Inspections
oil requires replacing, you should remove the oil, clean
the interior with kerosene, and refill the dashpot to the
Circuit breakers require careful inspection and
proper level with new oil. Ensure that the dashpot is free
cleaning at least once a year. If they are subjected to
of dirt, which may hinder the time-delay effect, and that
unusually severe service conditions, you should inspect
the tripping device is clean, operates freely, and has
them more frequently. Also, if a circuit breaker has
enough travel to trip the breaker. Do not change the
opened due to a heavy load, it should be inspected.
air-gap setting of the moving armature because this
would alter the calibration of the tripping device.
Calibration
Lubricate the bearing points and bearing surfaces
(including latches) with a drop or two of light machine
Perform calibration of circuit breakers following
oil. Wipe off any excess oil.
the Naval Ships' Technical Manual, chapter 300,
recommendations.
The sealing surfaces of circuit-breaker contactor
and relay magnets should be kept clean and free from
Metal Locking Devices
rust. Rust on the sealing surfaces decreases the contact
force and may result in overheating of the contact tips.
Metal locking devices are available that can be
Loud humming or chattering will frequently warn of this
attached to the handles of AQB circuit breakers to
condition. Alight machine oil wiped sparingly on the
prevent accidental operation. All breaker handles are
sealing surfaces of the contactor magnet will aid in
provided with a 3/32-inch hole that permits the locking
preventing rust.
device to be fastened with a standard cotter pin. Naval
Ships' Technical Manual, chapter 300, provides a list of
If wiping arc chutes or boxes with a cloth is not
the stock numbers for three different sizes of breaker
sufficient, clean them by scraping with a file or cleaning
handle locking devices.
pad. Replace or provide new linings when arc chutes or
box linings are broken or burned too deeply. Be certain
SELECTIVE TRIPPING
that arc chutes are securely fastened and that there is
sufficient clearance to ensure that no interference occurs
The purpose of selective tripping is to isolate the
faulty section of the system and, at the same time, to
when the switch or contact is opened or closed
maintain power on as much of the system as possible.
If the shunt and flexible connectors are worn broken
Selective tripping of circuit breakers is obtained by
or frayed, they should be replaced. The shunt and
coordination of the time-current characteristic of the
flexible connectors are flexed by the motion of moving
protective devices so that the breaker closest to the fault
parts.
will open first. The breaker farthest from the fault and
closest to the generator will open last.
If working surfaces of circuit breakers, contractors,
Figure 2-59 shows a portion of a distribution system
with circuit breakers employing selective tripping. The
equipment show signs of rust, you should disassemble
so-called instantaneous tripping time is the minimum
the device and clean the rusted surfaces. Use a light
time required for a breaker to open and clear a circuit
application of oil over the cleaned parts to prevent
when the operation of the breaker is not intentional y
further rusting. The oil should always be used sparingly
delayed. Each circuit breaker will trip in less than 0.1
when wiping over rusted parts that have been cleaned to
second (almost instantaneously) when the current
prevent further rusting. Remember, oil has a tendency
exceeds the instantaneous trip current setting of the
to accumulate dust and grit, which may cause
breaker. In a shipboard selective tripping power system,
unsatisfactory operation.
the individual circuit breakers (generator, bus tie, shore
power, or feeder breakers) differ from each other
Before returning a circuit breaker to service, inspect
depending on the following factors:
all mechanical and electrical connections, including
The available load current
mounting bolts and screws, draw-out disconnect
devices, and control wiring. lighten where necessay.
The available short-circuit current
Operate the circuit breaker manually to make sure that
The tripping time band and trip current settings
all moving parts function freely. Check insulation
selected
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