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Time Overcurrent Relays Type
lAC
A·C TRIPPING ELEMENT
INTRODUCTION
The a-c tripping element is a Type REA relay
element 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 c u r r e n t from the secondary
of
the
current transformer into the trip coil and removes
the c u r r e n t from the trip coil when the breaker
trips.
The tripping element is mounted on the rear of
the frame opposit� the
operating coil
of
the
induction u n it (see Fig.
The operation of this
e
1
e m e n t is illustrated in Fig. 9. The secondary
current circulates through the induction unit cur
rent coil and the main coil of the REA auxiliary
tripping element, returning through the REA con
tacts 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 cur
rent flows in this coil by transformer action, causing
a redistribution of flux which actuates the armature
andthe REA contacts. The opening
of
the REA con
tacts causes the secondary current to flow through
the trip coil which trips the breaker.
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CURRENT
TRANS.
F i g .
9
l AC TARGET
HOLD I HG
CO I L
BREAKER
TR I P CO I L
NORNALL
Y
CLOSED
REA CONTACTS
REA SHOAT I NG CO I L
REA MA I N CO I L
D i agram I l l ustrat i ng Ope rat i on o f Type
l AC
Re l ay s H�v i n g An A-C Tr i pp i ng Un i t
\
I
9
NULTI � L £S O F � I CK-U�
F i g.
1 0
T i me-cu r rent Characte r i s t i c of l n s tan tan,eou s Un i t
9
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