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TM 5-685/NAVFAC MO-912
Table 5-l. Low voltage circuit breaker
the pin in place, the contacts will close slowly when
the breaker is manually operated. Inspect connec-
tions for signs of arcing or overheating. Replace
faulty connectors and tighten all connections. Clean
Refer to manufacturer's l i t e r a t u r e for specific information on
the connecting surfaces. An infrared (IR) survey is a
individual circuit breakers.
recommended inspection procedure. The IR survey
should be performed when the circuit breaker is
under load and closed to detect overheating of con-
nections. Perform general troubleshooting of the
Check application of overcurrent
Overcurrent pick-up too low
breaker (refer to the following table) if a problem
trip device
develops. If the trouble cannot be corrected, refer to
Check application of overcurrent
Overcurrent time setting too short
the manufacturer's literature for specific informa-
trip device
tion on individual breakers. Instrument transform-
Mechanical binding in over-
Correct binding or replace over-
ers require no care other than keeping them dry
current trip device
condition current trip device
and clean. Refer to manufacturer's literature if spe-
cific information is required. Information related to
Captive thumbscrew on power sen-  Adjust power sensor. Tighten
sor loose. Fail safe circuitry reverts thumbscrew on desired setting
control circuit components is provided in paragraph
characteristics to minimum setting
5-3e of this chapter.
and maximum time delay
Ground sensor coil improperly con- Check polarity of connections to
coil. Check continuity of shield
nec ted
Table 5-l. Low voltage circuit breaker troubleshooting.
and conductors connecting the ex-
ternal ground sensor coil
Refer to manufacturer's l i t e r a t u r e for specific information on
Binding in attachments preventing  Realign and adjust attachments
individual circuit breakers.
resetting of latch
Latch out of adjustment
Adjust latch
Replace spring
Latch return spring too weak or
Adjust contacts
Contacts not aligned
Clean contacts
Contacts dirty, greasy, or coated
Hardened or gummy lubricant
Clean bearing and latch surfaces
with dark film
Safety pin left in push rod
Remove safety pin
Replace contacts
Contacts badly burned or pitted
Replace motor
Motor burned out
Clean surfaces of current-carrying
Current-carrying surfaces dirty
Replace or adjust faulty device
Faulty control circuit component
Corrosive atmosphere
Relocate or provide adequate en-
Increase arcing contact wipe Adjust
Improper contact sequence (main
contact opening sequence Refer to
contacts not sufficiently parted
Increase capacity of bus or cable
Insufficient bus or cable capacity
when arcing contacts part)
opening. Refer to manufacturer's
literature for contact maintenance
Tighten, but do not exceed, elastic
Bolts and nuts at terminal connec-
and adjustment information. Also
limit of bolts or fittings
tions not tight
refer to paragraph 5-3a( I )(,g)
Current in excess of breaker rating
Check breaker applications or
Requires system study and possible
Short-circuit current level above
modify circuit by decreasing load
replacement with breaker having
interrupting rating of breaker
adequate interrupting capacity
Correct bus or cable arrangement
Inductive heating
5-4. Medium voltage elements.
Travel of tripping device does not  Adjust or replace tripping device
provide positive release of tripping
a. Circuit breakers. Medium voltage switchgear
uses oil, air-blast, or vacuum circuit breakers. Usu-
Worn or damaged trip unit parts
Replace trip unit
ally the circuit breakers have draw-out construction
to permit removal of an individual breaker from the
Correct binding condition or re-
Mechanical binding in overcurrent
place overcurrent trip device
trip device
enclosure for inspection or maintenance without de-
energizing the main bus. All of these circuit break-
Tighten, connect, or replace electri-
Electrical connectors for power
ers can quickly interrupt and extinguish the electric
cal connectors
sensor loose or open
arc that occurs between breaker contacts when the
Loose or broken power sensor con- Tighten or re-connect tap coil tap
contacts are separated.


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