5-11. Replacement of Contacts
5-9. Measuring Contact Wipe and Pressure (fig.5-2).
a. Stationary Arcing Contacts.
a. Contact Wipe.
(1) Remove the plate by removing the two
(1) Remove arc quenchers, see TM 5-6120-
screws (32, fig. 5-2).
(2) Loosen screws holding spring (leaf contact)
(2) With the breaker open, measure the
horizontal distance from the edge of the stationary contact
(3) Remove pin (30) freeing the stationary
to the stationary block behind it. ("B" dimension for arcing
contacts (31) and springs (7).
contacts; "C" dimension for main contacts).
(4) Install new springs and stationary arcing
(3) Close the breaker and repeat step (2). The
contacts in reverse order.
difference between the readings in step (2) and (3)
(5) Adjust contact wipe and pressure. See
determines the wipe of the contacts.
The movable arcing contacts should be
Extreme care must be used not to trip the
replaced when the stationary arcing
b. Contact Pressure.
contacts are replaced.
(1) Remove arc quenches, see TM 5-6120- 250-
b. Movable Arcing Contacts.
(1) Separate front frame from rear frame. See
(2) Close the breaker and measure dimension
"B" figure 5-2.
(2) Remove pins (11. 24, fig. 5-2) and withdraw
(3) Open the breaker. Place a push-type scale
the contacts (29).
against the stationary arcing contacts (7) at a point
(3) Reassemble in reverse order of steps (2)
measured in line with the break between the contact stop
(9) and the contact pivot block. Push the c contact
c. Stationary, Intermediate Main Contacts.
backward until dimension "B" recorded in step (2) is
(1) Remove stationary arcing contacts as
described in preceding step a.
(4) Read the scale. If the pressure is not within
(2) Remove screws (2, fig. 5-2) and remove
the requirements listed in table 5-1 refer to paragraph 5-
bracket which holds pin (4) in place.
(3) Remove main and intermediate contact stop
5-10. Adjusting Contact Wipe and Pressure
19) which holds the lower part of the stationary contact.
a. Adjusting Center Pole. To obtain proper wipe and
Removal will be easier if the force on the contact
springs (8) is neutralized. This can be accomplished by
pressure, regulate dimension "A" in figure 5-2 to
operating the breaker with operating handle (with safety
increased to increase wipe and decreased to decrease
pin restraining closing springs ) slowly until the load on
the contact stop is relieved.
(1) To change dimension "A" remove the clevis
(4) Loosen the hardware which fastens the
pin (16) and rotate the clevis as necessary.
upper stud (6) to the pole unit base until the contact
(2) To prevent overstressing the clevis threads
spring load on pin (4) is relieved.
dimension "A" should not exceed 3/ 16 inch and space "A"
(5) Remove pin (4) and screws (2) and lift out
should be filled with shims to 0.005 inch of being solid.
contacts (27) and (3).
b. Adjusting Outside Poles.
(6) Reassemble contacts in reverse order,
(1) With the proper center pole wipe obtained,
being careful to replace the intermediate contact (27) in
moving the cross-bar adjusting plate on the center pole to
the proper position.
the right will simultaneously increase the wipe on both
d. Movable Intermediate and Main Contacts.
outside poles; moving the adjusting plate to the left will
(1) Remove the movable arcing contacts as
have the reverse effect.
described in preceding step b
(2) To increase the wipe on either outside pole
(2) Loosen spring (13, fig. 5-2).
individually, move the crossbar adjusting plate of that
(3) Remove braid if present by removing screw
pole to the left; to decrease the wipe move the adjusting
at bottom of contact.
plate to the right.
(4) Slide link (18) to the side and off of pin (25).
(5) Slide pins (25) and (12) far enough to the
If the proper contact pressure does not
side to allow the movable intermediate contacts to be
exist when the contact wipe is within its
limits, the stationary contact springs
(6) Reassemble parts in reverse order. Check
should be replaced.