Friday, February 13, 2009

Fuses maintenance

Low Voltage Fuse


Inspection.
Fuse terminals and fuseclips should be examined
for discoloration caused by heat from poor contact or
corrosion. Early detection of overheating is possible through
the use of infrared examination. If evidence of overheating
exists, the cause should be determined.


Cleaning and Servicing.
. All fuse-
holder connections should be tightened. All connections to
specifications should be torqued where available. Fuseclips
should be checked to be sure that they exert sufficient pressure
to maintain good contact. Clips making poor contact
should be replaced or clip clamps used. Contact surfaces of
fuse terminals and clips that have become corroded or oxidized
should be cleaned. Silver-plated surfaces should not be
abraded. Contact surfaces should be wiped with a noncorrosive
cleaning agent. Fuses showing signs of deterioration, such
as discolored or damaged casings or loose terminals, should
be replaced.

Replacement. When replacing fuses, fuse-
holders should never be altered or forced to accept fuses that
do not readily fit. An adequate supply of spare fuses with
proper ratings, especially those that are uncommon, will minimize
replacement problems.
.

Five characteristics should be considered

when replacing fuses:

Interrupting Rating. Fuses should have an interrupting
rating equal to or greater than the maximum fault
current available at their point of application. Fuses have interrupting
ratings from 10,000 amperes to 300,000 amperes.
Voltage. The voltage rating of the fuse should be at
least equal to or greater than the system voltage.
Current. Fuse ampere ratings should be adequate
for the applications. Ratings are determined by the service,
feeder, and branch-circuit conductors, and the loads served.
Time Delay. Time-delay fuses are especially useful on inductive circuits such as motor and transformer
circuits with inrush currents. Time-delay fuses are the
most commonly used fuses on power distribution and motor
circuits.

Current Limitation. Fuses are designated as either
current-limiting or non-current-limiting based on their speed
of response during short-circuit conditions. Non-currentlimiting
fuses can be replaced with current-limiting fuses, but
current-limiting fuses should not be replaced with noncurrent-
limiting fuses unless a review of the specific application
is undertaken.


Special Purpose. Special-purpose fuses are used for
supplementary protection of power systems and for utilization
equipment such as power rectifiers, variable speed drives, and
solid-state controllers. High-speed or semiconductor-type
fuses are most commonly used in these applications. These
fuses have unique performance characteristics and physical
size. They should be matched to the utilization equipment.

Fuses Rated Over 1000 Volts.

Introduction. Fuses rated over 1000 volts consist of
many parts, some current carrying and some non-current carrying,
all subject to atmospheric conditions. These fuses can
be current limiting or non-current limiting, sand or liquid
filled, or vented expulsion type.

Inspection and Cleaning.

The fuse should be disconnected and the mounting
de-energized from all power sources before servicing. Insulators
should be inspected for breaks, cracks, and burns. The
insulators should be cleaned, particularly where abnormal
conditions such as salt deposits, cement dust, or acid fumes
prevail, to avoid flashover as a result of the accumulation of
foreign substances on their surfaces.

Contact surfaces should be inspected for pitting,
burning, alignment, and pressure. Badly pitted or burned
contacts should be replaced.

The fuse unit or fuse tube and renewable element
should be examined for corrosion of the fuse element or connecting
conductors, excessive erosion of the inside of the fuse
tube, discharge (tracking) and dirt on the outside of the fuse
tube, and improper assembly that might prevent proper operation.
Fuse tubes or units showing signs of deterioration
should be replaced.

Bolts, nuts, washers, pins, and terminal connectors
should be in place and in good condition. Lock or latch
should be checked.

.

.


How to Test Contact-Resistance This test is used to test the
quality of the contacts on switches and circuit breakers. A test
set designed for this purpose is available with direct-scale calibration
in microhms, capable of reading contact resistances of
10 microhms or less. An alternate method is to pass a known
level of direct current through the contact structure and to
measure the dc millivolt drop across the contacts. The data
obtained can then be converted to resistance by applying
Ohm’s law. When millivolt drop data is used directly to describe
contact resistance, it is normally stated in terms of the
continuous current rating of the device. Millivolt drop data
obtained at currents lower than the rated continuous current
rating can be converted to the continuous current rating basis
by multiplying the actual millivolt readings by the ratio of the
continuous rated current to the actual test current. The alternate
method requires a source of at least 100 amperes with a
millivolt meter of approximately 0 mV to 20 mV range. The
contact resistance should be kept as low as possible to reduce
power losses at the contacts with the resultant localized heating,
which will shorten the life of both the contacts and near by insulation.

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