17
PAINOS
PAINOS
RI-type characteristic
The RI-type characteristic is a special characteristic used mainly for time grading with existing mechanical
relays. The characteristic is based on the following mathematical expression:
t [s] = k / (0.339 - 0.236 x I> / I) wheret = operating time in seconds
k = time multiplier
I = phase current
I> = set starting current
The graph of the characteristic is shown in Fig.5.
RXIDG-type characteristic
The RXIDG-type characteristic is a special characteristic used mainly for earth-fault protection where a high
degree of selectivity is needed also for high-resistance faults. With this characteristic, the protection need not
to be directional and the scheme can operate without a pilot communication.
The time / current characteristic can be expressed as:
t [s] = 5.8 - 1.35 x log
e
(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.6.
Note! If the setting is higher than 2.5 x I
n
, the maximum continuous carry (3 x I
n
) and the levelling out of
the IDMT-curves at high current levels must be noted.
Note! The high-current end of any inverse time characteristic 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 also of course be linked to a trip output relay.
C
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