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GEH-1788 Type IAC Time Overcurrent Relay
APPLICATION
The instantaneous unit is used on certain Type
IAC relay models to provide instantaneous tripping
for current exceeding a predetermined value.
OPERATING CHARACTERISTICS
*
The instantaneous unit operates over a four-
to-one range
and has its
calibration stamped on
a scale mounted beside the adjustable pole piece.
Time-current
characteristics
are shown in Fig.
7A for
the
instantaneous
unit used
before
1977.
In
1977
the
instantaneous
unit
was
modified
to
make
it
have a
higher
seismic rating.
The new
instantaneous unit can be identified by the mark-
ing "Hi -G" moulded
into
the
front
mouldi ng
jus t
above
the
target
windows.
The
time-current
characteristic
for
the
"Hi-G"
unit
is
shown
in
Fig. 7B.
RATINGS
The instantaneous unit is designed to use
the coils listed below;
Coil
Freq
Amp
Volt-
Imp.
P.F.
Amp
Ohms
0.5-2
60
5
360
14.4
0.74
1-4
60
5
90
3.6
0.77
2-8
60
5
22.5
0.90
0.83
25
5
8.0
0.32
0.83
4-16
60
5
5.63
0.225
0.87
50
5
4.77
0.191
0.87
40
5
3.93
0.157
0.88
25
5
2.37
0.095
0.88
7-28
60
5
1.68
0.067
0.88
10-40
60
5
0.83
0.033
0.95
50
5
0.80
0.032
0.95
25
5
0.68
0.027
0.98
20-80
60
5
0.20
0.008
0.95
50
5
0.20
0.008
0.95
25
5
0.18
0.007
0.98
A-C TRIPPING UNIT
INTRODUCTION
The a-c tripping unit is a Type REA relay unit
designed to energize a circuit-breaker trip coil
from its associated cuz:rent transformer upon the
operation of the main unit of the Type IAC relay.
It
transfers the current from the secondary of the
current transformer into the trip coil and removes
the current from the trip coil when the breaker trips.
The tripping unit is mounted on the rear of the
frame opposite the tapped operating coil of the in-
duction unit (see Fig. 5). The operation of this unit
is illustrated in Fig. 11.
The secondary current
8
*
Indicates revision
circulates through the induction unit current coil and
the main coil of the Type REA auxiliary tripping unit,
returning through the Type REA contacts to the cur-
rent transformer. Normally, most ofthefluxgener-
ated
by
the main Type REA coil passes through the
upper limb of the magnetic structure and holds the
armature firmly against this limb. When the contacts
of the induction unit close, the shorting coil of the
Type REA unit is short-circuited and current flows
in this coil by transformer action, causing a redis-
tribution of flux which actuates the armature and the
Type REA contacts.
The opening of the Type REA
contacts causes the secondary current to flow through
the trip coil which trips the breaker.
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