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1 General Information
Cradle
Assembly
1 .5 The Instantaneous Overcurrent
Function(Option 1 -1 )
One or two instantaneous overcurrent outputs are available,
Options 1-1 and
1 -2.
Normally one is selected. The instanta
neous pickup point is adjustable from 0.50 times to 20 times
the time overcurrent pickup point. When the sensed input
current exceeds an instantaneous overcurrent pickup setting,
an instantaneous output relay is energized. Also a target
indicator is set to show that an instantaneous output was
tripped. (On multi-phase relays, an additional indicator denotes
the phase or ground element which initiated the tripping.)
If an instantaneous option is selected for a relay which includes
both phase and ground sensing, then an instantaneous output
is supplied which operates for both phase protection and
ground protection. The instantaneous phase and ground pickup
points are independent of each other.
The desired instantaneous overcurrent pickup values for phase
and for ground are entered via the data entry controls at the front
panel. (This can also be done remotely by the data link if the
communications option is present. )
Figure 1 .
Relay Construction
1 .6 The Ground Time Overcurrent and
Instantaneous Functions
When the sensing input type includes ground current sensing,
the relay is supplied with a separate and independent time
overcurrent function for ground, as previously described for the
phases. When instantaneous is selected, it is supplied for both
phase and ground.
The signal for the ground function can be obtained by connect
ing the ground element in the residual circuit of three current
transformers connected in the phases (51 Nor SON /51 N ) , from
a current transformer in series with the connection from the
power transformer neutral to ground (51 G ) . or from a core
balance current transformer encircling all phase conductors,
often called a zero sequence CT (51 GS or SOGS/51 GS ) . Refer
to
Figures 8
1 .
Ground Trip Selectivity
7
This standard feature of the SCOR relay allows the ground
element to be enabled or disabled on the timed and INST 1
output.
Shown with
Front Cover
Assembled
through
1 2.
2
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