Quantcast Network protectors

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TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
(4) Trip devices. The trip devices on low voltage
mechanisms, and the control wiring for correct op-
erations, insulation condition and tightness of con-
circuit breakers provide the electrical decisions
nections. Check on-off indicators, spring-charge in-
needed to detect the difference between normal and
dicators, mechanical and electrical interlocks, key
abnormal conditions of current flow. The mainte-
interlocks, and lock-out fixtures for proper opera-
nance and adjustment of these devices is just as
important as the work performed on the main con-
tion and lubricate where required. In particular,
tacts and operating mechanism. The trip devices
test the positive interlock feature which prevents
are either electro-mechanical or solid-state. Both
the insertion or removal of the breaker while it is in
types are responsible for providing various degrees
the closed position. Check control devices for free-
of fixed, short, or inverse time delays based on the
dom of operation. Replace contacts when badly worn
amount of current they sense. The electro-
or burned. After the breaker has been serviced,
mechanical type, with an air or fluid dashpot for
manually operate it slowly with a closing device to
check for tightness or friction and to see that the
time delay, should be tested as part of the mainte-
nance work performed. A dashpot is a pneumatic or
contacts move to their fully open and fully closed
positions. Electrically operate the breaker several
hydraulic device used for cushioning or damping of
times to check the performance of the electrical ac-
movement to avoid mechanical shock and consisting
cessories using the "TEST" position, an external
essentially of a cylinder containing air or liquid and
a piston moving in it. Testing of the electro-
test/control cabinet, or a test coupler.
mechanical devices requires the use of a low voltage
2-7. Network protectors.
(about 0-20V) but high current (usually O-
The current-carrying parts, main contacts, and op-
50,000A) primary injection test set designed specifi-
erating mechanism of a network protector are very
cally for this purpose. Calibration tests should be
similar to those of the air circuit breaker. This simi-
made to verify that the performance of the device is
larity usually ends with the principal mechanical
within the values shown on the manufacturer's pub-
devices. Unlike the usual feeder circuit breaker, the
lished curves; taking into account that the time-
network protector is more like a tie circuit breaker;
current curves are plotted as a band of values
that is, it is almost always energized on both sides.
rather than a single line (fig 27). Pay careful atten-
This condition requires that extreme care be taken
tion to how the manufacturer has presented the
during installation or removal of the unit from ser-
curve data. There is a wide variety of formats.
vice. The network protector is equipped with special
Check to see that the current is in amperes or mul-
relays that sense the network circuit conditions and
tiples of a pickup value and whether temperature
command the mechanism to either open or close
ranges or previous conditions will affect results.
automatically in response to those conditions. Net-
Usually, the trip devices are tested one unit at a
work protectors are used where large amounts of
time. There are some devices which may use a ther-
power must be distributed to high density load ar-
mal element for time delay. These may have to be
eas such as commercial buildings and office com-
tested all at once to get results similar to those
plexes. To form a network, several incoming power
published by the manufacturer. Check the test con-
sources may be connected. As a result, a short cir-
ditions carefully. If the trip devices do not operate
cuit at any point in the system usually involves very
properly, the calibration and timing components
high fault currents.
should be adjusted or replaced per the manufactur-
a. Safety precautions. Due to the construction
er's recommendations. If repair or replacement of
and purpose of the network protector, taking it out
the electro-mechanical devices is being considered,
of service or placing it back in service is a procedure
then thought should be given toward retrofitting
that must be done while the circuit is energized.
the existing breaker with solid-state trip devices.
During this work, always use the special insulated
This newer technology is generally more reliable
tools provided with the particular model to be ser-
because the parts used to make the trip unit do not
viced. Alternate or make-shift tools are not recom-
drift out of adjustment or suffer the effects of aging
mended unless they have been laboratory tested
or contamination to the same degree as their
and are known to have good safety performance.
electro-mechanical forerunners. If the breakers are
Electrical grade, safety gloves should still be worn
already equipped with solid-state trip devices, they
by the person servicing the unit regardless of the
should also be checked for proper operation and
type or condition of the tools used.
time delay in accordance with the manufacturer's
b. Maintenance. A routine maintenance schedule
published curves. The test procedure recommended
for network protectors should be observed. The fre-
by the manufacturer should be followed.
quency of inspection will vary based on the location
(5) Auxiliary devices. Inspect the closing motor
and environment in which the unit is installed, and
or solenoid, shunt trip coil and mechanism, alarm
2-10





 


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