8
GEH-1753
Time Overcurrent Relays Type
lAC
A·C TRIPPING UNI1
INTRODUCTION
The a-c tripping unit is a Type REA relay unit
designed to energize a circuit breaker trip coil from
its associated current transformer upon the operation
of the main unit of the
lAC
relay. It transfers the
current from the secondary of the current trans
former 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
induction unit
(
see Fig.
8).
The operation of this
unit is illustrated in Fig.
9.
The secondary current
circulates through the induction unit current coil
and the main coil of the REA auxiliary tripping unit,
returning through the REA contacts to the current
transformer.
Normally, most of the flux generated
by the main 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 REA is
short-circuited and current flows in this coil by
transformer action, causing a redistribution of flux
which actuates the armature and the REA contacts.
The opening of the REA contacts causes the secondary
current to flow through the trip coil which trips the
breaker.
CURRENT
TRANS
laS I
BREAKER
TRIP COIL
AI:.
TRIP UH�T
::HORT I PiG
COIL
AC TRII" UNIT
MAIN
CO'IL
Fig.
9
(K-6154766-2)
Diagram Illustrating Operation
of Type
lAC
Relays having an
A-C
Tripping Unit.
.
�
&.U
1-
0..
�
0
0.0;30
0.025
OPERATING TIME
RANGE FOR
Nff
PICKUP
SETTING
O.C'r20
{).0!5
0.010
0.005
0
0
l
2
6
7
10
t.lJL T I PLES Of PI CK-uP
Tl.me-current Characteristics
of
Original Instantaneous Unit
Fig.
10 (&30&872
[S])
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