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GEK-45375
Once the current in the relay operating coil
is cut off, the relay contacts
will
open
in approximately 6 cycles
(0.1
second) with normal
adjustment of
contact
wipe.
This
permits
the
use
of
the
relay
in
conjunction
with
instantaneous
reclosing
schemes
without
risk
of
a
false
retrip
when
the
circuit breaker is reclosed on a circuit
from which a
fault has just been
cleared.
The instantaneous overcurrent unit present in the IFC51B, 53B and 77B relays
has a transient overeach characteristic as illustrated in Figure 13.
This is
the result of the DC offset that is usually present in the line current at the
inception of a fault.
When determining the pickup setting for this unit, the
transient overreach must be taken into consideration.
The percent transient
overreach
should
be
applied
to
increase
the
calculated
pickup
setting
proportionately so that the instantaneous unit will not overreach a downstream
device
and
thereby cause a
loss
of coordination
in
the
system
protection
scheme.
The operating time characteristics of this unit are shown in Figure
14.
CONSTRUCTION
The
IFC
induction
disk
relays
consist
of
a
molded
case,
cover,
support
structure
assembly,
and
a
connection
plug
to
make
up
the
electrical
connection.
See Cover Figure and Figures 1, 2, 3 and 19.
Figures 2 and 3
show the induction unit mounted to the molded support structure.
This disk is
activated by a current—operating coil mounted on either a laminated EE- or a
i-Magnet.
The
disk
and
shaft
assembly
carries
a
moving
contact,
which
completes the alarm or trip circuit when it touches a stationary contact.
The
disk assembly is
restrained
by a
spiral
spring to
give the
proper contact
closing current.
Its rotation is retarded by a permanent magnet mounted in a
molded housing on the support structure.
The drawout connection/test system for the Cl case, shown
in Figure 19, has
provisions for 14 connection points, and a visible CT shorting bar located up
front.
As the connection plug is withdrawn,
it clears the shorter contact
fingers
in the output contact
circuits
first.
Thus,
the trip circuit
is
opened
before
any other circuits
are
disconnected.
Next,
current
circuit
fingers on the case connection block engage the shorting bar (located at the
lower
front
of
the
case)
to
short-circuit
external
current
transformer
secondary
connections.
The
window
provides
visual
confirmation
of
CT
shorting.
The connection plug then clears the current circuit contact fingers
on the case and finally those on the relay support structure, to de-energize
the drawout element completely.
There is a High-Seismic target and seal-in unit mounted on the front to the
left of the shaft of the time overcurrent unit (see Figure 1).
The seal—in
unit has its coil in series and its contacts in parallel with the contacts of
the time overcurrent unit, such that when the induction unit contacts close,
the seal—in unit picks up and seals in.
When the seal—in unit picks up, it
raises a target into view, which latches up and remains exposed until released
by pressing a reset button located on the upper left side of the cover.
—5—
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