8
GEH-1753
Time
Overcurrent
Relays Type
lAC
A·C TRIPPING
UN11
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
a
c
t
ion, 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 tri
p
coil which trips the
breaker.
CURRENT
TRANS
T&SI
BREAKER
TRIP
COIL
AC
TlltP UNIT
SHOI!T I loG
COIL
AC TRII' UNIT
'MAIN CO'IL
Fig.
9
(K-6154766-2)
Diagram Illustrating
Operatio�
of Type
lAC
Relays having an
A-C
Tripping Unit.
.
�
w.J
....
a..
::l
�
0
0.0.)0
0.025
o.c;2o
{).01�
0.010
0.005
0
0
1
2
)
4
OPERATING TIME
RANGE
FOR Nff
PICKUP
SffiiNG
6
7
t.t.IL T I PLES
OF' PI CK-uP
b
10
Tl.me-current Characteristics
of
Original Instantaneous Unit
Fig.
10 (6306872 [5])
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