page 192

Section 7
Current protection
186
³ × ×
I 1.2 k I
t sc max
high
EQUATION1265 V1 EN
where:
1.2 is a safety factor
k is a factor that takes care of the transient overreach due to the DC component of the fault current and
t
can be considered to be less than 1.1
Iscmax is the largest fault current at a fault at the most remote point of the primary protection zone.
The operate times of the phase overcurrent protection has to be chosen so that the fault
time is so short that protected 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. Figure
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.
89 shows how the time-versus-current curves are
1MRK 506 334-UUS A
(Equation 109)
Application manual