Richards 137NP, Инструкция по эксплуатации, Страница: 26 / 53

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B. Contact Construction for 800 through 1875 Ampere Protectors
Each breaker pole has a set of main current-carrying contacts and a set of arcing contacts (see
Figure 6). The main current-carrying contacts consist of pieces of silver-alloy brazed to the
contact bars and to a bridging block. The silver-alloy pieces on the contact bars are
machined flat and the silver-alloy pieces on the ends of the bridging block are rounded
slightly to obtain line contact. The main current-carrying contacts open first and the current
is transferred to the arcing contacts which open last. In closing, the arcing contacts make
first and the main current-carrying contacts last.
With the breaker open, the arcing contacts should be separated by a gap of one inch
minimum. With the arcing contacts on one pole just touching, the maximum gap on the other
two poles should not exceed 1/32 inch. With the arcing contacts just touching, the gap at the
main current-carrying contacts should be 3/16 inch minimum and 3/8 inch maximum on the
top, and 1/8 inch minimum and 5/16 inch maximum on the bottom. During the closing
operation the crossbar and contact support should continue to move a minimum of 3/32 inch
toward the closed position after the main contacts touch. This compresses the heavy springs
behind the main bridging contact blocks to give the required contact pressure.
The contact surfaces should always be kept clean and smooth. After heavy short circuits they
should be carefully inspected and, if necessary, any rough or high spots should be removed.
Use a fine, clean file and wipe with a clean, dry cloth. When filing contacts remove the high
spots only, do not file to the bottom of deep pits. Care should be taken to maintain a line
contact as much as possible. To check the amount of contact obtained, take contact
impressions by holding, between the contacts, a piece of thin carbon paper with tissue paper
on the carbon side and closing the breaker. Open the breaker and examine the impressions
on the paper. Good contact is indicated if a well defined impression shows for 75 percent or
more of the length of the contact. Good contact is also indicated if a 0.001 inch feeler gage
cannot be inserted between the main silver contacts for more than 25 percent of the length of
the contact.
The movable arcing contacts are held in place by high-strength hexagon-head screws which
are locked in position with locking plates (see Figure 6 and 7). When replacing the arcing
contacts make sure that all screws are tight and that the locking plates are bent back around
the screw heads.
The main current-carrying contacts have adequate capacity to carry full-load current even if
the full line contact has been somewhat reduced, and replacement under normal
circumstances is unnecessary. However, if it should be necessary to remove a main bridging
contact, the following procedure is suggested (see Figure 6):
1. Remove the barriers and arc quenchers.
2. Close the breaker