Westinghouse DK-15, Manual

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Type DK Air Circuit Breaker
Drawing 4-D-2515 shows the arrangement of adjustments on this
tripping attachment, and curve 261560 represents the characteristic inverse
time delay.
The 100% calibration setting by the pointer on the scale plate
is the normal rating of the trip unit, and that value of current will just
trip the breaker with no oil in the pot. Adjustment of the thumb screw to­
ward the 200% calibration setting raises the minimum tripping current in
minute steps as desired. Rotation of the molded pot, properly filled with
oil as explained on the drawing, varies the time delay between 0 and 100%.
Thus, any desired combination of minimum tripping current and time de
can be selected as desired. Drawing 4-D-2515 explains in detail the neces­
sary adjustments for maintaining this overload attachment in service.
Characteristics of Thermal-Magnetic Dual Overload
The general arrangement of this attachment is shown on Drawing
4-D-8316 with inverse time delay characteristics on curve 25.S872. The 100%
calibration setting of the pointer on the scale plate is the normal rating
of the trip unit and that value of current will just trip the breaker in
approximately 30 minutes or longer in a normal ambient temperature. Adjust­
ment of the thumb screw toward the 150% calibration setting raises the mini
mum tripping current in minute steps as desired.
Ambient correction is explained on drawing 4-D-8316 as l% increase
in calibration for each 2oc. rise in temperature. The attachment is cali­
brated for 24oc. ambient, but if it is desired to meet the calibration
characteristics of curve 258 872 at 40°C., for instance, the calibration ad­
justment should be set at approximately 108% for minimum tripping at normal
current rating.
The calibration adjustment can therefore be set as close to the
normal running current of the motor as is desired, depending on ambient
temperature, frequency of starting, etc. Complete protection is thus af­
forded under all conditions of operation.
Full details for adjustment and maintenance of the thermal-mag­
netic trip attachment are explained on drawing 4-D-8316.
Replacement of Overload Attachments
Both types of overload trip units may be easily removed. From the
rear, take out two screws that fasten the magnet assembly to the main panel
and disconnect the trip unit upper
terminal where it is bolted to the main
pole unit shunt section. In the case of drawout studs the finger cluster
and large nut must also be removed. The trip unit may now be removed. Re­
placement is simply the reverse of this procedure.
After the unit is securely mounted to the panel, only one adjust­
ment is necessary for correct operation with the breaker. This adjustment
consists of setting the screw in the outer end of the trip armature lever
to properly trip the breaker with a thin spacer between the armature and
magnet, as explained in both drawings 4-D-2515 and 4-D-8316.
Electric Closing Mechanism
Electrically operated DK circuit breakers are equipped with an
entirely self-contained motor mechanism mounted on the right side of the
breaker frame. Two of the toggle linkage pins serve as dowels to accurately
locate the mechanism insulating base. By removing three bolts, this com­
plete assembly can be taken off the breaker without disengaging any parts
except the wiring.
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