MAINTENANCE
On larger overload currents, the action is
essentially the same as above except that the
moving armature (B) will close completely as
soon as the overload is applied.
When the
armature closes, it compresses spring (C)
which applies a force to diaphragm (D). After
a time delay determined by valve (F), the dia
phragm movement permits the spring to rotate
the trip crank
(J)
f ar enough to trip the
breaker by moving the trip lever
(K) .
Large fault currents cause the instantane
ous armature (M) to close immediately.
This
armature lifts the trip crank
(J)
without any
delaying action and trips the breaker.
Selective Overcurrent Tripping Device
(Refer to Figure 9A)
For small and intermediate overloads, the
operation of this device is the same as for
the standard overcurrent tripping device.
However, the selective o vercurrent tripping
device operates differently when large fault
currents occur.
When the fault current is large enough to
close the short delay armature (M), the link
age attached to the armature opens valve (P)
which permits air to enter the diaphragm cham
ber at a much faster rate than through the
long delay valve (F). Tripping is then accom
plished by the same means as though a small
overload had occurred.
That is; the main
armature (B) has closed, pulling on the com
pression spring (C) which is restrained by
diaphragm (D) until sufficient air has entered
valve (P) to permit the spring assembly (C)
to lift crank
(J)
and trip the breaker by ro
tating the trip finger
(K) .
A discriminator arm is used on the selective
overcurrent trip units to make the unit behave
as an instantaneous type trip unit while the
breaker is being closed and for a short inter
val of time after closing.
This is achieved
b� having a discriminator latch connection
between the short delay armature bracket and
the trip crank.
When this latch is engaged,
the short delay armature will lift the trip
crank directly if the current is greater than
20
the short delay pick-up setting.
If the cur
rent does not rise above this value, then the
breaker remains closed and the discriminator
arm disengages the discriminator latch so that
the trip unit will then revert to its normal
f unction as one having long and short time
d elay characteristics.
Instantaneous Overcurrent Tripping Device
(Single Element)
This device operates in an instantaneous
manner to trip the breaker at any time when
the current rises above the calibrated setting.
The main armature (B), of Figure 7A, is modi
fied so that it lifts the trip crank-
(J)
and
trjps the breaker directly.
The operation is
similar to the instantaneous trip of the stand
ard overcurrent tripping device.
FIG. lOA - SCHEM ATIC DIAGRAM OF
OVER CURRENT TRIPPING DEV ICE WITH
INSTANTANEOUS ELEMENT ONLY
I.B.
35-240-1
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