8
GEH-1753
Time Overcurrent Relays Type lAC
A-C
TRIPPING
UN I1
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.
o.
0..30
0.025
•
�
o.cr2o
�
0.0!5
0..
�
u
0.010
0.005
0
0
1
2
3
CURRENT
TRANS
I&SI
BREAKER
TRIP COIL
AC
TRIP
U�
�T
SHORTIIiG
COIL
AC
TRII' U"IT
MAIN COIL
Fig.
9
(K-6154766-2)
Diagram Illustrating Operatic�
of
Type
lAC
Relays having an
A-C
Tripping Unit.
OPERATING TIME
RANGE FOR Nff
PICKUP
SElliNG
6
7
b
WLT I PLES OF PICK-uP
Tl·me-current Characteristics of Original Instantaneous Unit
Fig.
10
(6306872 [5])
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