LB. 35-240-1
TYPE DB-75
DE-ION AIR CIRCUIT BREAKER
PICK-UP
ADJUSTMENT
CURRENT MAGNITUDE
FIG.
108-
TYPICAL TIME-CURRENT
C HARACTERISTICS OF OVERCUR RENT
TRIPPING DEVICE WITH INSTANTANEOUS
ELEMENT ONLY
Time-Current Characteristics
STANDARD OVERCURRENT TRIPPING DEVICE
(Refer to Fig. 7B)
The long delay pick-up adjustment can
change the position of the upper part of the
curve through the r ange indicated by the
number
(1).
This adjustment is accomplished
by changing the tension on the spring which
controls the force the long delay armature
must oyercome in order to close.
The long delay time adjustment can be used
to shift the knee of the curve over the range
indicated by the Number (4).
This adjustment
is changed by turning the knob, located at
the bottom of the molded calibration box,
which opens or closed the valve to control
the amount of air entering the diaphragm
chamber.
The instantaneous pick-up adjustment can
shift the vertical part of the curve to the
l eft or right as indicated by number (2).
This is achieved by changing the spring force
applied to the smaller instantaneous armature.
The flat portion of the curve at the bottom
represents the minimum time for the breaker
to clear when fault currents exceed ten times
the trip unit rating.
S ELECTIVE OVERCURRENT TRIPPING DEVICE
(Refer to Figure 9B)
The time-current characteristic of this
trip unit is the same as the standard device
except that the position of the flat portion
of the curve can be shifted as indicated by
the number (3).
This adjustment can be made
b y changing the setting of the short delay
time on the calibration box.
The adjustment
controls the maximum opening of the short de
lay valve and thereby controls the tripping
time when currents are high enough to operate
the short delay armature.
Calibration
Overcurrent tripping devices of this gen
eral type must be calibrated by using a defi
nite procedure and technique, as well as spe
cialized equipment.
Because few customers
have access to such equipment, it is highly
recommended that trip units be returned to
the factory if it appears that they need to
be calibrated.
Maintenance
In ordinary use, this trip unit needs very
little maintenance.
Any accumulation of
dust should be blown off occasionally.
No
oil or lubricant should be applied to any of
the pins or links.
Do not disassemble the
unit for cleaning purposes. In the event that
major repair work is needed, it is advisable
to return the unit to the factory.
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