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REJ 523
16
Technical Reference Manual
RI-type characteristic
The RI-type characteristic is a special characteristic that is principally used for
obtaining time grading with mechanical relays. The characteristic can be expressed
mathematically:
k
t [s] =
−
0 339 0 236
.
.
where
t = operate time in seconds
k = time multiplier
I = phase current
I> = set start current.
The RI-type characteristic is illustrated in figure 4.1.3.2-5.
RD-type characteristic
The RD-type characteristic is a special characteristic that is principally used in earth-
fault protection which requires a high degree of selectivity even at high resistance
faults. In this case, the protection can operate in a selective way even if it is not
directional. Mathematically, the time/current characteristic can be expressed as
follows:
t [s] = 5.8-1.35 x log
e
where
t = operate time in seconds
k = time multiplier
I = phase current
I> = set start current.
The RD-type characteristic is illustrated in figure 4.1.3.2-6.
>
I
×
I
I
× >
k I
ABB Substation Automation
Products and Systems
MRS750940-MUM
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