ABB SPAC 536 C User Manual page 113

RI-type
characteristic
RXIDG-type
characteristic
The RI-type characteristic is a special charac-
teristic used mainly for time grading with exist-
ing mechanical relays. The characteristic is based
on the following mathematical expression:
t [s] = k / (0.339 - 0.236 x I> / I)
The RXIDG-type characteristic is a special
characteristic used mainly for earth-fault pro-
tection where a high degree of selectivity is
needed also for high-resistance faults. With this
characteristic, the protection need not to be di-
rectional and the scheme can operate without
a pilot communication.
The time / current characteristic can be ex-
pressed as:
t [s] = 5.8 - 1.35 x loge (I / (k x I>))
where t = operating in seconds
k = time multiplier
I = phase current
I> = set starting current
The graph of the characteristic is shown in Fig.8.
where t = operating time in seconds
k = time multiplier
I = phase current
I> = set starting current
The graph of the characteristic is shown in Fig.7.
Note!
If the setting is higher than 2.5 x I
mum continuous carry (3 x I
ling out of the IDMT-curves at high current
levels must be noted.
Note!
The high-current end of any inverse time char-
acteristic is determined by the high-set stage
which, when started, inhibits the low-set stage
operation. The trip time is thus equal to the set
t>> for any current higher than I>>. In order
to get a trip signal, the stage I>> must of course
be linked to a trip output relay.
, the maxi-
n
) and the level-
n
19