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 TM
MO-91
2
are designed to perform up to 5000 and 3000 opera-
ment is not possible. The operating mechanism is
tions for 1200 ampere or 200 ampere breakers, re-
similar to that used in other medium voltage circuit
spectively, without major overhaul. More frequent
breakers, and the general outlines are the same for
servicing may be necessary if operating conditions
maintenance work. The enclosures are similar. Fig-
are severe. Inspection and servicing should be per-
ure 5-11 shows a breaker with the primary electri-
formed after every fault clearing operation. Refer to
cal contacts exposed. The stationary contact is sol-
instructions provided by the manufacturer. Wipe
idly mounted; the moving contact is mounted in the
insulating parts, including bushings and the inside
enclosure with a bellows seal. Contact erosion is
of box barriers; clean off smoke and dust. Repair
measured by the change in external shaft positions
moderate damage to bushing insulation by sanding
after a period of use. Consult the manufacturer's
smooth and refinishing with a clear insulating var-
instruction book. High voltage applied during test-
nish. Inspect alignment and condition of movable
ing may produce X-ray emission. Personnel per-
and stationary contacts. Check their adjustment as
forming a hi-pot test must stay behind a protective
described in the manufacturer's instruction book. To
shield during testing. Condition of the vacuum is
check alignment, close the breaker with pieces of
checked by a hi-pot test applied every maintenance
tissue and carbon paper between the contacts and
period. Consult manufacturer's instruction book for
examine the impression. Do not file butt-type con-
test procedures. The contacts in a vacuum circuit
tacts. Contacts which have been roughened in ser-
breaker cannot be cleaned, repaired or adjusted.
The vacuum bottle is usually replaced if the test
vice may carry current as well as smooth contacts.
Remove large projections or "bubbles" caused by un-
indicates a fault.
usual arcing, by filing. When filing to touch up, keep
5-5. Transfer switches.
the contacts in their original design; that is, if the
contact is a line type, keep the area of contact lin-
During actual or threatened power failure, transfer
ear, and if ball or point-type, keep the ball or points
switches are actuated to transfer critical electrical
shaped out. Check arc chutes for damage. Replace
load circuits from the normal source of power to the
damaged parts. When arc chutes are removed, blow
auxiliary (emergency) power source. When normal
out dust and loose particles. Clean silver-plated
power is restored, the transfer switches either auto-
breaker primary disconnecting devices with alcohol
matically retransfer their load circuits to the nor-
or silver polish (refer to the manufacturer's instruc-
mal supply or must be transferred manually. Volt-
tion book). Lubricate devices by applying a thin film
of approved grease. Inspect breaker operating
monitor each phase of the normal supply. The relays
mechanism for loose hardware and missing or bro-
initiate load transfer when there is a change in
ken cotter pins, retaining rings, etc. Examine cam,
voltage or frequency in any phase outside of prede-
latch and roller surfaces for damage or excessive
termined limits. Additionally, the relays initiate
wear. Clean and relubricate operating mechanism
retransfer of the load to the normal source as soon
(refer to the manufacturer's instruction book). Lu-
as voltage is restored in all the phases beyond the
bricate pins and bearings not disassembled. Lubri-
predetermined pick-up value of the relay. A transfer
cate the ground or polished surfaces of cams, rollers,
switch obtains its operating current from the source
latches and props, and of pins and bearings that are
to which the load is being transferred.
removed for cleaning. Check breaker operating
a. Types of transfer switches. There are two types
mechanism adjustments and readjust as described
of transfer switches: electrically operated or manu-
in the manufacturer's instruction book. If adjust-
ally operated. Electrically operated transfer
ments cannot be made within specified tolerances,
switches also come with an optional bypass func-
excessive wear and need for a complete overhaul is
tion.
indicated. Check control device for freedom of op-
( 1) Electrically operated. An electrically oper-
eration. Replace contacts when badly worn or
ated switch obtains its operating current from the
burned. Inspect breaker control wiring for tightness
source to which the load is being transferred. A
of connections. After the breaker has been serviced,
separate voltage supply is used in some systems.
operate it slowly with closing device to check ab-
Electrically operated switches consist of three func-
sence of binding or friction and check that contacts
tional elements: main contacts to connect and dis-
move to the fully-opened and fully-closed positions.
connect the load to and from the sources of power;
Check electrical operation using either the test cabi-
sensing circuits to constantly monitor the condition
net or test couplers.
of the power source and provide the information
(g) Service vacuum circuit breakers. This
necessary for switch and related circuit operation;
breaker has primary contacts enclosed in vacuum
and transfer mechanism to make the transfer from
containers (flasks), and direct inspection or replace-
source to source.
5-13
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